A Survey On Weather Condition Effects on Free Flow Speed and Capacity in Freeways (Case Study: Tehran-Qom Freeway)

Decrease in traffic speed will result in freeway’s passing capacity, and this means decrease in performance level of service quality. Factors resulting in traffic speed decrease are categorized as permanent or temporary. Raining parse is one of temporary factors decreasing free flow speed. In this research effect of weather conditions (rainy weather) are surveyed as a temporary cause of decrease in capacity of freeway and on freeway’s free flow speed in Tehran-Qom freeway. So it can calculate the rain fall decrement index in the geographic region of Iran, (previously not calculated). If the time of traffic considered in design of freeway in the time of rain, by using decrement index the design capacity can be optimized.
In this research. In the beginning rainy hour and non-rainy hour were identified and separated, and also hours with less than 0.01 millimeter of rainfall due to insignificant effects were ruled out and considered as non-rainy hour .Studies show that light rain has no significant effect on flow speed and capacity and in some points its effects are neglected. But in case of heavy rainfall due to decrease in driver's vision range and decrease in surface friction between vehicle's wheels and road surface, flow speed will be significantly less than light rain situation and the changes between free flow speed of in 2 different conditions , rainy hour and sunny hour would be calculated. And with use of multi regression analysis and validating data from statistics test, result would be evaluated. traffic data and weather data from the last 6 months of hijri shamsi year 1391 in Tehran - Qom freeway was gathered and results obtained also confirms that rainy weather will result in 3.5 to 8.9 % decrease in capacity and 4 to 10% decrease in free flow speed.