Observational study on correlations between speeding and road traffic accident along major highways in Thailand

The main objectives of this study were to conduct observational surveys designed to explore correlations between speed and accident occurred on expressways and motorways in Thailand and develop adequate Crash Modification Factors which can used to forecast expected number of crashes at different speeds after implementing a given countermeasure at a specific site.

The main cause of road traffic accidents is “speeding”, which either implies as driving at a speed exceeding an adequate speed or at a speed limit allowed by law. Thailand’s speed limits for personal vehicles and motorcycles have been set at 80 km/hour in Pattaya city, Bangkok Metropolitan and on rural roads, at 90 km/hour in other areas, and 120 km/hour on motorways such as motorways number 7 and 9. Nevertheless, despite having such speed limit law in place, both the number and severity of speeding related road traffic accidents have not yet been subsided.

Objectives

• To conduct an appropriate theoretical analysis framework, as well as conclusive findings on impacts of speeding on road traffic accidents, classified by severity levels, such as fatality and injury.
• To explore correlations between speeding and road accident on toll ways and motorways
• Develop adequate Crash Modification Factors (CMF) for Thailand, which can used to forecast expected number of crashes at different speeds after implementing a given countermeasure at a specific site.

Term of Reference (TOR)

Collect traffic and accident data between 1 February – 31 August 2014 on 2 motorways and 1 expressway including Bang Phili- Bang Khun Tien expressway, and motorway numbers 7 and 9 in order to be used in the analysis of correlations between speeding and road traffic accident as well as in the development of CMF.

Findings

The difference between the 15^th and 85^th percentile operating speeds had direct impact on road accident possibility, in which the more the percentile operating speed difference, the higher the probability of road accidents. Therefore, an adequate bottom and top speed percentile design and determination that can lower the speed difference between the 2 percentiles can significantly reduce road traffic injuries and fatalities to a great extent. According to this study, it is suggested that the difference between the top and bottom speed percentiles in Thailand should not exceed 35 km/hour. In addition, it was also found that by lowering the 85^th speed percentile by at least 5 km/hour would meaningfully reduce road traffic accidents on expressways and motorways as the average CMF was less than 0.5.