1. A system comprising a computer programmed to: receive data about a current and historical trajectory of each of the one or more second vehicles; select from a set of three or more pre-defined policies, a set of potential policies for each second vehicle, each of the potential policies in each respective set of potential policies including a respective possible second vehicle trajectory and one or more rules for the respective second vehicle trajectory; identify a set of change points for each second vehicle's historical trajectory, wherein each change point specifies a location and a time along the respective second vehicle historical trajectory at which it is determined that the respective second vehicle changed from a first policy from the set of potential policies for the respective second vehicle to a second policy from the set of potential policies for the respective second vehicle; determine, for each segment between a respective, adjacent pair of change points along the respective historical trajectory for each second vehicle, a respective policy from the set of potential policies for the respective second vehicle that fits to the historical trajectory of the respective second vehicle; identify, based at least in part on the policy from the set of potential policies for each segment between the respective adjacent pair of change points that fits to the historical trajectory of each respective second vehicle, a distribution of probabilities for each second vehicle at a current time; each probability of each respective distribution of probabilities associated with one of the potential policies from the set of potential policies for the respective second vehicle; select a policy for the first vehicle from the set of three or more pre-defined policies, based at least in part on the respective distributions of probabilities at the current time for the respective one or more second vehicles; and control the vehicle via an instruction provided by the computer to a controller based on the selected policy for the first vehicle.