| Summary: | This paper summarizes the work conducted to estimating a U-turn adjustment factor at selected signalized intersections in Doha, Qatar. Unfortunately, the Highway Capacity Manual (HCM) 2010 does not consider the effect of U-turn traffic on the capacity of signalized intersection; therefore, a few researchers attempted to develop U-turn adjustment factors to be added to the HCM analysis of signalized intersections when heavy U-turn traffic exists. To achieve this, the average headway was measured for U-tuning and left-turning vehicles at three different intersections in Doha. To estimate the U-turn adjustment factor, 198 queues with 2,327 U-turn vehicles and 1,564 left-turn vehicles were used to develop a regression model with the headway as the dependent variable and the U-turn percentage as the independent variable. The resulting quadratic model shows an increase in the average headway as the percentage of U-Turn traffic increases, with an R2 of 0.591. This indicates that the presence of U-turning vehicles has a significant effect on increasing the average headway thus reducing the capacity of signalized intersection. The result shows the U-Turn adjustment factor ranges from 0.76 for 100% of U-turn traffic to 0.95 for 10% of U-turn traffic.
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