Addressing traffic breakdown at merge sections in mixed traffic

Abstract

In the future, Human Driven Vehicles (HDV's) are expected to interact with Automated Vehicles (AV’s) and Connected AV’s (CAV’s). Due to the differences in the expected driving behavior of AV’s (ACC) and CAV’s (CACC) compared to HDV’s, the nature of traffic breakdown phenomena in the future can be expected to change. AV’s (ACC) and CAV’s (CACC) refer to AV’s enabled with Adaptive Cruise Control functionality and CAV’s enabled with Co-operative ACC functionality respectively. Currently, there are traffic management measures which address traffic breakdown for the current situation. With the expected changes in breakdown phenomena in the future, will the current measures be effective in addressing the different nature of breakdown in the future? This research answered this question through simulation (Vissim), by focusing into the effectiveness of one of the current measures. The current measure whose effectiveness was analyzed is Variable Speed limits (VSL) applied through the concept of feedback Mainstream Traffic Flow Control, MTFC-VSL, at on-ramp merge sections. Before conducting simulations, it was hypothesized that MTFC-VSL control effectiveness in addressing traffic breakdown increases as CAV’s (with ACC and CACC functionality) penetration rate increases in mixed traffic, because CAV’s can be expected to precisely follow the speed limits. Mixed traffic in this research comprised of HDV’s and CAV’s (with ACC and CACC functionality). CAV’s (with ACC and CACC functionality) implies that CAV’s majorly differ with that of HDV’s in car following behavior and not in lane change behavior. Simulation results and analysis revealed that the hypothesis doesn’t hold good. Improvements in average Travel Time (TT) of mainline vehicles and average network speed due to the presence of MTFC-VSL control compared to the absence of it, deteriorated as penetration rate of CAV’s (with ACC and CACC functionality) increased until 20% in mixed traffic. For further penetration rates the improvements fluctuates. On-ramp vehicles for most of the scenarios of mixed traffic, are better off without MTFC-VSL control as the presence of it increases the vehicles average TT. MTFC-VSL control doesn’t effectively address the capacity drop phenomenon for various scenarios of mixed traffic. Lastly, it was found that for less than 20% CAV’s (with ACC and CACC functionality) penetration rate in mixed traffic, MTFC-VSL control effectiveness can be expected to overall increase if Intelligent Speed Adaptation is installed as an On-Board Unit in HDV’s, as it limits HDV’s exceeding the speed limits.It must be noted that, MTFC-VSL control was set up considering the practical considerations of implementing in real life, which can also be expected to play a significant role in hypothesis not being valid. Given the ineffectiveness of MTFC-VSL control for various scenarios of mixed traffic, the future traffic management measures should focus on the causes of breakdown phenomena which aren’t addressed by MTFC-VSL control. One of the proposed measures is a combination of a merging assistant strategy & MTFC-VSL control to better address traffic breakdown than MTFC-VSL alone. Merging assistant strategy utilizes the connectivity feature of CAV’s to foster smoother merging of on-ramp vehicles which isn’t addressed by MTFC-VSL control.