Institute of Transportation, MOTC

Tai-Chieh Chen, 2008.06
Institute of Traffic and Transportation National Chiao Tung University

　　The blossom of economic growth in Taiwan associated with the continuous construction of highway infrastructures for convenient movements of people and freights has inevitably brought a rapid growth of private vehicles over the past decades. In 1990, for instance, there were only 2.3 million cars and 7.1 million motorcycles registered; in 2007, the numbers of registered cars and motorcycles have increased to 6.7 millions and 14.0 millions, respectively, which were almost three times and double of the 1990 figures. The trend toward greater use of private vehicles has not only created ubiquitous congestion on the urban roads and intercity highways, but also resulted in excessive emissions that have broken the ecological balance. In metropolitan areas, the poor air quality has been mainly resulted from the mobile emissions of these vehicles. Thus, the Environment Protection Authorities at the central and local levels have endeavored to vehicle inspection and maintenance programs (I/M) and other control initiatives aiming to effectively restrain the air pollution from these mobile polluting sources.

　　Many studies even attempted to identify the high-polluting vehicles based on the I/M database. However, only vehicle-related factors (i.e., direct factors) such as vehicle age, engine size, type of fuels, automatic/manual, brand, number of cylinders, catalytic converter, and gas mileage were recorded in the database. Other indirect factors such as drivers’ demographics, usage of vehicles, type of regions may also cause these vehicles high-polluting. To propose effective control strategies, a clear understanding of who, where and why these high-emitters are still in use is imperative. The objectives of this paper are twofold: (1) to identify the key direct factors significantly affecting the emissions of hydrocarbon (HC) and carbon monoxide (CO) of cars and motorcycles, according to the I/M database; (2) to further identify the key indirect factors making these vehicles so high-polluting.

 　　The direct factors significantly affecting the emissions of HC and CO can be identified by regressing HC and CO, respectively, on the characteristics of car and motorcycle. It is found that the key direct factors significantly affecting car emissions are age, engine size, number of cylinders, weight, mileage traveled, and brand. In contrast, the key direct factors for motorcycle emissions are age, engine size, mileage traveled, 2-stroke/4-stroke, and brand. The profiles of some high-polluting vehicles are clearly identified, accordingly. Furthermore, this study also conducted a nationwide post-mail questionnaire survey on some 90 thousand car and motorcycle owners, with 5981 valid samples returned. The I/M dataset and the returned questionnaire dataset are matched via the vehicle license-plate number so as to scrutinize who, where, and why the identified high-emitters are still in use. 　　Structural equations modeling (SEM) was then employed to test the hypothesized cause-effect relationships among five constructs: driver demographics, vehicle usage, regions, vehicle characteristics, and emissions. The results show that the constructs of driver demographics and regions have significant effects on the vehicle characteristics. The construct of vehicle characteristics has then significant influence on the construct of vehicle usage. Moreover, the direct factors causing the high emissions levels of HC and CO are the two constructs: vehicle usage and vehicle characteristics. In terms of total effect to the construct of emissions, the construct of vehicle characteristics has the highest total effect of -0.76 for both cars and motorcycles, followed by the construct of driver demographics with total effect of 0.24 for cars and 0.08 for motorcycles. Based on the direct/indirect factors found as well as their relationships identified, some relevant emission control strategies are proposed and discussed.