This study attempts to investigate underlying factors that could affect traffic sign visibility, which can be measured by road user’s detection distance when driving at night. Such factors may be due to physical characteristics of traffic signs, types of vehicle, and characteristics of road users. Significant factors under prevailing road conditions in Thailand are identified. The findings would help developing a more appropriate guideline for installation of traffic signs so that such road furniture can guarantee the maximum safety of road users, particularly on the visibility at night. In terms of study design, several factors are investigated, i.e., types of vehicles, coefficient of retroreflectivity, driving speed, height of traffic signs, and characteristics of road users. Systematic experimental deign is applied to study these effects on legibility distance. Subjects were recruited from road users with different age groups. The controlled variables in the experiment consist of road condition, road geometry, traffic volume, and time of the day during the experiment. Findings indicate that the passenger car provided statistically longer legibility distances and detection distances than the motorcycle. Coefficient of Retroreflection and Sign height was significant for T-junction sign stop sign and speed limit sign. The researcher also recommends that Department of Rural Roads (DRR) installs and maintains traffic sign to be higher than minimum retroreflectivity levels or increase the advance placement of traffic signs.
