An Overview of Automated Highway Suystems
Report 2004.TL.6859, Transport Engineering and Logistics.
This paper gives an overview of automated highway systems (AHS), including
a discussion of the four main system architectures, the layered control
system (particularly focusing on the coordination and link layers), and
sensing and communication technologies used. Automation of highway
vehicles and traffic oversight systems to create AHS is a candidate
solution for growing congestion and safety problems. Automation can
outperform human drivers by always remaining alert and reacting
predictably, faster and with greater precision. This performance advantage
along with the feedback control introduced by AHS would improve highway
capacity and safety. Automated vehicles can maintain small following
distances and eliminate human error to reach the main AHS goal of
increased capacity and safety. Although AHS is supposed to prevent the
need for expansion of the existing highways, researchers often call for
the construction of separate highway facilities for automated vehicles.
Of the four main system architectures for AHS, the synchronous moving cell
concept is the simplest, but also yields low performance. The infrastructure
manages cells or slots on the highway in both time and space, which can be
allocated to vehicles. A vehicle maintains its position relative to its slot
rather than other vehicles as is done in the other concepts.
The autonomous system architecture involves vehicles that operate completely
independent of other vehicles or the infrastructure. Any information about
other vehicles is generated by a vehicle's own sensors and although the vehicle
may receive information from the infrastructure it still makes its own
decisions. This concept requires sophisticated technology while yielding
performance no better than the synchronous moving cells concept.
The cooperative operating scheme basically adds intervehicle communication
to autonomous vehicles to improve performance with respect to capacity and
safety. The better-informed vehicle can maintain shorter spacing to
increase capacity, while also improving safety.
By further utilizing intervehicle communication, the platooning
architecture organizes vehicles into groups called platoons with small
intervehicle spacing. This relatively small following distance yields high
capacity and makes for low impact speed collisions, improving safety. The
California PATH research group and others conclude that this concept
yields the highest capacity and safety.
A layered control system is used in AHS. The bottom layer is vehicle
dynamics, including sensors and actuators. The regulation layer makes use
of feedback to control vehicle dynamics. The coordination layer
coordinates the maneuvers required by the assigned path with neighboring
vehicles, generating maneuver trajectories which are sent to the
regulation layer for execution. The link layer controls traffic flow in
sections or links of highway, according to the desired characteristics
determined by the network layer. Each link calculates and communicates
speed, spacing and path commands to vehicles in its highway section. The
network layer views the entire traffic system as a network and uses
optimization techniques to find desired traffic flow characteristics
throughout the network.
AHS requires communication and sensing capabilities. The proximity of
other vehicles can be sensed with millimetre-wave radars or laser
rangefinder. Wireless, two-way communication is used both for intervehicle
and vehicle-infrastructure communication. Differential GPS (DGPS) or
magnetic nails buried in the road can be used to detect the location of a
vehicle for navigation purposes.
Reports on Transport Engineering and Logistics (in Dutch)
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