ORIGIN AND DESTINATION (O - D) STUDIES:

These studies are carried out to collect factual information about desired lines of travel with a view to provide the most effective transportation system for the traffic. The purpose of this study is to get answers to the following questions.

(i) Why people travel ? (Purpose of trip) (ii) When people travel ? (time and direction)

(iii) How people travel ? (mode of transport - bus , car , cycle , foot etc.,) (iv) Where the people want to go (Origin and destination)

(v) Where and why people stop ? ( To determine concentration of vehicles warranting parking facilities).

These studies are most essential in planning new highway facilities and in improving some of the existing systems.

If this study , for example , indicates a high percentage of through traffic , a by-pass can be considered. This survey also helps in solving zonal as well as regional traffic problems. The uses of this study may be summarised as follows:

(i) To provide wide and better roads (express ways) along the maximum desired lines of travel.

(ii) To plan by - passes and establish preferential routes for various types of traffic.

(iii) To plan public transportation systems in cities.

(iv) To evaluate the existing facilities and plan for an improvement or new facilities as the case may be , and

(v) To fix dimensions and design standards for road bridges etc., 5.7.1. METHOD OF COLLECTING O - D DATA:

Some of the popular methods of collecting O and D data are as follows:

(i) Road side Interviews with Drivers: Trained personal are stationed at pre-selected stations. The vehicles are stopped and each driver is asked to answer the following questions (sample questions are only given).

(a) Origin (b) Destination (c) Purpose of Trip

(d) Route selected to reach the destination (e) Location of stops and purposes.

(f) Also note is made of the type of vehicles and (g) number of passengers in the vehicles.

This method is quick , but the main drawbacks of this method are wastage of time of traffic , possibility of traffic congestion and possible resentment from the road users.

(ii) Post-Card Survey - When the traffic is heavy and cannot be stopped long enough for interviews , prepaid business reply Post Cards with return address and on which a questionnaire to be filled in , along with a request to answer them and also the purpose of the study are distributed to the drivers as they pass the station. The stations can be located where the vehicles have to proceed slowly as at toll gates. A return of 10% or less is considered too small. Return of well planned studies range from 25-50%. Further if the questionnaire is not answered properly , the information hence compiled may not reflect the true picture.

(iii) Home Interviews: In this method about 10% of the population is interviewed by trained personnel. On the basis of the interview travel data are compiled and used for carrying out necessary improvements and planning new strategy.

(iv) Work-Place Interviews: In this method the place of interview is shifted to the work place so that time and manpower required for conducting the survey may be minimised. Trip particulars could be collected from many employees at one instant.

5.7.2. PRESENTATION OF O - D DATA:

Two methods of presentation of O - D survey data O - D Matrix method and Desire line chart method are discussed.

(i) O - D Matrix: This is the most convenient form , in which the origin zones and

destinations are presented as a matrix. (Fig 5.6). The horizontal axis of the matrix represents the destination zones and the vertical axis of the matrix represents the origin zones. The zones may be further classified into internal and external zones , if the survey covers both internal and external zones. The number of trips are entered in the cells of the matrix. In this (Fig 5.6) t2-3 represents the number of trips originating in zone 2 and terminating in zone - 3.

(ii) Desire - Line Chart: This is the most popular pictorial line representation by means of desire line chart. In this chart the trips between any pair of zones are represented by a straight line connecting the controids of the two zones and having a width drawn to a suitable scale to represent the actual volume of traffic. A typical desire - line chart is shown in Fig 5.7.

Fig 5.6 D - Matrix

Fig 5.7. Desire Line Chart 5.8. TRAFFIC CAPACITY:

The term capacity is used here to define the ability of a road to accommodate traffic under given circumstances. Some of the terms related to traffic capacity studies are defined below.

(i) Traffic Volume: This represents the number of vehicles moving in a specified direction on a given road way that pass a given point during specified unit of time. Traffic volume is expressed as vehicles per hour or vehicles per day.

(ii) Traffic Density: This is the number of vehicles occupying a unit length of lane of a road way at a given instant , usually expressed as vehicles per kilometre.

Traffic Volume = Traffic Density ×Traffic speed 5.6 The highest traffic density will occur when the vehicles are practically at a stand still on a given route and in this case the traffic volume will approach zero.

(iii) Traffic Capacity is defined as the number of vehicles passing a point on a highway in a unit period of time. In other words it is the ability of a road way to accommodate traffic volume. It is expressed as vehicles per hour per lane or road way.

Capacity and volume are measures of traffic flow and have the same units. Volume, represents an actual rate of flow and responds to variation in traffic demand where as capacity indicates the maximum rate of flow with a certain level of service characteristics that can be carried by the road way.

(iv) Basic Capacity is the theoretical capacity of a lane or a road way. Two roads having the same physical features will have the same basic capacity irrespective of traffic conditions , as they are assumed ideal.

(v) Possible Capacity: This is defined as the maximum number of vehicles that can pass on a lane or roadway during one hour under prevailing roadway and traffic conditions. This capacity is generally much lower than the basic capacity of the road way as the prevailing road way and traffic conditions and seldom ideal. Under the worst road way and traffic conditions , the possible capacity of the road way may approach zero. When the prevailing roadway and traffic conditions approach the ideal conditions , the possible capacity would approach the basic capacity. Thus the possible capacity varies from zero to basic capacity.

(vi) Practical Capacity or Design Capacity: This is the maximum number of vehicles that can pass a given point on a lane or road way during one hour without traffic density being so great as to cause unreasonable delay , hazard or restrictions to the drivers freedom to manoeuvres under the prevailing road way and traffic conditions. It is the practical capacity that is of primary interest to the designer.

5.8.1. DETERMINATION OF BASIC CAPACITY:

Basic capacity of a single lane may be obtained from the relation

C = 1000V / S 5.7

where V is the speed of the vehicle in kmph.

S is the average centre of the spacing of vehicles in meters C is the capacity of a single lane in Vehicles / hour.

The average spacing ‘S’ between centre to centre of vehicles is equal to the average length of vehicles plus the clear spacing between the vehicles in the stream. Clear-spacing may be assumed as the safe stopping sight distance with the reaction time assumed as 0.70 to 0.75 seconds. If the average length of the vehicles is ‘L’ (metre) ; then

S = Vt + L = 0.278 Vt + L 5.8

Considering a reaction time of 0.7 seconds (i.e.,) t = 0.7 seconds.

Smetres = (0.7 v + L) = (0.2V + L) 5.9

Theoretical capacity may also be obtained if the minimum time head way (Ht) seconds is known. The time interval between the passage of successive vehicles moving in the same line and measured from head to head as they pass a point on the road is known as the time head way. It has been observed that with the increase in speed of the traffic stream ,

the time head way decreases and after reaching a minimum value (Ht) at an optimum speed , starts increasing (Fig 5.8). The maximum theoretical capacity is then given by

C = 3600 / Htwhere C is the capacity per hour 5.10

Fig 5.8 VARIATION OF MINIMUM SPACING AND HEADWAY WITH SPEED The relationship between speed and maximum capacity of a traffic lane is shown in Fig 5.9. The peak value of the theoretical maximum capacity is reached at an optimum speed.

As the speed is increased further , the maximum capacity of the lane starts decreasing due to an increase in the time headway at the speed range.

Fig 5.9 Speed and Capacity

The practical capacity of a traffic lane is affected by factors like lane width , lateral clearance , width of shoulders , commercial vehicles , alignment , presence of intersections , stream speed , number of lanes of traffic movement , driver characteristics and composition of traffic etc.,. The practical capacity values suggested by the IRC for the purposes of design of different types of roads in rural areas are given in Table 5.3.

Table 5.3 Capacity of Roads in Rural Areas:

Type of Road Capacity PCU / day Both Directions 1. Single lane with 3.75m wide carriage way

and normal earth shoulders

1000

2. Single lane roads with 3.75m wide carriage way and 1.0m wide hard shoulders

2500

3. Roads with intermediate lanes of width 5.5m with earth shoulders.

5000

4. Two lane roads with 7.0m wide carriage way and earth shoulders

10000

5. Four lane divided highway (depending on traffic access control etc.)

20000 to 30000

On typical urban roads of pavement width 7.0 to 7.5m , the practical capacity may be assumed to be 600 to 1100 PCUs per hour depending on the various factors affecting the capacity.