TRANSPORTATION ENGINEERING
UNIT - 1
HIGHWAY PLANNING AND DEVELOPMENT IN INDIA CONTENTS:
Aims / Objectives:
1.1. Introduction
1.2. Different Modes of Transportation 1.3. Characteristics of Road Transportation 1.4. Highway Planning and Development in India 1.5. Summary
1.6. Self Assessment Questions 1.7. Books for Reference.
Aims / Objectives:
Transportation system is required for carrying passengers and goods from one place to the other. The different modes of transport are Roads , Railways , Airways and Water ways. Of all communications , highway or road is the nearest communication to man and is the only means of transportation that offers to the whole community alike. Merits and demerits of roads as a means of transport are presented.
Due to lack of rational and scientific planning road development in India has suffered a major set back. This set back has become a major hurdle in Nation’s march towards progress and prosperity. The government of India formed a Road Development Committee , under the chairman ship of Mr. M.R. Jayakar , in 1927 to examine the desirability of developing the road system of India and the means by which such development could be achieved. The committee submitted its report in 1928 which may be considered as a major land mark in the planned development of roads in our country. The recommendations of the Jayakar Committee and their impact on the planning and development of roads in modern India have been explained.
1.1. Introduction:
Transportation contributes to the economic , industrial and cultural development of any region or country. Transportation is also essential for strategic movement in emergency for the defence of the country and to maintain better law and order. A study of the economic , industrial and cultural development of the advanced nations like the United State , United Kingdom , Japan , Germany and others indicate that the progress and prosperity of any
country or nation may be linked up with the efficiency and adequacy of its transportation systems.
1.2. Different Modes of Transport:
The three media surrounding us , Land , Water and Air , have been used effectively for the development of transportation modes. Land has given scope for the development of road and rail transport. Water and air have developed water ways and air ways. Water ways include seas , rivers , canals and lakes for movement of ships and boats.
The choice of a transportation system depends upon (i) length of haul (ii) weight and size of consignment (iii) traffic density (iv) nature of route and (v) quality of service.
1.3. Characteristics of Road Transport:
Some of the important characteristics of this mode of transport are as follows: (i) Of all communications , road communication is the nearest to men.
(ii) Road transport is the only means of transport that offers itself to the whole community alike.
(iii) Roads are used by various modes of transport , that is , by-cycles , rickshaws , animal drawn carts and carriages , automobiles , etc. ; but railways , airways and waterways are used by rolling stock , aeroplanes and by ships and boats respectively.
(iv) Construction and maintenance cost of roads is cheaper than that of railway tracks , docks and harbours and airports.
(v) Stage Construction is feasible for roads.
(vi) Roads can be constructed to penetrate interior of any region and to connect villages. This advantage becomes particularly evident when planning the communication system in hilly regions and scarcely populated areas. Provision of railways in such areas become uneconomical.
(vii) Road transport offers a complete freedom to the road users to transfer the vehicle from one lane to another and from one road to another according to the need and convenience. (viii) Road transport offers a flexible service , free from fixed schedules.
(ix) In particular for short distance travel and short hauls road transport saves time and is economical.
(x) Road transport offers door to door service.
(xi) Road Transport has a high employment potential.
(xii) Road Transport causes parking problems of serious proportions in city streets. (xiii) One of the serious disadvantages of road transport in its poor record of safety.
(xiv) Road transport has been one of the major causes of environmental pollution. Noise , fumes , vibrations , loss of aesthetics , ribbon development. Cluster of advertisements along highways - are some of the ill-effects.
1.4.0. High-way Planning:
Planning is a pre-requisite for any development programme. This is particularly necessary when long range comprehensive plans have to be drawn.
Before going to the stage of planning , one should fix up the main objectives of a programme. Following are some of the main objectives of highway planning.
(i) To provide for efficient , safe , economical , comfortable and speedy movement of goods and people.
(ii) To plan anticipated future developments.
(iii) To plan for a road system having maximum utility within available resources. (iv) To phase the road development programme from financial considerations. (v) To evolve a financial system.
1.4.1. Planning and Development in India:
The history of roads dates back to the period before the advent of recorded history. The various civilisations of the world that are known for their excellence and attainments have left traces of their art of road building.
In Mauryan age , considerable importance began to be attached to roads as trade , agriculture and cultural activities flourished. Rajapaths (main highways) and vanijapathas (trade routes) were constructed. Kautilya , Prime minister of Emperor Chandragupta Maurya , laid down rules for the construction of roads for different types of traffic in his book ‘Arthashasthra’. During the Pathan and Mughal periods , roads were greatly improved. Some of the highways either built or maintained by Mughals and other rulers received great appreciation from the foreign visitors who visited India during the reign. Roads were built running from North-West to Eastern areas through Gangetic plains , linking also the coastal and central parts. Later the fall of the Mughal empire led to scant attention to communications ; and the conditions of the roads deteriorated considerably. At the beginning of the British rule , roads were constructed by British Military Engineers on the remains of old roads which existed. These roads connected important military and business centres.
Military maintenance of roads was not quite adequate and in 1865 Lord Dalhousie , the then Governor General , formed the public works department in more or less the same form that exists today. Engineering Colleges were established to train civil engineers.
Important roads were provided with metalled beds and were bridged. Specifications were formed for construction of roads. By the end of 19th century , these efforts resulted in the establishment of a good system of trunk roads in the country. With the development of railways (in the latter part of 19th century) a set back to the rapid development of roads occurred. Government of India act 1919 put a further damper on road development as the subject of roads was purely a provincial charge and central government remained concerned with roads of strategic importance.
After world war I , motor vehicle traffic on roads increased. The existing roads were not capable to withstand both bullock-cart traffic and motor vehicles. This demanded better road network which can carry the mixed traffic. In November 1927 , Government of India appointed a committee called the Road Development Committee under the chairmanship of Mr. M.R. Jayakar. The committee , known popularly as the Jayakar committee was required to:
(i) examine the desirability of developing the road system of India and the means by which such development could be achieved.
(ii) examine the possibility , having regard to the distribution of functions between the centre and state governments , of co-ordinating the activities of the different governing authorities in the country by the formation of a central road board or otherwise.
1.4.2. Recommendations of Jayakar Committee: The committee’s report (1928) may be
considered as a major landmark in the planned development of roads in our country. The most important recommendations made by the committee are:
(i) The road development in the country should be considered as a national interest as it has become beyond the capacity of state governments and local bodies.
(ii) An extra tax should be levied on petrol from the road users to develop a road development fund called ‘Central Road Fund’.
(iii) A semi-official technical body should be formed to pool the technical knowledge from various parts of the country and to act as an advisory body on various aspects of roads.
(iv) A research organisation should be instituted to carry research and development work and to be available for consultation.
Most of the recommendations of Jayakar committee were accepted by Government of India and were implemented subsequently.
1.4.3. Central Road Fund: Following the recommendations of Jayakar Committee for
levied on petrol and was intended for road development. Twenty percent of the same will be retained by the central government for meeting the expenses of road development of the country and the remaining will be distributed among the states in the ratio of actual consumption of petrol or revenue collected.
1.4.4. Indian Roads Congress (IRC): A semi-official technical body known as the Indian
Roads Congress (IRC) was formed in 1934 by the government of India. This , it may be recalled , was one of the recommendations of the Jayakar Committee.
Now IRC is an active body of national importance controlling standardisation , specifications and recommendations as regards design and construction of roads and bridges. The IRC publishes journals , research publications and standard specifications on various aspects of Highway and Traffic engineering. The IRC works in collaboration with the Roads wing of the Ministry of Shipping and Transportation , Government of India. It is responsible for the various highway development plans of our country.
1.4.5. Motor Vehicles Act:
In-order to have control over the driver , vehicle ownership and vehicle operation on roads , the Government of India broughtout for the first time the Motor Vehicles Act in 1939. This act has been thoroughly revised in the year 1988.
1.4.6. Nagpur Conference:
At the initiative of IRC , a conference of the chief engineers of all states was held at Nagpur in 1943 to finalise the road development plan for the country as a whole. This may be considered as a landmark in the history of road development in India , as it was the first attempt to prepare a co-ordinated road development programme in a comprehensive and scientific manner. In this conference a 20 year plan , for the period 1943-63 , popularly known as Nagpur Plan , was finalised. All roads were classified into five categories and a target of 16 KM of road per 100 sq. km area of the country was aimed at.
1.4.7. Central Road-Research Institute (C.R.R.I):
One of the recommendations of the Jayakar Committee was to set up a central organisation of research. Accordingly , an institute for carrying out research in various fields of highway engineering , called the Central Road Research Institute (CRRI) was started at New-Delhi in 1950. This institute is mainly engaged in applied research and offers technical advice to state governments , other organisations and industries on various problems concerning highways.
The National Highway Act was passed in the year 1956. According to this act , the development and maintenance of National Highways is the responsibility of the Central government. The Central government is empowered to declare any other highway as National Highway or to omit any of the existing National Highway from the list.
1.4.9. Border Roads Development Board:
This board was set up in March 1960 under the chairmanship of the prime minister. A General Reserve Engineering Force (GREF) consisting of unskilled and skilled labour , supervisors and engineers from all parts of India was raised. This organisation is to take up construction of roads at high altitudes in extremely varied , difficult and hostile terrain under unfavourable climatic conditions.
1.4.10. Second Road Development Plan (1961 ‘ 81):
The second twenty year road development plan for the period 1961 - 81 was initiated by the IRC and was finalised in 1959 and is also known as the Bombay Road Plan. The plan gave due considerations to the developments those are taking place and developments that have to take place in our country in various fields during the plan period. The target of road length contemplated during this plan period was 32 per 100 Sq. Km of area covered.
Though the road length envisaged in this plan has been exceeded , the standards to which these roads have been brought , leave scope for considerable improvement. The position of rural roads is far from satisfactory , Constraint of resources have been the major bottleneck for this state of affairs.
1.4.11. Highway Research Board:
Recognising the need for faster research on highway matters , the Indian Roads Congress has established a Highway Research Board on October 24 , 1973. Its main functions are to advise the government about the road research programme required for the conditions prevailing in our country , correlate the research information from various organisation in India and recommend priorities about various road research problems. It will also obtain feedback of research findings and evaluate the same , collect and disseminate the results of research.
1.4.12. National Transport Policy Committee (NTPC):
This committee was formed in the year 1978 to prepare a comprehensive national transport policy for the country , keeping in view the objectives and priorities set out in the Five Year Plans. This Committee submitted its report in 1980 and most of the recommendations of the Committee have been accepted by Government of India. Some of the
important recommendations include (i) Liberalisation of transport sector , inclusion of transportation in priority sector and optimal inter-modal mix between railway and road transport based on resource - cost consideration and energy conservation.
(ii) Development of roads in rural , hilly and tribal areas ; strengthening of national highways ; increasing the funds for maintenance of roads ; to connect all the villages with all weather low-cost roads within the next twenty years , and
(iii) Separate recommendations for various factors connected with development and growth of road transport by the year 2001.
1.4.13 Third Twenty Year Road Development Plan (1981 - 2001): This plan is also called
‘Lucknow Road Plan’ and has been prepared keeping in view the growth pattern envisaged in various fields by the turn of this century. Some of the points which were given due consideration while formulating the plan are improvement of transportation facilities in villages , towns and small cities , conservation of energy , preservation of environmental quality and improvement in road safety.
1.4.14. National Highway Authority: National Highway Authority was constituted by the
Ministry of Surface Transport with effect from June , 15 , 1989 with the following objectives: (1) National Highways carry 1/3 of the total road traffic and to cope up with the increasing demands of traffic widening of existing sections to four lanes and construction of express ways on the high traffic density corridors are required. The funds for this are met with from external financing institutions like the World Bank and Asian Development Bank. Construction , maintenance and operation of National Highways , hither to done by State P.W.D’s will ultimately be taken over by National Highway authority.
(2) Construction of toll based expressways having grade separated , divided carriage ways will serve as a vailable alternative facilities. These require initial budgetary provision. Within 10 to 15 years the cost of the facilities along with interest is recovered through tolls ; then the revenues generated through tolls will be very high in comparison with maintenance and operation costs. As such ultimately the authority will have enough internal resources of its own for construction of an expressway net work.
(3) With the National Highways Authority of India , taking over execution of National Highway Projects , it will be possible to ensure uniformity and continuity in the improvement to National Highway system , introduction of modern management and operation techniques , and optimum mechanisation and new technology in road construction besides deployment of modern equipment for energy saving and pollution control.
(4) Because of economics better equipment will be deployed and a continuous flow of large scale projects would be feasible with one executing agency handling National Highway Projects.
1.5. Summary:
The basic function of a transportation system is to carry passengers and goods from one place to another. Social , cultural and economic development of a region or country depends upon its transportation system. An efficient transportation system is necessary for maintaining law and order with in the country and for boarder security.
The different modes of transport are roads , railways , airways and waterways. Out of these roads are the most popular and major means of transport and are considered as vital means of communication.
The Jayakar Committees report may be considered as a major land mark in the planned development of roads in our country. Various phases of planning and development of highways in India , during the 20th century , have been dealt with in detail.
1.6. Self Assessment Questions:
(1) Briefly out line the historical development of highways in India. (2) Discuss the merits and limitations of roads as transportation means. (3) Write notes on
(a) Jayakar Committee’s Recommendations (b) Indian Roads Congress
(c) National Transportation Policy Committees (d) Objectives of highway planning.
1.7. Books of Reference:
(1) Bindra , S.P. (1977) - A course in Highway Engineering , Dhanpat Rai and Sons , New - Delhi.
(2) Kadiyali , L.R. (1984) - Principles and Practice of Highway Engineering - Khanna Tech Publications , New - Delhi.
(3) Khanna , Dr. S.K. and Justo , Dr. C.R.G. (1991) - Highway Engineering , Nem chand and Bro., Roorkee.
TRANSPORTATION ENGINEERING
UNIT - 2
ROAD DEVELOPMENT PLANS OF INDIA
CONTENTS:Aims / Objectives:
2.1. Introduction
2.2. Classification of Roads 2.3. Road Development plans 2.4. Rural Road Patterns 2.5. Master Plan Preparation 2.6. Summary
2.7. Self Assessment Questions 2.8. Reference
Aims / Objectives:
Highways have been universally recognised as vital means of transport every where. Three long term plans have been drawn for the development of roads in India for the period 1943 to 2001. Important features of these three plans - Nagpur Plan , Bombay Plan and Lucknow Plan have been explained. Classification of roads based on a rational basis is quite essential for planning of highways in a region or country. Classification of roads based on their location and importance , adopted in the Nagpur Plan , and the modified system of classification of roads adopted in the third 20 year development plan have been discussed. Road patterns generally adopted for rural roads have been included in all long term planning programmes. ‘Master Plans’ play a very important role. Various stages in the preparation of master plans are also indicated in this unit.
2.1. Introduction:
Nagpur Plan is the first plan prepared in India on scientific principles , for the development of highways for the period 1943 - 63. The targets of this plan have been achieved by the end of the second five year plan i.e., by 1961. As such a perspective plan for the period 1961 - 81 , known as ‘Second Twenty Year Plan’ was drafted by the Roads wing of Government of India. The roads in these two plans have been divided into five groups based on their location and importance. Star and Grid pattern of roads have been adopted. Transport planners realised that prosperity of the country takes place only when transportation facilities are extended to rural and undeveloped areas. Further , aspects like
environmental protection , energy conservation and improvement in road safety have also to be included because of the expected very high rate of growth of road traffic by the turn of this century. The Lucknow plan for the period 1981 - 2001 has been drafted including all the above aspects.
2.2. Classification of Roads:
Roads are classified based on a number of considerations , some of the important consideration are ;
(a) Structural Behaviour:
Based on their structural behaviour roads are classified as Flexible pavements and Rigid pavements. These are discussed in units 9 and 10.
(b) Material of Construction:
Based on the materials of construction roads are classified as Earth roads , Gravel roads , Water Bound Macadam roads , Bituminous roads and Cement Concrete roads. These are discussed in Units - 11 and
(c) Service Conditions:
Based on their service conditions during different seasons of a year , roads are divided into two categories - all weather roads and fair weather roads.
(d) Traffic Volume or Load transported or Location and Function:
Classification systems based on the traffic volume or the load transported have been arbitrarily fixed by different agencies and there is no common agreement regarding the limits for each classification group. However , in the system based on location and function , different categories may be defined clearly.
The Nagpur plan and the Lunknow plan have classified the roads based on their location and importance. In the Nagpur plan roads have been classified into five major categories as follows:
(i) National Highways (NH) (ii) State Highways (SH) (iii) Major District Roads (MDR) (iv) Other District Roads (ODR) and (v) Village Roads (VR).
The above system of classification of roads has been modified in the Lunknow plan (1981 - 2001) and the roads in the country have been classified into three groups.
(i) Primary System: - This system consists of two categories of roads (a) Express ways and National Highways (NH).
(ii) Secondary System:- The secondary system consists of two categories of roads , namely , State Highways (SH) , Major District Roads (MDR).
(iii) Tertiary System or rural roads:- This system includes Other District Roads (ODR) and Village Roads (VR).
Each of the above categories are explained below.
2.2.1. Express Ways:
Express ways are separate class of highways with superior facilities and design standards and are meant as through routes having very high volume of traffic. The express ways are to be provided with divided carriage ways , controlled access , grade separations at cross roads and fencing. These highways should permit only fast moving vehicles. Express ways may be owned by the Central government or a State government depending on whether the route is a national highway or state highway.
2.2.2. National Highways:
Main highways running through the length and breadth of India , connecting foreign highways , capitals of large states , ports and including roads required for strategic movements for the defence of India are classified as National Highways. They constitute the frame on which the entire road communication system of the country based. They must give uninterrupted road communication throughout the year and should be of fairly high grade construction. All National Highways vest in the union Government of India as per the National Highway Act 1956 and is the responsibility of the centre to develop and maintain properly all national highways.
2.2.3. State Highways:
These highways are other main trunk or arterial roads of a state , connecting up with the National Highways or highways of adjacent states and linking the district head quarters and important cities within the state. The state highways are the main arteries of traffic within a state. They are to be of the same standards as National Highways.
2.2.4. District Roads:
District roads are roads traversing each district serving areas of production and marketing and connecting these with each other or with national and state highways or railways or important navigational routes. They should be capable of taking road traffic into the heart of rural areas throughout the year with only minor interruptions. District roads are divided into two classes on the basis of traffic.
(i) Major District Roads (M.D.R) for higher order of traffic. (ii) Other District Roads (O.D.R) for lower order of traffic.
Village roads are roads connecting villages or groups of villages with each other and to the nearest district , state or national highway or railway or navigational routes. They are in essence roads from villages to a market or to a main route.
The road systems within the Urban areas are classified as ‘Urban Roads’ and will form a separate category of roads to be taken up by the respective urban authorities.
2.3. Long Term Road Plans:
Selient features of the three plans are presented briefly.
2.3.1. Nagpur Plan:
This is the first plan prepared on scientific principles for the development of highways in India. At the initiative of the IRC , chief engineers of the various states met at Nagpur for drafting a highway development plant for the period 1943-63. According to this plan the road net work in the country was divided into five categories:
(i) National Highways (ii) State Highways (iii) Major District roads (iv) Other district roads and (v) Village roads. Requirements of each of these categories of roads are given in article 2.2.
The target for the total length of the roads was fixed as 16 km per 100 Sq. Km of the area covered. Based on ‘Star and Grid pattern’ of road network two sets of formulae have been developed. One for the total length of NH , SH and MDR and another for the length of ODR and VR. These formulae have been developed taking into consideration the geographical , agriculture and population conditions.
2.3.2. Second Twenty Year Road Plan:
The length of roads envisaged under the Nagpur plan was found to have been achieved by the end of the second plan i.e., by 1961 , but the road system was deficient in may respects. The changed economic , industrial and agricultural conditions in the country warranted a review of the country’s rapidly growing economy. Accordingly roads wing of Government of India received the situation and drafted a perspective plan for road development for the period 1961 - 81 , known as second twenty year road plan.
Five different formulae based on the ‘star and Grid’ pattern of roads have been formulated for the five categories of roads proposed in Nagpur plan. The target of this plan has been fixed as 32 km of total length of road per 100 Sq. km of area covered.
Comparison of Nagpur Road Plan and Second 20-Year Road Plan:
(i) The Nagpur plan has a target length of 16 km per 100 Sq. Km area covered where as the second 20 year plan has double this length.
(ii) Nagpur road plan gives two formulae. One for total length of fast category of roads , namely N.H , S.H and M.D.R and the second formula for finding the total length of O.D.R. and V.R. Hence it is not possible to get the road length for each category of the roads separately. In the second 20 - year plan it is possible to find out the length of each category of roads as five different formulae are available for the five different categories.
(iii) The Nagpur plan formulae for the road lengths divide the area into two categories - Agricultural area and Non-Agricultural area. In the second road plan , the area is divided into three categories - developed and agricultural area , semi-developed area and undeveloped and uncultivated area.
(iv) In developing formulae for the different categories of roads in the Nagpur plan , villages and towns are divided into six groups based on population. All towns with population greater than 5000 , are grouped together. In the second 20 year plan , villages and towns have been divided into nine different population groups. All towns with population greater than 1,00,000 are kept in one group.
(v) In Nagpur plan the length of railway track in the area was deducted from the total length of road required. Such a deduction was not allowed in the second 20 year formulae , as it was realised that the highway system should develop independently.
(vi) In the second 20 year plan , a development factor of 5% only is allowed where as in the Nagpur plan this factor is 15%.
(vii) In the second 20 year road plan , provision was made for express highways. (Highways provided for the movement of heavy volumes of motor traffic at higher speeds and have atleast four lanes).
(viii) In general it may be said that the second 20 - year road plan has been developed on a more rational and scientific basis than the Nagpur road plan.
Deficiencies of the Road Plans:
The two plans are mainly centered around planning a network of roads. These plans are hardly based on and evolved from principles of transport planning. The plans are not correlated to transport needs and are not based on systematic transport surveys.
The plan formulae for evolving the road length indicate that greater length was apportioned for developed areas than semi-developed and underdeveloped area. The
back-wardness tended to be perpetuated rather than conditions of communications and accessibility improved.
2.3.3. Third Twenty Year Road Development Plan (1981 - 2001):
Salient features of the Lunknow plan are as follows:
(i) The future road development should be based on the modified classification consisting of primary , secondary and tertiary road systems as mentioned in Article 2.2.
(ii) The road network should be developed so as to preserve the rural oriented economy and to develop small towns with all essential facilities. All villages with population over 500 (based on 1981 census) should be connected by all weather roads by the end of this century. (iii) The N.H. net work should be expanded to form square grids of 100 km sides so that no part of the country is more than 50 km away from a N.H.
(iv) The over all road density in the country should be increased to 82 km per 100 Sq km area by the year 2001.
(v) The lengths of SH and MDR required in a state or region should be decided based on both the area and the number of towns with population above 5,000 in the state or region.
(vi) Express ways should be constructed along major traffic corridors to provide fast travel. (vii) All the towns and villages with population over 1500 should be connected by MDRs and villages with population 1000 to 1500 by ODRs. There should be a road within a distance of 3.0 km in plain and 5.0 km in hilly terrain connecting all villages or groups of villages with population less than 500.
(viii) Roads should be constructed in less industrialised areas to attract growth of industries. (ix) Long term plans for road development should be prepared at various levels. The road network should be scientifically decided to provide maximum utility.
(x) The existing roads should be improved by rectifying the defects in road geometrics , improving the riding quality of the pavement surface and strengthening of the pavement structure to save vehicle operation cost and thus to conserve energy.
(xi) There should be improvements in environmental quality and road safety.
Determination of Road Lengths: I. Primary System:
(i) Express ways of total length 2000 Km to be developed for fast travel based on traffic requirements.
(ii) Total length of NH in the country or in a state in Km = Total Area of the country or state in Sq. Km / 50.
II Secondary System:
(i) Length of State Highways (SH) in Km.
(a) By total area: S.H , Km = Area of the State in Sq . Km / 25 (b) By total number of towns and area in the state ,
SH , Km = 62.5 x no. of towns in the state - area of state , sq . km
50
(ii) Length of MDR in a State in Km.
(a) By total area , MDR Km = Area of State in Sq. Km / 12.5
(b) By no. of towns in the State , MDR , Km = 90 x no. of towns in the state. (III) Tertiary System or Rural Roads:
(i) Length of Rural Roads (ODR and VR) in each state
= Total length of roads in the State - Length of (NH + SH + MDR) in the state.
Note:
Total length of roads in a state = Area of the State in Sq Km
100 ×82
2.4. Road Patterns:
The following are the road patterns (fig 2.1) used for rural roads. (i) Grid or Rectangular or Block pattern.
(ii) Radial pattern (a) star and Block (b) star and circular (c) star and grid. (iii) Hexagonal pattern (iv) Minimum travel pattern.
The choice of a road pattern depends upon: layout of town showing industrial , agricultural and production centres , terrain and topography and choice of the planner.
2.4.1. Grid pattern:
In this type (fig 2.1A) roads are perpendicular to each other. It is easy to set out this pattern and is suitable for flat countries without any predominant natural features. This system has been adopted in the city roads of Chandigarh , This pattern produces monotonously long sets flanked by dull blocks of buildings. It encourages an even spread of traffic over the entire grid. It is easy for the through traffic to bypass a definite control area in the middle of the grid. This pattern is not quite convenient from traffic operation point of view.
2.4.2. Radial pattern:
Fig 2.1 B , 2.1 C and 2.1 D show the various systems of radial road patterns.
In this system a number of roads radiate from central core known as focal point. This system of roads lead to congestion of centre (fig 2.1B)
The ring roads (figs 2.1C and 2.1D) are circumferential highways to permit traffic to avoid centre of town. The location , number and design of ring roads depend upon the population of the town , size , layout and usage of central area.
The inner ring road deflects traffic which has no need to traverse the central area and the outer ring road is used by through traffic of the town as distribution between radials. The outer ring roads are located within the outer fringe of present and future development.
The Connaught place in New-Delhi has radial and circular pattern of road network. The Nagpur Road Plan (1943 - 63) and the II Road Plan (1962 - 81) were formulated on the basis of star and grid pattern.
The advantages of this pattern are not much because (i) Towns are not circular (ii) It is not possible to join a ring route at any point and (iii) the relative advantages of routes are different.
2.4.3. Hexagonal Pattern:
In this pattern the roads are arranged to form a hexagonal shape (fig 2.1 E). Each system has one of its road common with another system of hexagon.
2.5. MASTER PLAN:
Master plan is the final road development plan for the area under study - a city or a district or a state or a country. It is an ideal plan showing the full development of the area at a future data. It serves as a guide to the planner to improve some of the existing roads and plan net work of new roads. Master plan of an area helps in controlling the industrial picture of the fully developed area in a planned and scientific manner.
The various stages in preparation of a master plan are
(a) Data collection: This include data regarding existing land use , population , industrial and agricultural growth , traffic flow , topography and future trends.
(b) Preparation of draft plan based on future trends and invite suggestion from public and experts.
(c) Revision of draft plan in the light of discussions and comment from public and experts , and
(d) Compare the various alternate proposals of road system and determine the sequence in which the master plan will be implemented.
In India , in the various development plans considered , targets for the lengths of roads have been fixed based on population , area , number of towns , agricultural and industrial growth. Similar system may be adopted in preparing master plan also.
2.6. Summary:
For the development of roads in our country , three long term plans have been drawn. The first plan , popularly known as Nagpur plan , drawn for the period 1943 - 63 may be considered as a land mark in the development of roads in our country.
Depending on the development of the area and population served , roads have been divided into five groups - National Highways , State Highways , Major District Roads , Other District Roads and Village Roads. A target of 16 Km of road for every 100 Sq.Km of area covered was fixed for this plan period. As the targets of Nagpur plan could be achieved by 1961 , a second road development plan for the period 1961 to 1981 , also known as Bombay plan , was formulated. The targets of this plan were double that of the Nagpur plan. In both of these plans star and gird pattern of roads has been used for developing the formulae for the different categories of roads proposed in the Nagpur Plan.
Both the Nagpur plan and Bombay plan have been hardly based on and evolved from principles of transportation planning. The plan formulae for evolving the road length indicate that greater length was apportioned for developed areas than for semi-developed or under developed areas. The backwardness tended to be perpetuated rather than conditions of communications and accessibility improved. Taking care of these short coming and keeping in view the growth pattern envisaged in the various fields by the turn of the century , a third road development plan for the period 1981-2001 has been prepared. Attention was paid while drafting the plant to present the quality of environment , to improve road safety and to conserve energy.
Fig 2.1 ROAD PATTERNS
Roads have been classified into three groups - primary system consisting of the Express ways and National Highways , Secondary System comprising of State Highways and Major
District Roads and Tertiary system or Rural roads which include other district roads and village roads for the purpose of the Lucknow plan.
Road patterns adopted in rural roads have also been discussed in this unit. Data to be collected and the various steps in the preparation of Master Plans for highway development have also been dealt with in detail.
2.7. Self Assessment Questions:
1. Compare the I and II - 20 year Road development plans clearly bring out the deficiencies in these two plans.
2. Discuss important aspects of Lucknow plan indicating how the total lengths of different categories of roads can be calculated ?
3. Write notes on (a) Rural Road Patterns (b) Master plans for Roads (c) Classification of Roads.
2.8. BOOKS FOR REFERENCE:
1. Bindra , S.P. (1977) - A course of Highway Engineering. Dhanpath Rai & Sons , New Delhi.
2. Kadiyali L.R. (1984) - Principles and practice of Highway Engineering - Khanna Tech. Publications , New Delhi.
3. Khanna , Dr. S.K., and Justo , Dr. CEG - Highway Engineering , New Chand and Bros., Roorkee.
***
TRANSPORTATION ENGINEERING
UNIT - 3
HIGH WAY GEOMETRIC DESIGN - 1
CONTENTS:AIMS / OBJECTIVES:
3.1. Introduction
3.2. Pavement Surface Characteristics 3.3. Width of Pavement or Carriage way 3.4. Right of way
3.6. Sight Distance - Stopping Sight -Distance 3.7. Sight Distance - Over Taking Sight -Distance 3.8. Sight Distance at Intersections.
3.9. Sight Distances - Miscellaneous 3.10. Criteria for Sight Distance
3.11 Factors Effecting Horizontal Alignment Design 3.12. Summary
3.13. Self Assessment Questions 3.14. Books of Reference.
AIMS / OBJECTIVES:
Various geometric features of a highway have to be designed on scientific principles so as to provide maximum efficiency to traffic operations with safety , comfort and economy. The geometric features discussed in this unit are Cross-sectional elements , sight - distance requirements , and factors that affect the control and design of horizontal alignment.
3.1. INTRODUCTION:
Geometric design of a highways deals with dimensions of various highway features such as alignment , slopes , widths , sight distances , gradients etc. In the early phase of road development , greater emphasis was used to be laid on structural design of road way rather than on the geometric design. With the increase in number and speeds of motor vehicles emphasis has been shifted to the geometric design. The geometric layout should be so designed as to provide maximum efficiency to traffic operations with safety , comfort and economy.
It is possible to design and construct the pavement of a road in stages ; but it is very expensive and rather difficult to improve the geometric elements of a road in stages at a later time. Therefore , it is important to plan and design the geometric features of the road during the initial alignment itself taking into consideration the full growth of traffic and the possibility of the road being upgraded to a higher category.
Geometric design of a highway , in general , deals with the following elements. (i) Cross - sectional elements
(ii) Sight -Distance Considerations
(iii) Horizontal and Vertical alignment details (iv) Intersection elements.
In this unit , design aspects of the cross-sectional elements , sight distance and factors affecting the horizontal alignment are considered.
Highway geometrics depend upon the topography , locality and type and intensity of traffic for which the road is intended. Comprehensive design standards for roads have been evolved after considerable thought by the Indian Roads Congress (IRC) and the Roads Wing of Ministry of Transport (MOT).
3.2. PAVEMENT SURFACE CHARACTERISTICS:
The important surface characteristics of the pavement are friction , roughness , light reflecting characteristics and drainage of surface water.
3.2.1. Friction:
The friction between the vehicle tyre and the pavement surface is a very important factor. It affects the operating speed , distance requirements in stopping and accelerating vehicles , and the force resisting centrifugal force while a vehicle negotiates a curve. Important factors affecting the frictional resistance of the pavement surface are (i) The type of road surface , (ii) condition of the pavement (iii) the type and condition of the tyre (iv) the speed of vehicle (v) the extent of brake application and (vi) the load , and the tyre pressure.
Longitudinal friction comes into play when brakes are applied for stopping a vehicle and it depends on the speed of the vehicle and the surface conditions of the pavement. I.R.C. recommends the longitudinal friction coefficient values of 0.35 to 0.40 depending on the speed. In the case of horizontal curve design , IRC recommended lateral coefficient of friction of 0.15. This low value has been suggested for the worst possible surface condition such as mud on pavement surface on horizontal curve with super -elevation (as it is essential to prevent lateral skid).
3.2.2. Pavement Unevenness:
Pavement unevenness increases fuel consumption and operating cost of the vehicles , reduces the speed , safety and comfort of travel. Uneven surfaces increase fatigue and accidents. As such pavement surfaces should be maintained as even as possible.
The pavement surface condition is measured by “UNEVENNESS INDEX’ , which is the cumulative vertical undulations of the road. It has been found from tests that it is desirable to keep the unevenness index low and preferably less than 150 cm/km , for good road surfaces of high speed highways. Values of more than 350 cm/km is considered very uncomfortable even at a speed of 50 Kmph.
Night visibility very much depends upon the light reflecting characteristics of the pavement. Light reflecting characteristics of the pavements are colour and surface condition (dry or wet) of the pavement. The glare caused by the reflection of head lights is considerably more on wet pavement surface than on a dry pavement. Though light coloured or white pavement surface give good visibility at night , they produce glare and eye strain during bright sunlight. Black top pavement surface on the otherhand provides very poor visibility at nights , especially when the surface is wet.
3.3. WIDTH OF PAVEMENT OR CARRIAGE - WAY:
The pavement or carriage way width depends on the width of a traffic lane and number of lanes. The carriage way width intended for one line of traffic movement may be called ‘traffic lane width’. This lane width is determined on the basis of the width of the vehicle and the minimum clearance to be provided for safety. When the side clearance is increased (upto a certain limit) , there is an increase in the operating speed of the vehicle and hence an increase in the capacity of a traffic lane. Keeping all these in view , a width of 3.75m is considered desirable for a road having single lane for vehicles of 2.50m width (width of design vehicle). For pavements having two or more lanes , width of 3.5m per lane is considered.
In the case of a single lane carriage way of width 3.75m a side clearance of 0.625m would be obtained as shown in Fig 3.1 (A) and in the case of two lane pavement of width 7m, a minimum clearance between two lanes of traffic would be 1.00m for standard vehicle as shown in Fig 3.1 (B).
The number of lanes required in a highway depends on the predicted traffic volume and the traffic capacity of one lane. In developing countries like India , the traffic is of mixed nature consisting of both slow and fast moving vehicles between which there are wide variations in speed. In such cases , it is common practice to express the traffic capacity in terms of PASSENGER CAR UNITS (PCUS’).
The weightage values for various classes of vehicles as recommended by the MOT are given in Table 3.1. Factors affecting PCU are (i) average speed of the vehicle class , under the prevailing roadway and traffic conditions , (ii) average width and length of the vehicle class , and (iii) average transverse gap and longitudinal gap between vehicles of the same class in the speed range under consideration of a compact stream flow.
TABLE 3.1. PASSENGER CAR UNITS (PCU):
Sl.No. Type of vehicle PCU
(1) Bicycle 0.5
(2) Motor cycle , scooters 0.75 (3) Light Commercial Vehicles 1.0
(4) Automobiles 1.0
(5) Cycle Rickshaws 1.5
(6) Trucks and Busses 3.0
(7) Animal drawn Vehicles 4.0 to 8.0
Technical group of the Ministry of Transport recommended the following criteria for designing carriage way width used on highway capacity expressed in passenger car units. (Table 3.2).
TABLE 3.2
Sl.No. Road Type Recommended Capacity
PCUS’ / day 1. Single lane road with satisfactory earth
shoulders
Upto 1000 2. Single lane road with 1.0m wide all
weather shoulders on either side
Over 1000 but less than 2500 3. Two lanes under ideal conditions with
eathen shoulders
Over 2500 but less than 10,000 4. Four lanes , divided highway (depending
on traffic , access control etc.)
The traffic at a future data (end of design period) in PCU / day for design purposes is generally computed by using the following formula:
A = P (1 + r)n + m 3.1
The width of carriage way for various classes of roads as standardised by the IRC are given in Table 3.3.
Table 3.1. WIDTH OF CARRIAGE WAYS FOR VARIOUS CLASSES OF ROADS:
Sl.No .
Class of Road Width of carriage-way in metres
1. Single lane 3.75 for all roads ; may be decreased
to 3.0m for village roads.
2. Two lane without raised kerb 7.0
3. Two lane with raised kerb 7.5
4. Intermediate carriageway (Except on important roads)
5.5
5. Multiple lane pavements 3.5 / lane
Where A = Design number of PCU / day P = Existing number of PCU / day
r = Annual rate of increase in traffic (taken as 7.5% in the absence of any data)
n = Number of years between last census and the year of construction or improvement and m = design period in years.
The value of P in formula 3.1 , should be seven day average of heavy vehicles found from 24 hours counts.
3.3.1. Traffic Separators or Medians:
In some highways traffic separators and medians are provided between two sets of traffic lanes intended for traffic moving in opposite directions. The main function of the traffic separators is to prevent head - on collision between vehicles moving in opposite directions in adjacent lanes. In such highways the road width depends on the pavement width (or lane width and number of lanes) and the width of traffic separators. Apart from preventing head-on collision of vehicles , separators may also help to
(i) Channelise the traffic into streams at intersections. (ii) Shadow the crossing and turning traffic , and
The traffic separators may be in the form of pavement markings , physical dividers , or area separators. Pavement markings are the simplest of these. The mechanical separators should be designed in such a manner that even if the wheels of a vehicle encroach , no part of the vehicle body should be damaged.
Area separators may be medians , dividing islands or parkway strips dividing the two directions of traffic flow , (fig 3.2). It is desirable to have a wide area separators of 8 to 14m width. A minimum of 6m is required to reduce head light glare. The glare can be reduced in narrow strips by planting trees or shrubs.
For medians desirable minimum width of 5.0m on a rural highway may be reduced to 3.0m where land is restricted. On long bridges the width of the median may be reduced upto 1.2 to 1.5m. The medians should preferably be of uniform width throughout. On urban highways with 6 lanes or more , medians should invariably be provided ; absolute minimum width being 1.2m and desirable minimum being 5.0m.
3.3.2. Kerbs:
Kerbs (curbs) indicate the boundary between the pavement and shoulder or sometimes islands or footpaths or parking space. These are classified as ‘barrier’ and mountable kerbs. Barrier kerbs are designed to discourage ,
Fig 3.3. TYPES OF KERBS
vehicles leaving the pavement. The face may be vertical or sloping and the height may range from 15-25 cm. (Fig 3.3 a). A smaller height may be adopted for pedestrian or refuge islands. Mountable kerbs are those which can be easily crossed by vehicles if required (fig 3.3 B). They are used at medians and channelising islands.
In rural areas submerged kerbs are provided at pavement edges between the pavement edge and shoulders of rural roads. These kerbs provide lateral support for the granular base course of flexible pavements.
3.3.3. Shoulders:
The shoulder is that portion of the roadway contiguous with carriageway and is intended for accommodation of stopped vehicles , emergency use and lateral support of base and surface courses. The width of the shoulder should be adequate to accommodate stationary vehicles fairly away from the edge of the adjacent lane. The shoulder should have sufficient load carrying capacity to support a loaded truck even in wet weather.
Fig 3.4. CROSS SECTION DETAILS
Cross sectional details of a highway are shown in fig 3.4. Width of formation or roadway is the sum of widths of pavement or carriage-way including separators if any , and the shoulders. Formation width is the top width of highway embankment or the bottom width of cutting excluding the side drains as shown in fig 3.4. The widths of road way as standardised by the IRC are given in Table 3.4.
3.4.1. Right of Way:
The right of way is the area of land acquired for the road along its alignment. The width of this acquired land , is known as ‘Land -width’ or ‘Right of Way Width’ and depends on the importance of the road and possible future development. The recommended land width for different categories of roads as per the IRC are presented in Table 3.5.
NOTES: (i) In multilane highways , roadway width should be adequate for the requisite
number of traffic lanes besides shoulders and central median.
(ii) The minimum roadway width on single lane bridge is 4.25m. While acquiring land for a highway it is desirable to acquire more width of land as it may be difficult to acquire land at a future date for widening or other improvements.
In order to prevent ribbon development along the highways , it is sometimes necessary to establish ‘Building lines’ and ‘control lines’ with the following definitions.
Roadway width , m:
Sl.No. Road classification Plain and Rolling
terrain
Mountainous and Steep terrain 1. National and State Highways
(a) Single lane (b) Two lanes
12.0 12.0
6.25 8.80 2. Major district road
(a) Single lane (b) Two lanes
9.0 9.0
4.75 ---- 3. Other District Roads
(a) Single (b) Two lanes 7.5 9.0 4.75 ----
4. Village roads - single lane 7.5 4.00
Tables 3.5 RECOMMENDED LAND-WIDTH FOR DIFFERENT CLASSES OF RURAL ROADS (METRES):
Plain and rolling terrain Mountainous and steep
Terrain Sl.
No.
Road
Classification
Open areas Built-up ares Open areas Built-up
areas
Normal Range Normal Range Normal Normal
1. National and State Highways 45 30-60 30 30-60 24 20 2. Major distric Roads 25 25-30 20 15-25 18 15 3. Other district Roads 15 15-25 15 15-20 15 12 4. Village Roads 12 12-18 10 10-15 9 9
Control line is a line which represents the nearest limits of future uncontrolled building activity in relation to a road. This signifies that though building activity is not totally banned between building line and control line , the nature of buildings permitted here are controlled.
Table 3.6. RECOMMENDED STANDARDS FOR BUILDING LINES AND CONTROL LINES:
Plain and rolling terrain Mountainous and steep
terrain Road
classification
Open areas Built-up areas Distance between building
line and road boundary (set back) , m Overall width between building lines , m Overall width between control lines , m Distance between building line and round boundary (set-back) , m
Open areas Built-up areas
N.H and S.H. 80 150 3 to 6 3 to 5 3 to 5
M.D.R 50 100 3 to 5 3 to 5 3 to 5
O.D.R. 25/30* 35 3 to 5 3 to 5 3 to 5
V.R. 25 30 3 to 5 3 to 5 3 to 5
Note: *If the land width is equal to the width between building lines indicated in this column
, the building lines should be set back 2.5m from the road land boundary.
3.5. CAMBER OR CROSS SLOPE:
Camber or cross - slope is the slope provided to the road surface in the transverse direction to drain off rain water from the road surface. Usually camber is provided on straight roads by raising the centre of the carriage way with respect to the edges forming a CROWN , or highest point on the centre line. At horizontal curves , the surface drainage is affected by raising the outer edge of the pavement with respect to the inner edge while providing the desired superelevation.
The rate of camber is usually designated by 1 in n which measures the transverse slope in the ratio 1 vertical to n horizontal. Camber is also expressed as a percentage. If a camber is Y% , the cross slope is Y in 100.
The rate of camber depends on:
(i) The type of pavement surface and (ii) the amount of rainfall.
The minimum camber needed to drain off surface water may be provided keeping in view the type of pavement surface and the amount of rainfall in the locality. The values of camber recommended by the I.R.C. are given in Table 3.7. A range of values are given with a
view that in localities with lesser rainfall , a flatter cross slope may be adopted and in places of higher rainfall a steeper camber may be adopted.
Table 3.7. RECOMMENDED VALUES OF CAMBER (IRC):
Sl. No. Type of road surface Range of camber in areas of rainfall range Heavy to light
1. Concrete and high type
bituminous surface 1 in 50 (2%) to 1 in 60 (1.7%)
2. Thin bituminous surface 1 in 40 (2.5%) to 1 in 50 (2%)
3. Water bound macadam and
gravel pavement 1 in 33 (3%) to 1 in 40 (2.5%)
4. Earth 1 in 25 (4%) to 1 in 33 (3%)
The cross slope for shoulders should be 0.5% steeper than the cross slope of adjoining pavement , subjected to a minimum of 3 percent: maximum value of 5.0% for earth shoulders.
3.5.1. SHAPES OF CAMBER:
The camber is given a parabolic , elliptic or straight line shape. In parabolic or elliptic shape the profile is flat at the middle and steeper towards the edges which is preferred by fast moving vehicles as they have to cross the crown line during overtaking operations frequently. When very flat slope is to be provided as in cement concrete pavements , straight line shape of camber may be provided as shown in Fig 3.5. Some times a combined camber with parabolic central portion and straight line at the edges (Fig 3.5) is preferred when animal drawn vehicular traffic with steel tyres is heavy.
3.5.2. Camber Boards:
For providing the desired amount and shape of camber , camber boards or templates are prepared with the specified camber. These are used to check the lateral profile of the finished pavement during construction.
Fig 3.5 SHAPES OF CAMBER WORKED OUT EXAMPLE:
3.1. Give dimensions of the camber board for two lane bituminous surface to be constructed
in an area of heavy rainfall.
Solution: Since it is a two lane highway , width = 7m = 700cm
parabolic camber is proposed.
In an area of heavy rainfall , camber is 1 in 40 (Table 3.7) Height of pavement at the centre over edges = 700
2 1
40 8 75
× = . cm. Equation for parabolic shape of camber is
Y = 2x2/ nw 3.2
where y is the ordinate at a distance x from the crown of the pavement. (Various terms are as shown in Figure).
For various values of x , y is calculated from the above formula.
x in cm y in cm 50 0.1785 100 0.714 150 1.607 200 2.857 250 4.464 300 6.428 350 8.750 These values are plotted as shown in figure of camber board.
Fig Camber Board 3.6. SIGHT DISTANCE:
Sight distance is the length of the highway ahead visible to the driver. It is the element , which has the most important influence on highway safety and efficiency in operation. A knowledge of the sight distance requirement is needed in the design of straight lengths , intersections as well as on the horizontal and vertical curves.
Sight distance available from a point is the actual distance along the road surface , which a driver from a specified height (1.20 m) above the carriage way has the visibility of a stationary (0.15 m height) or moving object (1.20 m height). In other words , it is the length of the road visible ahead to the driver at any instance. Restrictions to sight distance may be caused at horizontal curves by objects obstructing vision at the inner side of the road or at vertical summit curves or at intersections.
Three sight distance situations are considered in the design. (i) Stopping or absolute minimum sight distance.
(ii) Safe overtaking or passing sight distance , and
(iii) Safe sight distance for entering into uncontrolled intersection.
Stopping sight distance is the distance required by the driver of a vehicle travelling at a given speed to bring the vehicle to stop after an object on the roadway becomes visible. Because of the importance to safety all highways must be designed to provide minimum stopping sight distance throughout their length. This is also called sometimes ‘non-passing sight distance’.
The stopping distance is the sum of (i) the distance travelled by the vehicle during the total reaction time known as LAG DISTANCE and (ii) the distance travelled by the vehicle after the application of brakes , before coming to a dead stop position , known as the ‘BRAKING DISTANCE’.
(i) Lag Distance:
The total reaction time of the driver is the time taken from the instance the object is visible to the instant the brakes are effectively applied. The amount of time gap , total Reaction time , depends on several factors.
Some traffic engineers have split the total reaction time into four parts based on ‘PIEV’ theory. According to this theory the total reaction time of the driver is split into the following four parts.
(a) Perception time , is the time required to perceive an object or situation.
(b) Intellection time , is the time required for comparing the different thoughts , regrouping and registering new situation.
(c) Emotion time is the time required during emotional sensation and disturbance , and (d) Volition time is the time required for the final action.
It is possible that the driver may apply the brakes or take any avoiding action by the reflexive action even without thinking. This is shown in Fig 3.6. Which illustrates the PIEV process.
P - Perception I - Intellection E - Emotion V - Volition.
The total reaction time depends upon the physical characteristics of the driver , psychological factors , environmental conditions , purpose of trip and speed of the vehicle.
Fig 3.6. Reaction Time and PIEV process
If ‘v’ is the design speed in m / sec and ‘t’ the total reaction time of the driver in seconds , then
Lag distance (metres) = vt 3.3a
If V is the design speed in kmph , then the lag distance , metres
=
V
×
1000
60
×
60
×
t
= 0.278 Vt 3.3bA total reaction time of 2.5 seconds is recommended. As such under most situations , the lag distance , (metres)
= 2.5 v = 0.278 x 2.5 x V
(ii) Braking Distance:
Braking distance is obtained by equating the work done in stopping the vehicle to the kinetic energy of motion.
Case (a):
Let the vehicle move on an ascending gradient of +n% . The forces acting against motion and helping to stop the vehicle are
(i) Frictional force F acting down the gradient.
(ii) The component of gravity of the vehicle W , acting parallel to the surface and acting downwards and equal to W sin = W tan = Wn / 100.
Figure
If l is the braking distance then the work done in stopping the vehicle = F. l + W n
100 l
= (f + n / 100) Wl.
The kinetic energy at the design speed v , m / sec will be =
1
2
mv2=
1
2
(W/g) v2(g
being the acceleration due to gravity).
Equating the work done in stopping the vehicle to the kinetic energy. (f + n / 100) Wl =
1
2
(W / g) v2
or braking distance l (metres) = v2/ 2g (f + n / 100) 3.4a
Case (b):
In a descending gradient of -n% , the braking distance ‘l’ increases as the component of gravity now opposes the braking force. Hence , the braking distance may be obtained from the equation.
l = v2/ 2g (f - n / 100) 3.4b
Case (c):
On a level surface n = 0 , then the braking distance may be obtained from the equation
l = v2/ 2gf 3.4c
In general , the expression for braking distance may be written as
l =
v
2
2g f
(
±
n / 100
)
=
v
22g f
(
±
0.01 n
)
3.4dStopping distance = Lag distance + Braking distance
S.D. (m) = vt +
v
2
2g f
(
±
0.01 n
)
3.5aSince the total reaction time is taken as 2.5 seconds and V is the design speed in Kmph , equation , 3.5a , may be written as
S.D (metres) = 0.278 Vt +
(
0.278 V
)
22
×
9.81 f
(
±
0.01 n
)
(Since g = 9.81 m / sec). = 0.278 Vt +V
2254 f
(
±
0.01 n
(
)
3.5bThroughout the above analysis , it was assumed that the brakes are quite effective , i.e., of 100% efficiency. If the efficiency of the brakes is only n% , then
S.D (metres) = 0.278 Vt +
V
2
254 f
(
±
.01 n
)
×
.01 n
3.5cThe coefficient of friction ‘f’ depends upon a number of factors like the type and condition of the pavement surface , tyres and speed of the vehicle The IRC recommended the following values of f based on the speed of vehicles.
Table 3.8. Recommended values of Coefficient of Friction:
Speed in Kmph 20-30 40 50 60 65 80 100
Long. coefficient of friction f. 0.40 0.38 0.37 0.36 0.36 0.35 0.35 The minimum sight distance should be equal to the stopping distance in one way traffic lanes and also in two - way traffic roads when there are more than one lane of traffic. In roads with restricted width , in single lane roads , when two-way movement of traffic is permitted , the minimum sight distance should be equal to TWICE the stopping distance to enable both the vehicles coming from opposite directions to stop. The Stopping Distance should invariably be provided throughout the length of the road and hence this is also known as Absolute Minimum Sight Distance.
Worked Example:
3.2. Calculate the minimum stopping sight -distance on a highway at a descending gradient of
6%. Design speed may be taken as 80 kmph. If the road is single lane one , with two way traffic , what is the sight distance to be provided ?
Solution:
Total reaction time may be taken as 2.5 sec.
For the speed of 80 kmph , f = 0.35 (from Table 3.8). Minimum stopping sight distance on a descending gradient is
= 0.278 Vt +
V
2254 f
(
0.01 n
)
= 0.278×80×2.5 +80
2254 0.35 0.01
(
×
6
)
= 142.5m.As there is two way traffic in a single lane , minimum sight distance = 2 (stopping sight distance).
3.7. OVERTAKING SIGHT DISTANCE:
It is the minimum distance open to the view of the driver of a vehicle intending to overtake a slow vehicle ahead with safety against the traffic in the opposite direction. The overtaking sight distance is measured along the centre line of the road which a driver with his eye level 1.2m above the road surface.
Important factors on which the minimum overtaking sight distance or the safe passing sight distance depends are:
(i) Speeds of the overtaking , overtaken and the vehicle coming from the opposite direction. (ii) Spacing between the vehicles.
(iii) Skill and reaction time of the driver , and the (iv) Slope of the road.
3.7.1. Analysis of Overtaking Sight Distance:
Figure 3.7 shows the elements that go to make up the overtaking sight distance. In Fig 3.7 A is the ‘overtaking vehicle’ travelling at the design speed and B is the ‘overtaken vehicle’ moving slowly on a two lane road. The vehicle C is the ‘on coming vehicle’ coming in the opposite direction at design speed.
Fig 3.7 Elements of taking manoeuvre
The overtaking distance may be divided into three parts:
(i) d1 - the distance travelled by the overtaking vehicle A. during the reaction time ‘t’ from
position A1to A2.
(ii) d2 - the distance travelled by vehicle , A from A2 to A3 during the actual overtaking
operation.
(iii) d3 is the distance travelled by the on-coming vehicle C , from C1 to Cz during the
overtaking operation of A.
The overtaken vehicle B is moving slowly at a speed of Vbkmph or vbm / sec.
In a two-way lane road , the opportunity to overtake depends on the frequency of vehicles coming from the opposite direction and overtaking sight distance available at any instant.
The overtaking phenomenon may be assumed as follows:
(i) When it is decided to overtake , the overtaking vehicle A , reduces its speed to the speed of the slow moving vehicle B and moves behind it during the reaction time t , till there is opportunity for safe overtaking.
This distance d1, shown in fig 3.7 = vbx t (metres) where t is the reaction time of the
driver in seconds.
This reaction time is taken as 2 seconds as an average value.
Then d1= vbx t = vbx2 metres 3.6
(ii) From position A2 , the vehicle A starts accelerating , shifts to the adjoining lane ,
overtakes the vehicle B , and shifts back to its original lane ahead of B in position A3 . The
distance between the positions A2and A3is taken as d2.
From the geometry of the figure d2 = b + 2s where s is the spacing to be maintained
between the vehicles and is given by the formula.
s = (0.7 vb+ 6) metres 3.7
Let the time taken by vehicle A to overtake vehicle B be T sec. During this time the vehicle A moves from A2to A3over a distance of d2and the vehicle B , moves from B1to B2
over a distance b.
Then b = vb. T (metres) since vbis the speed of vehicle B.
Thus d2= vbT + 2s (from the geometry , fig 3.7) 3.8.
The vehicle A , travelling with an initial velocity of vb, accelerated at ‘a’ metres / sec
, travels a distance of d2in time T seconds , then
d2 = vbT + 1 / 2 (aT2) 3.9
Equating equations 3.8 and 3.9. d2= vbT + 2S = vbT +
1 2 aT
2
or Tsec =
4s / a
3.10where s = (0.7 vb+ b) from equation 3.7.
