During the 1980s and 1990s, California witnessed the widespread adoption and rejection of a policy known as mandatory employer-based trip reduction (EBTR). Mandatory EBTR was implemented largely through city and county ordinances and air district rules. EBTR rules and ordinances required employers to implement programs to reduce the number of employees driving vehicles to their worksite. The programs were adopted to reduce traffic congestion, pollutant emissions, or both. However, opposition to mandatory programs from the business community led to their prohibition in California in 1995. The purpose of this paper is to examine the history of mandatory EBTR and to help answer the broad question, What happened? The research found that key factors in the demise of mandatory EBTR included issues of problem definition, goal setting, adoption and implementation process, regulatory approaches, costs, analysis of results, and context.