The number of motorcycles in Bangkok has grown tremendously during the past two decades. They are widely used throughout the city and have become an important means of travel for low-income people. However, their use has caused several problems such as air and noise pollution and an increase in the number of accident rates and in various respiratory problems. This study investigates why the use of motorcycles as an urban transport option in Bangkok has grown in popularity as opposed to bus use. Despite the negative impacts, motorcycle use continues to increase, while bus ridership decreases dramatically. The investigation analyzes survey data of bus and motorcycle users in Bangkok using one-way ANOVA and binomial logistic regression to model modal choice. Characteristics of bus and motorcycle trips such as travel time and cost, trip frequency, origin-destination data and demographic characteristics of bus and motorcycle users are compared. The results indicate various factors contributing to the popularity of motorcycle use. These include shorter travel time, greater accessibility and more flexibility in accommodating higher trip frequency. In addition, the regression model shows that the most important factors differentiating modal choice are gender and trip frequency. Policies aimed at improving public transport services and minimizing the impacts of motorcycle use on the environment in the city and the health and safety of the Bangkok population are suggested. These include policies to create transit use incentives such as improving bus performance through the designation of the Right of Ways (ROW), and promoting greater accessibility to the bus system by increasing walkability to bus stops and integrating feeder services with bus routes. In addition, policies to create disincentives for motorcycle use by applying Travel Demand Management (TDM) strategies are also considered, together with enforcement of safety and emissions regulations on motorcycle use.