Effect of Vertical Deflections in Vehicle Speed: A Case study from Khon Kaen University

Abstract

Vertical deflections are widely used traffic calming techniques. Due to a change in height, drivers must reduce their speed when crossing the device to maintain an acceptable level of comfort. As the measure is based on the experiences of developed nations, its effectiveness in mixed traffic environments is still questionable. Therefore, this study aims to 1) determine the effects of vertical deflections on vehicle speed on collector roads in Khon Kaen University and 2) identify the factors that contribute to speed reductions. This study observed driver behavior on a section of collector road that had vertical deflection installed, including 3 sections with speed humps and 6 sections with speed tables. The vertical deflections vary in width from 4 to 14 meters, in height from 5 to 14 centimeters, and in slope from 1:43 to 1:20. The drone was used to video free-flowing traffic characteristics. During post-recording analysis, speed profiles of 1,068 passenger cars and 840 motorcycles were plotted every 10 meters for 50 meters prior to and after the location of the device. In addition, the technique of Linear regression analysis was utilized to determine the contributing factors in speed reductions. The results revealed that the mean and 85th percentile crossing speeds were 25-37 km/h and 29-44 km/h, respectively. The speed reduction ranged from 18 to 46 percent for passenger cars and 12 to 34 percent for motorcycles. According to the speed profile data, vertical deflection can control the vehicle's speed 20 to 30 meters before and after the device. In addition, contributing factors to speed reduction involve device geometry, approaching speed, and vehicle types.