Our research design required two non-smokers to take exposure measurements. We assigned commuters to start their trip at the same time according to the predetermined schedules. Each person used a different mode of travel on one of four specified routes in Taipei. For a given route and sampling day, the two trip-makers took simultaneous mea-surements so that exposure comparisons between travel modes could be made by maximizing the variation in the explanatory variable of travel mode and controlling the variation of daily traffic volume, which fluctuated over the hours of day and displayed double peaks in the morning and afternoon rush hours (07:00–09:00 and 17:00–19:00) (TEO, 2008).