Using Czech TCT to Access Safety Impact of Deceleration Lane at Thai U-Turns

Abstract

The purpose of this study is to evaluate the safety impact of the deceleration lane at the Upstream Zone of at-grade U-turns on 4-lane divided highways of Thailand. A substantial speed reduction is required by the vehicles for diverging and making U-turn, and the deceleration lanes are provided for this purpose. These are also provides a storage space for the U-turning vehicle to avoid unnecessary blockage of the through lanes and reduce the potential of rear-end collisions. The safety at the U-turn is greatly influenced by the proper or improper uses of the deceleration lanes. Depending on their length, not only either the full or partial speed adjustment can occur within the deceleration lane also it influence the road users' behavior. To assess the safety impact, the four groups of U-turns with the varying length of deceleration lanes were identified. Due to the limitations of availability and reliability of road crash data in Thailand a surrogate approach, based on traffic conflict was adopted for the study. Although the Traffic Conflict Technique (TCT) is widely accepted as an alternative and proactive approach but subjective nature of its parameters is debatable since its origin. The U-turns' geometric data, traffic conflicts and volume data were recorded in the field. The Severity Conflict Rate (SCR) is assessed by applying weighing factor (based on severity grades according to the Czech TCT) to the observed conflicts exposed to the conflicting traffic volumes. A comparative higher value of SCR represents a lower level of traffic safety at a U-turn facility and a significant relationship was obtained between the length of deceleration lanes of the U-turns and the level of road traffic safety.