Intelligent Vehicle Highway System Safety: Multiple Collisions in Automated Highway Systems

A comparison is drawn between the casualty rates per failure on an automated highway system (AHS) according to the longitudinal control configuration used. It is suggested that the most important failure of this system is one in which brakes fail full on. The inverse failure brakes will not apply—must be avoided by system designers. Incident for incident, it is clearly the more dangerous of the two, and the brakes-on failure is therefore the one selected in a "fail-safe" scheme. Further, unlike the "no-brakes" failure its consequences are sensitive to the control system configuration. This possibility was the original reason put forward for selection of platooning as the operational mode for AHS. The comparison is drawn between close-spaced platooning, vehicle following of the types used in Autonomous Intelligent Cruise Control (AICC) and Cooperative Intelligent Cruise Control, and a point-following configuration (PFC). The model used permits evaluation of the consequences of a failure, allowing for the multiple collisions that usually ensue. In terms of casualties per failure, short-spaced platooning is over ten times better than AICC at high capacities. Point Follower Control is better, in these terms, than platooning in ideal conditions (full, accurate knowledge by the system of the deceleration capabilities of all vehicles). However, if the information is not precise, casualties per failure increase many times. If it is desired that casualties due to this failure be much less than the present rate for all causes, the required reliability of the control system can be deduced. It is shown that the reliability of the control system against this form of failure for platooned systems needs to be around 0.1% failures/year/vehicle (mean time between failures = 100,000 hours operating time) if this target is to be achieved. Some general observations are made about the impact of AICC in mixed traffic. It appears that the safety record of this device will at first be good, but will diminish if such systems are mounted, and used on the great majority of cars.