Washington State requires large employers in its nine most populous counties to encourage their employees to reduce commuting vehicle trips and to monitor progress by surveying employees. The monitoring requirement yields roughly 250,000 surveys every 2 years. Analysis of the survey results for 1999 and previous years estimated that the program removed an average of 18,500 vehicles from the road during the morning peak in 1999, with 12,600 of these in the Seattle metropolitan area. Information from the surveys was used to construct an origin-destination table for these trips, and the table was then used in a four-step modeling process to estimate the corridors and links that realized the greatest effect of the trip-reduction program. The modeling compared a baseline, which assumed effects of the trip-reduction program, with a case in which trips removed were added back into the trip tables. Results indicate that the trip-reduction program has measurable effects on traffic volumes and delay, both areawide and in specific corridors. The analysis is unique in having these types of data on which to draw for a metropolitan area whose modeling system can use them.