Some solution strategies for automated highway exit bottleneck problems

Abstract

Transportation planners are exploring methods to ease congestion on urban freeways. The Automated Highway System (AHS) is a potential solution that could triple current freeway capacity. A three-fold increase in freeway capacity would translate into a significant increase in peak freeway traffic flow. The corresponding increase of flow on urban roads due to exiting AHS vehicles could quickly overwhelm the capacity of urban streets and lead to critical bottlenecking of AHS flow. If exiting AHS vehicles cannot sufficiently clear AHS exits because the local streets are jammed, traffic will spill onto the AHS mainline and reduce AHS capacity. The loss of AHS capacity due to congestion in urban destinations and the resulting back spill of traffic onto the AHS will greatly detract from the AHS system.The following Non-Intermodal Interface (NII) and Intermodal Interface (II) strategies are proposed as solutions to this critical problem: 1. (1) the dispersion of flow through AHS networks; 2. (2) the dispersion of flow at commuter destinations; 3. (3) the consolidation of flow at central locations. Specifically, smart street signals and prioritized street design, intermodal terminals, and automated garages are identified as potential solutions. This paper qualitatively discusses the advantages and disadvantages of each of the three general strategies, and quantitatively discusses the advantages and disadvantages of each of the four specific solutions through simulated scenarios which account for varying ramp volumes, trip attraction patterns, and urban structures. Our investigation shows that creative combinations of these methods can be useful to integrate future AHS with local road networks. Without such integration, the AHS cannot succeed.