Trip reduction policies are increasingly utilized in U. S metropolitan areas to address congestion and air quality problems. These policies typically focus on the journey to work and are aimed at reducing the amount of drive-alone commuting by providing transit and ridesharing incentrives. Severe air quality problems in Southern California have prompted the air polluation control agency for theLos Angeles metropolitan area to enact Regulation XV. The regulation requres employers to develop and implement a trip reduction program to achieve specified ridesharing goals. It si the most ambitious and far reaching such program implemented to date, and offers a unique opportunity to determine whether such programs can significntly affect travel behavior . This paper presents results from the first year of Regulation XV's implementation.