The objective of this research is to evaluate the performance of speed control measure by the use of automatic speed enforcement cameras along Highway 118 (Chiang Rai - Chiang Mai). Five cameras were installed along the 36-kilometer road section between Doi Nangkaew and Doi Saked to reduce the risk of road traffic accidents. This study is divided into two-folds. Part one investigates the driving speed during 1-month before and 1-month after the installation of automatic speed enforcement cameras. Part two examines the psychological factors that most influence driving speed on mountain roads using the Theory of Planned Behavior. The statistical results showed that automatic speed cameras can significantly reduce the speed of vehicles. The average driving speed of passenger cars is reduced by 7.7%, while that of buses is reduced by 8.8%. Further, the analytical results revealed that the most influential psychological factor toward speeding behavior is the Perceived Behavioral Control, which indicates that drivers on this mountainous road overspeed whenever it is possible and they can perceive such risks. Therefore, the proposed speed control measure using automatic speed enforcement cameras is the proper measure to close the opportunity gap of the driving risks. This results in the safe driving intention to follow the speed control.