Effects of gradient on speed of vehicles on an Iranian single carriageway road

The effect of an upgrade and its length is very significant for traffic flow characteristics. Road traffic consists of vehicles of wide ranging physical dimensions, weight and dynamic characteristics such as engine power, acceleration rate, etc. Due to these variations, the effect of grade on vehicles may vary significantly among vehicle categories. Variation in the level of the interaction between vehicles on upgrades may result in different sets of traffic flow characteristics. Hence, it is necessary to study speed of vehicle on upgrades. This study evaluates the influence of gradient in vehicle speed in a single carriage way in Iran. Quantify speed reduction for main types of vehicles on various road grades, and establish relationships between speed and road grades are steps to achieve the aim. Road of the study is two lane road located in north of Iran between Semnan and Firouzkouh. It was found that the effect of grade on the vehicle performance speed may not be significant beyond a length of 600 to 800 meter length of upgrade. Operation on a 2-3-percent upgrade has only a slight effect on passenger car speeds. On steeper upgrades (4-6percent upgrade), speeds decrease progressively with increases in the grade. The effect of grades on truck speeds is much more pronounced than on speeds of passenger cars. Speeds of trucks decrease by 6 percent or more on upgrades as compared to their operation on the level.