The Effect of Autonomous Vehicles on Traffic

Friedrich, Bernhard · 2016 · Crossref

DOI: 10.1007/978-3-662-48847-8_16

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Summary

This paper investigates the impact of autonomous vehicles (AVs) on traffic efficiency, specifically focusing on the capacity of existing road infrastructure. The author argues that while AVs offer individual benefits like comfort and accessibility, their broader social value lies in improving the safety and efficiency of road transport. The study aims to quantify how AVs affect the capacity of highway segments and signalized intersections, which are the primary determinants of traffic flow efficiency. The analysis employs macroscopic traffic flow theory, utilizing fundamental diagrams that relate traffic volume, density, and speed. The author derives mathematical models for capacity based on vehicle footprint (length plus safety distance) and time gaps between vehicles. The study compares human-driven traffic, characterized by empirical reaction times and safety distances, against purely autonomous and mixed traffic scenarios. Key parameters include assumed vehicle lengths (4.5 m for cars, 18 m for trucks) and time gaps. For human drivers, a mean following distance of 1.15 seconds is used, while AVs are assumed to operate with significantly shorter time gaps (0.5 s on highways, 0.3 s at intersections). The models account for mixed traffic by weighting the proportions of autonomous and human-driven vehicles and considering the larger safety margins AVs must maintain when following human drivers. The results indicate substantial potential for capacity increases. On highways, where capacity is limited by stability and high-speed flow, a fleet of purely autonomous passenger cars could increase lane capacity from approximately 2,200 vehicles per hour to nearly 3,900 vehicles per hour, an increase of roughly 80%. This gain is driven by the ability of AVs to maintain shorter headways safely. In mixed traffic, capacity gains are non-linear; at a 50% penetration rate, capacity increases by only about 36% of the maximum potential. At signalized intersections, where capacity is determined by saturation flow and clearance speeds, purely autonomous traffic could increase lane capacity by approximately 40%, from 800 to 1,120 vehicles per hour. The study also notes that AVs can enhance traffic stability, potentially eliminating the "capacity drop" phenomenon observed in human-driven traffic during congestion breakdowns. The significance of these findings lies in the demonstration that AVs can significantly optimize the use of existing infrastructure without new construction. However, the benefits are maximized only in homogeneous autonomous traffic. In mixed traffic, human-driven vehicles dictate slower speeds and larger gaps, limiting overall efficiency gains. The author concludes that while AVs must initially operate safely in mixed environments, the creation of reserved lanes for autonomous vehicles would be highly beneficial once penetration rates are sufficient. This separation would allow AVs to exploit their full potential for speed and density, leading to further significant capacity improvements and reduced congestion.

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