ESTIMATION OF CAPACITY FOR SINGLE, INTERMEDIATE AND TWO-LANE INTER-URBAN ROADS

Abstract

Estimation of capacity is one of the fundamental requirements for designing, operation and layout planning of a road network system. The knowledge of capacity is helpful to determine the maximum amount of traffic flowing in an hour and level-of-service (LOS) of a road section. More than 50 per cent of the length of National Highways in India is constructed two-lane and 24.13 per cent is of intermediate lane roads. Being a substandard two-lane road, the quality of service and performance level of an intermediate lane road can be highly improved by increasing the width of the carriageway. Also, as per IRC: 64-1990 some of the roads in rural areas are constructed merely with a single lane with at least 3.75 m width of paved surface due to lesser population or limited budget. Since augmentation projects are inter-related with funding and budgeting policies, prioritizations are made based on the studies conducted on the present and future demand state of a road system. This requires similar amount of consideration towards the importance of the knowledge over the capacity of single and intermediate-lane roads as required for two-lane inter-urban roads. The heterogeneous nature of traffic stream in India and many other developing countries makes the task of traffic analysis for a traffic engineer a complicated one. Different vehicle categories, which are the obvious characteristic of a heterogeneous traffic, impose consideration of different vehicular physique-related variables while studying the traffic flow behavior on a section of road. Generally, such variables can be considered by taking account of variation in speed, acceleration capability, area occupied on the road surface and the influence of all these variables on other vehicles present in the same traffic stream. This study is focused on determining the capacity of single, intermediate and two-lane inter-urban roads under heterogeneous traffic condition. Addressing the dynamic state of PCU values of different vehicles present in a heterogeneous traffic, the study further makes an attempt to derive relationships between capacity and operating speed of passenger cars travelling the proposed road types. The aim of developing such Abstract ii relationships is to measure the effect of width of carriageway and contribution of paved shoulder as well as the joint effect of gradient and curvature to their capacity. Data were collected at eleven sections of two-lane, seven sections of intermediate-lane and five sections single-lane roads in different parts of India to get information on traffic volume, vehicle composition and the speed of different vehicles present in the traffic stream. Parameters of the traffic stream like traffic volume and speed of different types of vehicles were extracted from the video data. The PCU value is the amount of interaction caused by a particular vehicle type to the traffic stream of passenger cars. This interaction will be different at different traffic volume levels. It will change with composition of the traffic stream as well. The amount of such interactions is measured using speed volume equations. But, development of PCU values using the speed volume equations has limitations. Such equations are effective in providing PCU values for all vehicle categories provided that all those categories are present with acceptable proportion in the traffic stream since the nature of such equations necessitates that presence of different vehicles must impose an impedance to the traffic. Such necessity is not met if all categories are not present or are very low in number. Added to this, is the large number of graphical representations handling of which can be very complicated and time consuming for a traffic engineer. To overcome such complexity, The concept of stream equivalency factor was used to provide a simple and fast method to calculate the flow of traffic in terms of PCU per hour based on the proportion of the individual vehicles present in the traffic stream on any given sections of two-lane, intermediate-lane and single-lane roads instead of going through the complicated method of manual data extraction and calculation of PCU values. After dealing with the PCU values, the capacity was estimated for sections where data collection was performed using Greenshields model. According to HCM the capacity of road varies with the FFS on the road but unlike multilane freeways, such influence is not quantified for two-lane roads in the manual. Therefore an attempt was made to develop a relationship between the 85th percentile FFS as the operating speed ofpassenger cars and capacity of different road categories in this study. By developing the cumulative frequency distribution of free flow speed of passenger cars the operating speed of passenger cars on each section was obtained. Plotting these Abstract iii operating speeds against the capacity values which were already estimated using the field data, an equation was formed for each road type. One section from each category was kept for validation of developed models and the results showed low difference between field data and theoretical models. The developed models provide the base section capacity of two-lane, intermediate-lane and single-lane interurban roads using the operating speed of passenger cars. As the capacity of a road is influenced by many factors, the effect of any single or multiple factor on the capacity of the proposed road types may be evaluated with regards to the change they inflict on the operating speed of those sections. The effect of width of carriageway for every 1.0 m increase was found to be 320.0 PCU/hr. Using the operating speed the effect of provision of 1.0 m paved shoulder to the both sides of a two-lane road was found to be 20.0 %. A model was also developed which related the operating speed to gradient and curvature data on hilly roads. Road construction on hilly areas is different from plain and level grounds. Due to environmental and budgetary concerns, such roads are generally constructed with less carriageway width. Therefore, to measure the influence of the proposed geometric factors, the operating speed-capacity model developed for intermediate-lane roads were used. Such models were also validated by comparing the results obtained using the developed model and those from manual field data collection. It was found that in the absence of curvature, the capacity of a hilly road decreases 2.0 percent for 1.0 percent increase in the gradient. On the other hand, on level ground, for every 10 degree/100 m increase in curve deflection angle, the capacity decreased 2.0 percent