What Characterizes a Vehicle as Follower on Two-Lane Intercity Highways under Heterogeneous Traffic Conditions

Abstract

The presence of a high proportion of vehicles in the following condition indicates that a particular roadway facility is performing poorly. Due to lack of related standard guidelines, planners and engineers in India use the same definition presented in U.S. Highway Capacity Manual regarding following vehicles, which may not be appropriate because of the heterogeneity in Indian traffic conditions. After performing graphical analysis, it was concluded that the previously used methodologies cannot be used for the present study because it could mislead the final findings. Hence, a new method was introduced to identify following vehicles in which speed difference (SD) and gap threshold value are used instead of the headway between two consecutive vehicles. It was found that after a cut-off gap threshold of 1 s, all the vehicles, irrespective of their categories, started traveling in the nonfollowing condition. A SD limit of −4 to +1 km/h was used to identify the following vehicles below the cut-off gap threshold value of 1 s. By using the acceptance curve method, different cut-off gap values ranging from 1.9 to 4.4 s were observed at all study sites, beyond which probability of not following (PNF) increased. After validation of the results, two-way traffic volume was identified as the dominating parameter that affects the cut-off gap value.