The author considers work by Hsu which described the normal operation of a system of automated freeways that minimizes the degree to which the infrastructure is involved in maneuvers. No account is given of procedures on entry and exit or of possible faults. The Partners for Advanced Transit and Highways (PATH) safety program demanded a second example of the process of full specification and fault tree analysis to determine if this process was generally applicable. Accordingly, Hsu's specification has been completed, retaining minimal infrastructure-based involvement in the maneuvers. In this design the safety criterion was adopted that no hazard shall arise unless there are two independent faults. The hazards were those used earlier. Because a hazard is the precursor of a catastrophe, no multivehicle high-relative-speed collisions should occur without three independent faults. A fault tree analysis was carried out in which there werenever more than four branches in any line of the tree. It is concluded that it is possible to produce a design that meets these safety criteria and that behaves during normal operation in the same way as the system defined by Hsu. Further, the method of complete specification and fault tree analysis suffices to produce a system of demonstrated safety with a practical allocation of resources and time. A comparison is made of the safety of this vehicle intelligence system and the earlier infrastructure intelligence one. (A)
Abstract