Safety impact of application of auxiliary lanes at downstream locations of Thai U-turns

- This study offers road safety assessment at downstream locations of U-turns on divided Thai highways.
- Four types of layout design of U-turn were identified on four-lane divided rural highways.
- The Traffic Conflict Technique is used because the available road crash data was unreliable.
- Number installed components is a major factor that influences the level of road safety.
- The application of a single component at downstream location of U-turns yields a highest level of road safety.

To assess the safety impact of auxiliary lanes at downstream locations of U-turns, the Traffic Conflict Technique was used. On the basis of the installed components at those locations, four types of U-turns were identified: those without any auxiliary lane, those with an acceleration lane, those with outer widening, and those with both an acceleration lane and outer widening.The available crash data is unreliable, therefore to assess the level of road safety, Conflict Indexes were formulated to put more emphasis on severe crashes than on slight ones by using two types of weighting coefficients. The first coefficient was based on the subjective assessment of the seriousness of the conflict situation and the second was based on the relative speed and angle between conflicting streams.A comparatively higher Conflict Index value represents a lower level of road safety. According to the results, a lower level of road safety occurs if two components apply or if a location is without any auxiliary lane. The highest level of road safety occurs if the layout includes only a single component, either an acceleration lane or outer widening.