The automated highway system (AHS) architecture of the California PATH program organizes traffic into platoons of closely spaced vehicles. A large relative motion between platoons can increase the risk of high relative velocity collisions. This is particularly true whenever platoons are formed or broken up by the join and split control maneuvers and by the decelerate to change lane control maneuver, which allows a platoon to create a gap before switching from one lane to another. In this paper we derive a safety region for the relative velocity between two platoons. By guaranteeing that the relative velocity between platoons remains in this region, impacts of high relative velocity can be avoided. Under normal operating conditions, there are four control laws for a platoon leader: leader law, join law, split law, and decelerate to change lane law. For each control law, a desired velocity profile for the platoon that satisfies safety and time-optimality requirements is derived. A nonlinear velocity controller is designed to track the desired velocity profile within a given error bound. When safety is not compromised, this controller keeps the acceleration and jerk of the vehicles in the platoon within comfort limits.
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