To investigate the significant factors affecting the usage of high-emitting cars and motorcycles in Taiwan, this paper conducts a nationwide post-mailed questionnaire survey on the owners of cars and motorcycles; each vehicle type contains 45 thousand samples drawn from Taiwan's Vehicle Registration Database. A total of 3,454 car samples and 2,856 motorcycle samples are returned. We further match these returned samples with Taiwan's Vehicle Inspection and Management Database according to the license plate number. Some 748 car samples and 1,322 motorcycle samples have been successfully matched, which are further used to build up structural equation models for testing the cause-effect hypotheses between the latent constructs (sets of factors) and the usage of high-emitting vehicles. Our results show that the most influencing construct is the vehicle characteristics, which include old, small engine, manual transmission, high mileage, and 2-stroke engine (only for motorcycle) vehicles. The second most influencing construct is the driver/rider demographics, which includes aged, male, low-educated, high-income, and long-driving-experience people. Our results also show that emissions of cars are more serious in small cities than in large and medium cities; however, this relationship is not significantly existent in the motorcycle model.