06-05-2011, 11:48 AM
This paper describes the design of an Automated Highway System (AHS) developed over the past ten years at the California PATH program. The AHS is a large, complex system, in which vehicles are automatically controlled. The design and implementation of the AHS required
advances in actuator and sensor technologies, as well as the design, analysis, simulation, and testing of large-scale, hierarchical, hybrid control systems. The paper focuses on the multilayer AHS control architecture and some questions of implementation. It discusses in detail
the design and safety verification of the on-board vehicle control system, and the design of the page link layer traffic flow controller.
1 Introduction
This paper describes the control architecture of an Automated Highway System (AHS), developed over the past ten years at the University of California Partners for Advanced Transit and Highways
(PATH) program, in cooperation with the State of California Department of Transportation (Caltrans) and the United States Federal Highway Administration (FHWA). This multilayer AHS architecture was first described in [43, 42], and the paper discusses aspects of design and verification
at several of those layers. The AHS architecture envisions a fully automated control system that leaves few vehicle driving decisions to the driver. It is argued in [42] that full automation can greatly increase highway capacity while improving
safety. A key to greater capacity is the organization of traffic in groups of up to twenty tightly-spaced
cars called platoons.1 Although the spacing between these platoons is large (about 60 meters), platooning
decreases the mean inter-vehicle distance to achieve a capacity of up to 8,000 vehicles per
hour per lane, as compared with a capacity of 2,000 in today’s highways with manually-controlled
vehicles. Because the maintained distance between cars within a platoon is small (1-2 meters), in
the event of a collision the relative impact velocity (and hence the impact energy) between colliding
vehicles is small. As a consequence, platooning can increase safety. An additional benefit is
that the tightly-spaced vehicles reduce aerodynamic drag. As a result fuel consumption and vehicle
emissions are lower [45, 5]. To maintain close proximity while traveling at relatively high speeds
(90 Km/h), the vehicles must be fully automated, since people cannot react quickly enough to drive
safely with such small headways.
1Although we speak of cars, we mean all vehicles including trucks and buses. It is likely that an AHS will be initially
deployed for trucks and buses.
1 INTRODUCTION
Because of its size, complexity, and large impact on everyday life, the design of an AHS control
system that is safe, reliable, and practical, poses major challenges, both in the development of new
advances in communication, computer, sensor and actuator technologies, as well as in the synthesis
and analysis of intelligent, hierarchical, large-scale hybrid control systems. Reference [39] provides
an overview of the Advanced Vehicle Control System (AVCS) research at the PATH program in
1990, while [23] describes the PATH AHS architecture design in 1994, focusing on the physical and
coordination layers of the architecture. This paper emphasizes progress since 1994.
We also present new results on the safety and performance analysis of the hybrid system formed by
the combined action of the coordination and regulation layer control systems and some results on
the control of the combined system formed by the link, coordination, and regulation layers of the
AHS architecture.
Table 1 summarizes the functions of the five-layer PATH AHS architecture, and the mathematical
framework used in the design of each layer. Section 2 presents an overview of the architecture and
describes each layer. Section 3 discusses the design and safety verification of the hybrid on-board
vehicle control system. Section 4 discusses the page link layer control system. Section5 summarizes the
main points of the paper and contains some remarks about the future of AHS.
The PATH AHS research program began in 1989 with Caltrans support. In order to carry out
AHS research PATH developed basic tools for hybrid system design, simulation, and verification.
Among these, the hybrid system simulation language and run-time system SHIFT [11] and related
theoretical and software tools have been used in other intelligent control projects.
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