Analysis of Geometric Design Impacts on Vehicle Operating Speed on Two-Lane Rural Roads

The rural road level of service quality has highly related to the vehicles operating speed. The vehicle operating speed has been generally influenced by geometric design characteristics. Previous studies have constructed various analytical models to measure the effect of miscellaneous geometric design variables on vehicle operating speed. This study investigates the main effect of road slopes and its interaction with other variables in Two-Lane rural roads. The vehicles operating speed in 42 sections of Two-Lane rural roads in north of Iran, a recreation area, has been recorded by video. The obtained films are analysed by image processing and operating speeds are measured by an appropriate software with acceptable precision. The distribution of vehicle speed along tangent sections was analysed and it was found speed had normal distribution. So it had two parameters: mean and variance. The relation between these two parameters and geometric characteristics of tangent sections was studied. The results show that the interaction between vehicle classification (passenger car or heavy vehicle) with slope has been significant parameter. Moreover, the initial speed of vehicle at start of tangent section is a significant parameter. This result demonstrates the effect of previous geometric section on vehicle speed. The proposed models are validated by the standard error of estimate and observation versus estimation graph. The models will help road designer to estimate the average operating speed of vehicles along tangent sections and useful for policy makers to enact policies to control vehicles operating speed. The estimated speed can be helpful for analysis the safety of road concepts such as design consistency.