Effect of Shoulder Width on Traffic Flow Parameters on Two-Lane Undivided Roads

Traffic flow on two-lane undivided facility is very different from other roadway facilities. In case of two-lane undivided highways, after a certain increase in traffic volume, the overtaking maneuver decreases considerably due to the presence of oncoming vehicles. Also, with further increase in traffic flow, it may result in platoon formation. Two-lane undivided road is a very common facility in both urban and rural settings in India, where such kind of facility can cater to the significant proportion of traffic of that area. Despite this fact, the research available on operation of these types of facilities is very limited. Moreover, very little empirical-based research has been conducted about traffic flow characteristics on two-lane undivided roadway, especially for Class III type facilities (HCM 2010) under Indian conditions. In the present study, three study sections with features like Class III facilities are considered for analysis. Geometric feature of the roadway has 7-m carriageway with additional paved shoulder having varying width. These paved shoulders were also utilized by the vehicles during heavy-flow conditions. For Section 1, formation width was observed as 10.5 m (7-m carriageway and 1.75-m paved shoulder on each side); Section 2, it is 9.7 m (7-m carriageway and 1.35-m paved shoulder on each side); whereas in Section 3, it is observed as 9.5m(7-m carriageway and 1.25-m paved shoulder on each side). Speed distribution is analyzed for the three study sections for different vehicle categories. It shows reduction in speeds of vehicle with decreasing shoulder widths. Further, to convert heterogeneous traffic in terms of vehicles/hour to PCU/hour, dynamic PCU concept is used. Thereafter, Speed-flow relationships are developed using Greenshields model for the study sections. The capacity of the three study sections-Bardoli (1st Section), Bardoli (2nd Section), and Olpad are found to be 3400, 2950, and 2800 PCU/h for both directions of movement. Considering Section 1 as standard basic section, the reduction in capacity of Section 2, and Section 3 were found to be 13.2 and 17.6%, respectively, due to the reduction in shoulder width. Finally, level of service analysis is done for the study sections using Percent of Free-Flow Speed (PFFS) as measure of performance (HCM 2010). The results of this study may be useful for studying.