Modeling Capacity and Delay at Signalized Intersections with Channelized Right-turn Lanes Considering the Impact of Blockage
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Right-turn channelization is used to improve the capacity at busy intersections with a lot of right-turns. However, under heavy traffic conditions the through lane vehicles might backup and block the right-turn lane. This will affect the discharge rate of right-turning vehicles and reduce the approach capacity and, consequently, increase the approach delay. So if the right-turn channelization is blocked frequently, its advantage is neglected and serious capacity problems can be overlooked. This issue is not addressed in the Highway Capacity Manual (HCM) and no separate model is provided to estimate the capacity and delay of approaches with channelized right-turn lanes. Using conventional methods for estimating the capacity and delay without considering the effect of potential blockage results in overestimation of the approach capacity and underestimation of the approach delay. This research presents probabilistic capacity and delay models for signalized intersections with channelized right-turn lanes considering the possibility of the right-turning vehicles being blocked from accessing the lane.The capacity model was developed by considering the capacity under blockage and non-blockage conditions with respect to the probability of blockage. Subsequently, a model was developed to estimate the probability of blockage. The capacity model is significantly affected by the length of the short-lane section and proportion of right-turn traffic. The proposed capacity model under blockage conditions and also the blockage probability model were validated through VISSIM, a microscopic simulation model. The validation process showed that both models are reliable. For operational purposes, the recommended lengths of the short-lane section were developed which would be useful in evaluating adequacy of the current lengths, identifying the options of extending the short-lane section length, or changing signal timing to reduce the likelihood of blockage. The recommended lengths were developed based on different signal timing plans and several proportions of right-turn traffic. The queue accumulation polygons (QAPs) were used to estimate the approach uniform delay and the HCM procedure was followed for the computation of the incremental delay caused by the random fluctuation of vehicle arrivals. To investigate the effect of blockage on the uniform delay, two different QAPs were developed associated with arrival scenarios under blockage and non-blockage conditions. The proposed delay model was also validated through VISSIM. It was found that, the proposed model can provide accurate estimates of the delay by reflecting the delay increase due to the right-turn channelization blockage. The results showed that the delay of an approach with a channelized right-turn is influenced by the length of the short-lane section and proportion of through and right-turn traffic.