Left-Lane Changes in Laterally Unbalanced Traffic

Lane changes caused by weaving traffic flows at freeway merging, diverging, and weaving sections can lead to a drop in capacity and can significantly disrupt the entire traffic facility. The lane-changing pattern of on-ramp vehicles has been studied by using vehicle trajectory data when traffic is laterally balanced. However, in reality, detailed vehicle trajectory data are hard to obtain, and traffic at freeway weaving sections can be laterally unbalanced. This study offers a simple but practical approach that uses data from lane-based loop detectors for estimating the number of left-lane changes when traffic is steady but laterally unbalanced. Stationary states with nearly constant flow rates and speeds were obtained from the loop detector data at a freeway weaving section on SR-91 in Buena Park, California. Under near-stationary traffic conditions inside the weaving section, (a) traffic flow conservation was found generally to hold, and (b) traffic was laterally unbalanced with dissimilar additions of vehicles in the lanes. The number of left-lane changes was estimated by using the lane flow-rate differences between the upstream and the middle main-line detectors at the weaving section. Results showed that (a) the number of left-lane changes within two adjacent lanes linearly increases from the leftmost lane to the shoulder lane and (b) the total number of left-lane changes is not sensitive to main-line traffic conditions but increases linearly with the on-ramp flow rate. This study further corroborates the validity of the macroscopic lane-changing model for the lateral distribution of left-lane changes in laterally unbalanced traffic, especially in congested traffic.