Sensitivity Analysis of the Effect of Rainfall on Road Traffic Speed in Bangkok, Thailand

Rainfall affects urban traffic flow. In rapidly urbanizing megacities in Asian countries, heavy rainfall causes roads to flood and traffic congestion to worsen due to weak drainage systems. This study statistically quantified the impact of rainfall on urban traffic speed in Bangkok, using probe vehicle data and rainfall data from 2018 to 2020. Traffic speeds are calculated based on the travel distance and travel time between districts, taking into account the detouring of flooded sections.

Results show that both the rainfall intensity at the time of driving as well as the amount of previous rainfall affect the traffic speed reduction. In particular, the impact of previous rainfall increases at times and areas where traffic is concentrated, such as during the weekday morning and evening peak hours and travel to/from the city center. The results of the analysis based on regional characteristics show that low-lying districts are more affected by the previous rainfall because the flood water tends to stay on the road surface, while districts with high vegetation index (NDVI) are less affected by the previous rainfall. In addition, the impact of previous rainfall increases with population density and the ratio of narrow streets. In Bangkok, urbanization has progressed while leaving behind a city block configuration with many narrow streets, called Soi, connecting to arterial roads. This result means that limited road space is prone to flooding, and once flooding occurs, combined with the concentration of traffic on adjacent roads, traffic congestion becomes more severe.

The results of this study showed the impact of rainfall on urban traffic in different areas and at different times of the day in the target site. Integrated improvements to the transport and drainage systems could have a greater benefit.