AIMS / OBJECTIVES:

TRANSPORTATION ENGINEERING UNIT - 5

TRAFFIC ENGINEERING - I

CONTENTS:

Aims / Objectives 5.1. Introduction

5.2. Scope of Traffic Engineering 5.3. Traffic Studies

5.4. Traffic Volume Studies 5.5. Speed Studies

5.6. Speed and Delay Studies 5.7. Origin and Destination Studies 5.8. Traffic Capacity

5.9. Road Accidents

5.10. Self Assessment Questions 5.11. Summary

5.12. References

AIMS / OBJECTIVES:

The fundamental objective of traffic engineering is to achieve safe , free , rapid and efficient flow of traffic. Factual studies of traffic operations provide the foundation for developing methods for improvement , in general , and for solving specific traffic problems.

Traffic volume , speed , speed and delay , origin and destination , capacity and accident studies which form the basis for traffic data are presented in this unit.

5.1. INTRODUCTION:

Traffic engineering is a comparatively new branch of engineering , and has grown with the increase in traffic in recent years. As vehicular traffic began to increase , the congestion on streets began to hamper the safe and efficient movement of traffic. More and

more accidents were caused and serious problems of parking and environmental pollution began to be felt. If was therefore necessary to give increasing attention to the operational characteristics of highway transportation.

Traffic engineering is the science of measuring traffic and travel , the study of the basic laws relating to traffic flow and generation and application of this knowledge to professional practice of planning , designing and operating traffic systems to achieve safe and efficient movement of persons and goods.

Highway engineering and traffic engineering are related subjects and the latter may be considered to be an off-shot of the former. However , now-a-days traffic engineering has been recognised as a specialised branch.

5.2. Scope of Traffic Engineering:

Traffic engineering may be considered as a special technique in highway design and control of traffic. Traffic engineering deals with the direction and control of vehicular traffic and pedestrians on existing highways. The planning , design and operation of all devices that aid in the highway safety and free flow of traffic fall within the scope of this subject.

Traffic characteristics are quite complex and mainly dependent upon the characteristics of the road user and the vehicle and require particular attention. The factors which affect the characteristics of road user are physical , mental , psychological and environmental. Vehicle size , power , speed and braking system effect the vehicular performance on the road and need special attention. Apart from these , the various studies to be carried out include volume , speed , speed and delay , origin and destination , capacity , parking and accident studies.

Various aspects that are covered under traffic operations are regulations , control and warrants for application controls. Regulations may be in the form of laws and ordinances or other traffic regulating measures such as speed limit , parking restrictions etc.,. Installation of traffic control devices like signs , signals and islands are the most common measures to regulate and control traffic. Actual adoption of traffic management measures such as traffic regulations and control need adequate attention.

Enforcement of traffic regulations and education of the masses towards a proper traffic behaviour are akin to Traffic Engineering and these three things are some times designated as the three E -S of traffic safety.

5.3. TRAFFIC STUDIES:

Traffic studies are carried out to collect factual data regarding traffic operations.

These studies help in deciding the geometric design features and traffic control for safe and efficient traffic movements. These studies are also known as ‘Traffic Surveys’ or ‘Traffic Census’. The following traffic studies are discussed in the following pages.

(i) Traffic Volume studies (ii) Speed studies

(iii) Speed and Delay studies

(iv) Origin and Destination studies (0 - D studies) (v) Traffic Capacity studies and

(vi) Accident studies

5.4. TRAFFIC VOLUME STUDIES:

The term ‘Traffic Flow’ and ‘Traffic Volume’ are used interchangeably to define the number of vehicles that pass a given point on the highway in a given period of time. It is this information that is of most value to the highway planner. When the traffic is composed of a number of types of vehicles , mixed traffic , it is the normal practice to convert the flow into’

Equivalent Passenger Car Units’ (PCUS) by using the specified equivalency factors. The flow is then expressed as PCUS per hour or per day.

A complete traffic volume study includes the classified volume study by recording the volume of various types and classes of traffic , the distribution by direction and by turning movement per unit time.

The objects and uses of traffic volume studies are given below:

(i) Relative importance of roads may be fixed in deciding the priority for improvement and expansion.

(ii) Helps to evaluate the existing facilities and plan new facilities in regard to traffic operation and control.

(iii) Helps in the analysis of traffic trends and patterns.

(iv) Classified volume study helps in structural design of pavements and geometric design of roads ; design of side walks , pedestrian crossings etc.,

(v) Helps in planning regulatory measures.

(vi) Turning traffic studies are useful in design of road intersections , light signal timings and other control measures.

The volume of traffic flow varies from time to time , hour to hour in a day , day to day in a week , and week to week in a year. Hence if a true picture is to be obtained , the hourly traffic volume should be known with daily and seasonal variation pattern.

5.4.1. COUNTING OF TRAFFIC VOLUME:

Traffic volume counts are made either by mechanical counters or manually.

(a) Mechanical counters: These automatically record the total number of vehicles crossing a section of a road in a desired period. These recorders are capable of recording impulses caused by the traffic movements on a pneumatic nose placed across a roadway or by electrically operated counters. In some cases , the impulses caused by vehicles of light weight , and in particular pedestrians traffic , may not be enough to actuate the counter. Other methods of working of the mechanical detectors are by photo-electric cells , magnetic detectors and radar detectors.

The main advantage of these counters is that they can work throughout the day and night for the desired period , recording the total hourly volume , which may not be practicable in manual counting. The main drawback in this method of traffic count is that it is not possible to get the traffic volumes of various classes of traffic in the stream and the details of turning movements.

(b) Manual Counts: This methods employs a field team to record traffic volume on the prescribed sheets. By this method it is possible to obtain data which cannot be collected by mechanical counters , such as vehicle classification , turning movements , and counts where the loading conditions or number of occupants are required. However , it is not practicable to have manual count for all the 24 hours of the day and on all the days in a year. Hence resort is made to statistical sampling techniques to arrive at the peak hourly volumes as well as the average daily traffic volume.