Preferential high occupancy vehicle facilities such as lanes restricted for transit and carpools are playing an increasingly important role in urban transportation systems. The preferential treatment poses, however, new operational problems for state and local transportation departments and enforcement officials. The minimum occupancy requirement for carpools must be set and effective enforcement of the facility must be maintained. A demonstration project in Seattle, Washington, tested the use of a public telephone hotline to reduce transit and carpool lane violations and also introduced the use of a variable carpool definition in order to maximize transit and carpool lane effectiveness. The variable carpool definition was tested by lowering the occupancy requirements from three to two persons per vehicle at selected locations in an Interstate corridor. Project data showed a 33 percent reduction in transit and carpool violation rates attributable to the public hotline. The change in carpool definition occurred smoothly. Vehicle volumes increased at locations where the occupancy requirement was lowered. Violation rates did not increase at locations where the requirement remained at three or more persons per vehicle. The key to the success of both elements of the project was an extensive, well-orchestrated public information campaign.