Increasing traffic volume during the semester starts was regarded as a main factor for
traffic congestions in Bangkok and big cities in Thailand [17], particularly during rush
hour periods: 6:30-8:30 and 15:00-17:00 [18]. Low speed of vehicles during congestion
could contribute higher PM emitted to the atmosphere. Gillies et al. (2001) [4] found that
vehicle speed of 42.6 km/h can increase PM10 concentrations around 1.7 times higher than
at speed of 72.6 km/h. Dropping off/picking up the school children by their parents may
lead to the idled vehicles that consequently would be another reason of higher PM
emissions.