Truck-only lanes and tollways are under study as tools to combat road congestion, enhance safety and reduce other external costs of road traffic. This paper investigates the potential benefits from separating cars and trucks onto different lanes or routes while treating road infrastructure as given. The benefits are found to depend on several factors: the relative volumes of cars and trucks, the congestion delay and safety hazards that each vehicle type imposes, values of travel time for each type, and lane capacity indivisibilities. The optimal assignment of vehicles to road capacity can be supported using tolls that are differentiated by vehicle type and lane. Lane access restrictions usually cannot support the optimum and may well provide no benefit at all. Creating a toll lane for one vehicle type is generally more effective. The benefits of all forms of intervention are sensitive to whether the proportions of cars and trucks are commensurate with lane capacities.
