Decentralised Cooperative Collision Avoidance with Reference-Free Model Predictive Control and Desired Versus Planned Trajectories

Connected and automated vehicles provide a new opportunity for highly advanced collision avoidance, in which several cars cooperate to reach an optimal overall outcome, that no single car acting in isolation could achieve. For example, one car may automatically swerve to allow another to avoid an obstacle. However, this requires solving the challenging problem of deciding what joint trajectories an ad-hoc group of cooperating vehicles should follow, with no obvious leader known in advance. To avoid the complexities of agreeing what plan to follow in an ever-evolving situation, a protocol requiring no leader and no explicit inter-vehicle agreement is desirable, which nevertheless yields cooperative, robust behaviour. One method is demonstrated here, in simulation. This uses the notion of "desired" versus "planned" trajectories, allowing vehicles to influence each other for mutual benefit, without requiring a leader or explicit agreement protocol. Essentially the desired trajectory is that which the vehicle would choose if other cooperating vehicles were not present, avoiding the predicted paths of non-cooperating actors. The planned trajectory additionally accounts for the planned trajectories of other cooperating vehicles, giving the safest currently available path. Both trajectories are broadcast. As each vehicle attempts to (weakly) avoid the desired trajectories of other vehicles, cooperative behaviour emerges. A simple form of model predictive control is used. The cost function penalises predicted collisions, accounting for severity. There is a weak preference for maintaining the current road lane, but no explicit reference trajectory. This decentralised planning and simple optimisation scheme results in effective handling of a wide range of collision scenarios, with no hard limit to the number of cooperating vehicles. The computing cost is linear in the number of vehicles.