2.2 Traffic and Street Characteristics
A comparison of traffic characteristics between the rush
hour and the non-rush hour was performed at different
monitoring stations (Table 1). The observed travel speeds at
Yaowarat and Din Daeng stations were found to be slower than at
other locations and corresponded to higher air pollutant
concentrations relative to traffic. Where traffic flows are slow,
more pollutants will be emitted by vehicles, especially dieselengined
buses, many of which are old and inefficient. In addition,
the finding also revealed that air pollution from traffic emissions
depends on the physical conditions in narrow streets enclosed by
tall buildings which govern the ability of atmosphere to disperse
the pollutants. The street geometry of Yaowarat station is flanked
by tall buildings and shows characteristics of a canyon street with
frequent traffic jams. Similarly, traffic at Din Daeng and
Ratchaprarop sites moves more slowly with the approach of
major junctions and this appears to increase the air pollutant
level. In contrast, a lower air pollutant concentration was
observed at Phahonyothin station, having lower traffic volume
and more stable traffic flow. Thus, marked differences for street
level pollutants in Bangkok have been found between canyon
streets with slow-moving traffic and more open radial streets with
fast-moving traffic. An obvious way to reduce the build-up of
pollutant concentration on Bangkok streets would be to speed up
the flow of vehicles and prevent long periods of idling in
congested traffic.