Proposed design parameters for raised transverse rumble strip profiles to abate discomfort on highways in Uganda
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Transverse rumble strips are used in highway design as speed calming measures in built-up areas, where a driver is expected to reduce travel speeds to a new advisory speed. There are diverse designs in terms of shapes, sizes, configurations and spacings on the highways each providing different outcomes in terms of lateral stability of the vehicle and discomfort. Significant lateral deviation at high speeds into the opposite lane could result in safety risks with consequences of a crash, with opposite vehicles. This study investigated the effect of rumble strip configurations (number of rumble strips per group), shape and spacing between strips on speed and lateral deviation reduction. The study further investigated the relationship between approach speed, lateral deviation and rumble strip configuration. The data used in the study was collected from 15 sections with configuration of 2, 3 and 4 rumble strips per group on two-lane major highways in Uganda. Speed and lateral deviation of each vehicle was collected using video recording for 2 – 3 hours per section. The lateral deviation of the vehicle with reference to its original path was determined by observing the vehicle front plate on hitting the first rumble strip and leaving the last rumble strip. Speeds and deviations of individual vehicles were extracted from videos using Kinovea 0.8.25 software after calibration of the image to a reference image. The approach and departure speed of each vehicle was determined at 5 metres before and after the rumble strips and was estimated based on the time taken for a vehicle to move over the 5 metre known distance. The results show that generally for all configurations, the effect of speed is minimal (difference between approach and departure speed of 0.00 to 16.67 km/h) although statistically significant at 95% confidence level. The 3-strip configuration is the most effective in speed reduction (mean = 2.20 km/h) compared to 2 (mean = 2.15 km/h) and 4-strip (mean = 1.95 km/h) groupings albeit to a moderate level. However in controlling lateral deviation, 2 strips (mean = 4.85, SD = 5.44) are the most effective, followed by 3 strips (mean = 9.26, SD = 7.76) and 4 strips (mean = 11.44, SD = 11.27). Rumble strips of width = 350 mm, gaps = 250 mm, height = 30 mm are the most effective in controlling approach speeds (mean speed reduction = 2.83 km/h). Vehicle deviation by vehicle type shows that buses are recommended for 4No rumble strips with dimensions of width = 350 mm, gaps = 250 mm, height = 30 mm. The deviation basically increases with approach speed with optimum values ranging from 4.12 to 13.59 cm and 28.59 to 78.83 km/h for deviation and approach speed respectively. This study therefore recommends that the rumble strip dimensions (for vehicles other than buses) whose width, height and spacing should be 400 mm, 620 mm and 20 mm respectively, with a value of 3.35m being adequate lane width for less potential for adverse effects.