Fully Automated Highway System

Imagine a world where your car drives you and you don’t drive it! A world in which your car will take you any place, any time and all you have to do is sit there. This world is not so far away. The need for fully automated highway system development is in high demand due to the factors stated above. Fortunately, there is one such technology in the works, vehicle platooning; vehicle platooning is method used in AHS to increase the capacity of highways. The eight-vehicle platoon demonstration at the National Automated Highway Systems Consortium Technical Feasibility Demonstration, held in San Diego from August 7-10, 1997, successfully demonstrated the technical feasibility of operating standard automobiles, Buick LeSabres, under precise automatic control at close spacing and at highway speeds. Riders experienced real travel in a fully automated AHS vehicle, and were shown that comfortable, high-capacity, automated travel is technically feasible in the near future. Since platooning enables vehicles to operate much closer together than is possible under manual driving conditions, each lane can carry at least twice as much traffic as it can today.

This should make it possible to greatly reduce highway congestion. Also, at close spacing aerodynamic drag is significantly reduced, which can lead to major reductions in fuel consumption and exhaust emissions. The high-performance vehicle control system also increases the safety of highway travel, reduces driving stress and tedium, and provides for a very smooth ride. At Demo ’97, the eight vehicles of the PATH platoon traveled at a fixed separation distance of 21 feet at all speeds up to full highway speed. At this spacing, eight-vehicle platoons separated by a safe inter-platoon gap of about 200 ft. and traveling at 65 mph would represent a “pipeline” capacity of about 5700 vehicles per hour. Reducing this by 25% to allow for the maneuvering needed at entry and exit points corresponds to an effective throughput of about 4300 vehicles per lane per hour. Throughput under normal manual driving conditions at this speed would be approximately 2000 vehicles per lane per hour. Such short spacing between vehicles can produce a significant reduction in aerodynamic drag for all of the vehicles (leader as well as followers).

These drag reductions are moderate at the 21 foot spacing of the Demo, but become more dramatic at spacings of half that length. Wind-tunnel tests at the University of Southern California have shown that the drag force can be cut in half when vehicles operate at a separation of about half a vehicle length. Analyses at UC Riverside have shown how that drag reduction translates into improvements of 20 to 25% in fuel economy and emissions reductions. The tight coordination of vehicle maneuvering is achieved by combining range information from a forward-looking radar with information from a radio communication system that provides vehicle speed and acceleration updates 50 times per second. This means that the vehicles can respond to changes in the motions of the vehicles ahead of them much more quickly than human drivers. As a result, the space between the vehicles is so close to constant that variations are imperceptible to the driver and passengers, producing the illusion of a mechanical coupling between the vehicles. The vehicle-vehicle communication capability is used to coordinate maneuvering.

These maneuvers include lane changing, in which a vehicle safely coordinates its lane change with adjacent vehicles, so that they do not try to occupy the same place at the same time, and platoon join and split maneuvers, thus decreasing the space between vehicles to form a platoon and increasing the space to separate from a platoon. Tight coordination among vehicles also facilitates responses to malfunctions, enabling all vehicles in a platoon to learn about a malfunction within a fraction of a second, so that they can respond accordingly. The vehicles are equipped with malfunction management software, to automatically implement such corrective actions as increasing the separation between vehicles while warning the drivers. The control system has also been designed with careful attention to passenger ride quality. Both the lateral (steering) and longitudinal (speed and spacing) control systems have been designed, tested, and proven to have higher performance than even highly skilled human drivers. The lateral control system keeps the vehicle to within a few inches of the lane center under virtually all conditions, which is much more accurate than human drivers’ steering.

The longitudinal control system maintains speed and spacing accuracy that exceeds that of all but virtuoso race-car drivers. The accuracy and fast response of the longitudinal control system provides a reassuring, smooth ride. Although some people are initially startled by the “tailgating” aspect of vehicle following at close separations, most of them quickly adapt and develop a sense of comfort and security because of the constantly maintained separation. The human/machine interface on the Demo LeSabres has been carefully designed to enhance user acceptability. The steering-wheel control buttons can be used to activate and deactivate automation functions, and the flat-panel display in the center of the instrument panel provides timely status information. The latter is important so that the driver can be given assurance during fully automated driving that the system really “knows” what it’s doing. Maneuvers that might be surprising are indicated in advance on the display so that there are no surprises and so that vehicle movements will seem natural and logical. Although the platoon scenario at Demo ’97 in San Diego did not include the full range of functions that would be needed for an operational automated highway system, it did include capabilities that would not be needed in normal AHS operations.

For example, the entire platoon started from a stop at the start of the demo, and decelerated to a stop at the end, because of the physical constraints of the Demo site. In an operational system, individual vehicles would accelerate on the onramps to merge into the traffic stream, and would decelerate on the exit ramps after lane changing out of a platoon, while the mainline traffic would be flowing continuously. Since the I-15 HOV facility does not have intermediate on- and off- ramps, the entire platoon started and stopped together. PATH researchers designed the operational concept and control systems for the platoon scenario, and specified the hardware performance requirements. They developed the magnetic reference sensor system for lateral control, the electronic throttle actuation system, the communication protocols for vehicle-to-vehicle communication, and the malfunction management software. PATH researchers also integrated all the in-vehicle software, and debugged and tested the complete vehicle control system.