In this article we will discuss about:- 1. Traffic Volume Studies 2. Traffic Flow Characteristics 3. Traffic Forecasting.
Traffic Volume Studies:
Systematic traffic surveys, traffic studies, and their scientific analysis are essential in traffic engineering and find wide application in planning for future needs of roads, improvement of existing facilities, geometric design aspects, pavement design, and traffic regulation and control.
Traffic surveys, also called traffic census, include types of traffic, size and weight, traffic flow, traffic volume per hour and per day, including seasonal variations/annual variations, distribution in different parts of a road network, and distribution in different directions at intersections.
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Traffic volume counts can be obtained by the following methods:
(i) Manual Method:
A team of observers can record not only traffic volume but also the types of vehicles, turning movements, directions of movements, laden weights of trucks and other such details that cannot be captured by using automatic methods. (IRC: 9-1972—Traffic census on non-urban roads, New Delhi, 1972).
However, it is not practicable to conduct manual counts for all the 24hours of the day and on all days of the year. But it is the most reliable method to obtain traffic volume by classified kinds of traffic and directional volume for short counts needed for intersection design. Statistical sampling techniques are used to extrapolate short-term counts.
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Data may be recorded on prescribed sheets with appropriate tabular format or using multibank hand tally counters. Daily summary and weekly summary sheets are prepared.
(ii) Automatic Counters:
Automatic counters use less number of observers and have the advantage of collecting a continuous record of traffic movement. The disadvantage is that details of classified traffic and other relevant information cannot be got.
Pneumatic Counters:
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In this type, the counter is activated by an air switch attached to a flexible hose set across the road over which vehicles pass.
Electrical Counters:
In this method, the counter is activated by the closing of an electrical circuit by the moving vehicle.
Photo-Electric Cell Counters:
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In this method, the counter is activated by the interruption of light rays falling on a photo-electric cell placed on the side of the road as the vehicle moves past it.
(iii) Moving Observer Method:
In this method, an observer in a car moves along with the traffic, and against the traffic, to count the number of vehicles met, number overtaken and the time of travel.
Volume of traffic is calculated from equations for q (eq.4.19) and the journey time t̅ (e.q.4.20).
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Presentation of Traffic Volume Study Data:
The data obtained from traffic volume studies are presented in different forms for arriving at appropriate conclusions depending on the purpose of studies:
(a) Hourly, daily, weekly, seasonal, and annual variations of traffic in the form of charts or graphs. These will help in evaluating the existing facilities and for traffic regulation and control.
(b) Annual average daily traffic (AADT) of the total traffic as well as classified traffic are determined. The concept of ‘passenger car unit’ (PCU) is used to convert all types of traffic into an equivalent number of passenger cars. Continuous traffic counts are needed for this. Extrapolation from short counts needs the application of sampling techniques.
If the flow is measured only for short period, and not for an entire year, it is called the average daily traffic (ADT).
These values help in assessing the relative importance of different routes in a road network, and also for planning improvements and phasing future development.
(c) Design hourly volume of traffic may be determined from a plot between hourly traffic volume (as percent of AADT or ADT) and the number of hours in a year when the particular traffic volume is exceeded.
(d) Traffic flow-lines are drawn along the route with the thickness of lines being proportional to the traffic volumes. This helps in knowing traffic distribution. Such lines help in fixing priorities for improvement of facilities along the routes.
(e) Traffic flow distribution at road intersections can be prepared, showing details of turning and crossing traffic volumes. This helps in the design of intersections and traffic control devices.
Typical hourly variation patterns are shown in Fig. 4.14.
Design Hourly Volume of Traffic:
The peak hourly volume in a year will be so high that it will not be economical to design highway facilities to cater to the needs of this traffic. The average hourly volume found from annual average daily traffic (ADDT) data will be inadequate for a considerable period of the year.
It has been established from considerable research that if the highway is designed for the 30th hourly traffic volume of the AADT plot, it will be satisfactory for a considerable period of the year. Since this volume will be exceeded only 29 times in the whole year, congestion will be caused only on these few occasions. This is designated as 30HV.
Thus, the thirtieth highest hourly volume is usually taken as the design hourly volume for achieving reasonable economy consistent with minimal inconvenience round the year. In some cases, this may not be strictly followed when spot study data are available and the traffic engineer uses his discretion. Sometimes, speed and O&D studies may influence the decision.
A typical plot from which the 30th highest hourly volume of traffic is found is shown in Fig. 4.15.
A design hourly volume of 8 to 10 per cent of the AADT has been suggested for Indian conditions (IRC: 64-1990), based on traffic surveys on some national highways.
Traffic Flow Characteristics:
The theory of traffic flow may be defined as the study of the movement of vehicles over a highway or a road network. The study involves a comprehensive knowledge of vehicular traffic, which enables a traffic engineer to evolve improved techniques for the regulation, control and management of traffic. This is a multi-disciplinary subject involving the application of probability and statistics, psychology and operations research, which is a relatively new area of knowledge.
Congestion is inherent to the movement of vehicular traffic, which automatically leads to queuing and delay, leading to economic loss. Therefore, the study of queuing and delay is also important to the traffic engineer.
Traffic flow theory is concerned with speed, traffic flow or traffic volume, and traffic density or concentration. Traffic flow is the number of vehicles passing a particular point at a specified period of time. This is commonly expressed as number of vehicles per hour.
Traffic density or concentration is the number of vehicles occupying a specified length of the road at a particular instant of time. It is usually expressed as the number of vehicles per kilometer length of roadway per lane.
The distance between the fronts of successive vehicles is called space headway, usually expressed in metres. The time interval between the passages of the fronts of successive vehicles at a specific point in seconds is called the time headway, or simply headway.
Transverse and longitudinal distribution of vehicles on the highway depends on the headway. With increase in speed of the traffic, the minimum space headway increases, whereas the minimum time headway first decreases and then increases after reaching a minimum value at optimum speed of the traffic stream (Fig. 4.16). Maximum capacity flow is attained at this optimum speed when the time headway is a minimum.
These are ‘diverging’, ‘merging’ and ‘crossing’ and are shown in Fig. 4.17.
Since in India we follow the Keep Left rule, diverging from the left and merging from the left are easy and do not involve traffic conflicts.
Transfer from one traffic lane to an adjacent lane, called lane change, involves diverging and merging, weaving and speed range of vehicles. The more speed range, the more is the demand for lane change. One-way or two-way traffic conditions also govern these lane-change manoeuvres.
Traffic flow characteristics including transverse and longitudinal distribution of vehicles in the stream are useful in the design of geometric elements such as traffic capacity, number of traffic lanes and carriageway width. Also the study of these is useful in planning traffic control measures such as one-way traffic and intersection design.
Traffic Forecasting:
Transport is a catalyst for the development of a nation as it is an important infrastructure facility. Transportation projects have to be planned carefully, considering not only the present requirements but also the demands for a reasonable period in future. This calls for traffic forecasts either for a new route or for the improvement of existing facilities. The feasibility or cost-benefit analysis will be facilitated by good traffic forecasting.
The factors which generate traffic are varied – population, vehicular explosion, industrial and agricultural output and other such developmental factors. Only limited accuracy can be achieved in estimates of traffic because of the difficulty of predicting these factors.
Current traffic includes the existing traffic and attracted or traffic diverted from other routes consequent to improvements of the highway under consideration.
Increase in traffic may be from the following:
(i) Normal growth due to the general increase in the number and usage of vehicles.
(ii) Traffic diverted on to this route from adjacent routes.
(iii) New or induced traffic because of new road users.
(iv) Development traffic, or the increase in traffic due to improvements of land use, industrial or otherwise, adjacent to this route.
In fact, the increase in traffic due to the reasons stated in (ii) and (iii) is called ‘generated traffic’.
Traffic Forecasts Based on Past Trends:
The past trends of traffic growth are simply extrapolated or projected forward. An annual growth of about 7 percent is considered reasonable. Principles such as a correlation index or a growth formula can be used; the former may include productivity, gross domestic product, and fuel consumption. The growth formulae used are straight line formula, geometric increase, compound interest formula and the like.
Analytical or Mathematical Models:
These are popular in travel demand forecasts used in transportation planning. If the chosen model is properly calibrated with the aid of the collected data, the traffic forecasts based on this approach are considered to reasonably accurate. Demand for travel being an economic activity, well known principles of economics related to consumer behaviour in relation to goods and services may be applied to predict travel demand.
The aspects of trip-making are trip frequency, mode, destination and route choice. These may be considered sequentially or simultaneously for the development of mathematical model for forecasting. The use of computers has made the adoption of complex models for traffic forecasting relatively easy.
Design Period:
The choice of design period is difficult; in general, a period of ten years—five years in detail and further five years in somewhat less detail—is considered sufficient. Long-range transport plans and forecasts are not attempted because of the indeterminacy and complexity of the factors involved.