AN EXAMPLE OF QUANTITATIVE EVALUATION OF AVCS SAFETY

A method has been devised for expressing in absolute terms the effect on safety of various features in the design of an AVCS system. It depends on gathering data from different sources and some modelling of vehicle motions in and accident. It is illustrated by an example concerning a postulated one-lane Automated Highway System on the Santa Monica Freeway. If no "divider" separates the automated lanes from the rest, a particular kind of secondary accidents will result in additional deaths amounting to 4-5 per year (present rate = 8 per year) on the 20km (12 mile) stretch between 1-405 and 1-110. If a divider is present the additional toll is 0.4 deathdyear. Automation may be expected to reduce other deaths by more than the latter figure.This is with manual flow at the present rate, and an additional 6 0 % automated flow. Figures can also be calculated for various degrees of injury, and variations in assumptions. Well-founded concrete data of this kind assists evaluation. 'Paper presented at the Pacific Rim Conference, Seattle, WA, July 1993. Quantitative Methods in Evaluation of AVCS Safety In designing or evaluating any Advanced Vehicle Control System (AVCS) it would be very useful to be able to assign absolute quantities to the effects on safety, as well as to those on, for example, cost, air quality or time savings. Such a method now exists. Safety can be evaluated in terms of casualties per year or per 100 M VMT. In this paper it is exemplified by application to a disputed issue about one feature of the physical layout of an Automated Highway System (AHS). This paper describes this application of the method, and identifies types of uncertainty which are common to many applications. Background A Safety-Evaluation Problem An AHS may well consist of automuted lanes (ALs) with traffic in platoons and manual lanes on the same freeway. With such a configuration there are arguments for and against the provision of a physical barrier (called a divider) between the automated lanes and the rest. On the one hand, a divider is unattractive to drivers. It is likely to make them feel "fenced in" and could generate claustrophobic reactions. While changing lanes through a gate, drivers may become alarmed or fearful. ("Gates" are the gaps that have to be left in the divider to permit vehicles to enter or leave the ALs.) Changing lanes between such gates could reduce capacity. The end of a divider would itself be an obstacle which a vehicle can strike, come to rest and be struck by other vehicles. Thus, it is argued, dividers create more casualties than they prevent. On the other hand, Hitchcock (1991) asserts that a divider is necessary to prevent the casualties which would otherwise occur in secondary accidents when a collision on the manual lanes intrudes on the ALs. With high-speed platoons, such an occurrence would lead to multiple casualties. Further, the divider will not generate serious casualties. Its end need be only a foot or so high. If a car hits the divider's end while changing lane, the passenger compartment would not be penetrated, and deceleration would not be very great. Because a vehicle would join a platoon only by entering at the rear, the following platoon could stop without running into a vehicle stranded on the barrier. Such an accident would interrupt traffic on the AL, but would generate no serious casualties. The secondary accidents voided by the divider, however, would generate serious casualties. This paper therefore puts numbers to the effect on secondary accidents of a divider or physical barrier between the AL and the manual ones on part of a hypothetical AHS, situated on the Santa Monica Freeway between 1-1 10 and 1-405. Data has been gathered from a number of sources: Anwar and Jovanis 1993 obtained the numbers of relevant accidents 1986-7 accidents in which vehicles (referred to as intruders) entered the left of the freeway. This is the area where it is supposed the ALs would be. The National Crashworthiness Data System gave data relating impact velocities to death and injury in 1990-1, and Caltrans District 7 gave data on existing (1992) flows.