UC Berkeley

The morning rush in two idealized settings are forecast into the future, assuming that most drivers will engage Adaptive Cruise Control (ACC) while driving their cars. To this end, careful measurements of ACC-equipped cars traveling on roads and a highway confirm an earlier finding reported by others: for a given traffic speed in ACC-rich congestion, the density tends to be smaller than in present-day congestion where all vehicles are manually operated. All else equal, the lower congested densities (i.e., larger car spacings), mean that queues will be less-densely-packed and will expand over longer physical distances in the future, as ACC-equipped vehicles become more prevalent. These uncompacted queues pose problems for urban areas, where queue storage is often already a problem during rush hours.Simulations calibrated using the field-measured data confirm this concern and contradict optimistic predictions of how ACC may lead to a congestion-free future. In a setting with already moderately high congestion and long-street links inspired by Downtown Los Angeles, it is predicted that networkwide vehicle hours traveled (VHT) will increase by up to 12% compared to present-day levels, despite assumptions that are largely favorable to ACC. In a setting inspired by Midtown Manhattan, with short-street links and already high congestion levels, networkwide VHT is predicted to increase by as much as 87%. The higher bottleneck capacities often promised by advocates of ACC are shown to be irrelevant when spillover queues restrict flows from reaching those capacities. Interventions tested through simulations include adjusting onboard ACC controllers to produce lower jam spacings, to prevent a problematic future.