This paper addresses the implementation, impact and future changes to the State Commute Trip Reduction (CTR) law in King County Washington, USA in the context of trip reduction ordinances in the United States, their future and their effectiveness, The paper indicates that the collaborative approach of Washington State＇s CTR law is a key factor in its success, basing success on the criteria of goal achievement and positive employer response, The legislation has met its short-term goal of gaining full participation from affected employers and is making progress toward achieving its long-term goals of reducing traffic congestion, air pollution and energy consumption, The paper concludes that mandatory trip reduction programs, particularly those stressing collaboration between the public and private sectors, have a greater impact than voluntary programs and that trip reduction ordinances are similar in cost-effectiveness to strategies involving alternative fuels.