What are the internal and external conditions of a work site that influence the success of its commuter choice program, aside from the trip-reducing services and incentives themselves? This article shares the results of two studies that used a more rigorous case study method for discerning the relative influence of conditions within and outside of the control of a work site. It also discusses the useful application and elements of the case study method as well as the location for finding properly documented case studies that describe successful programs. Recommendations are developed for work sites to implement commuter choice programs for TDM marketing professionals to identify receptive work-site candidates and for TDM researchers to investigate further using the case study method. By understanding the relative importance of various conditions, TDM professionals, employers, and employee transportation coordinators are better equipped to exert influence on factors within their control to develop more effective programs.