Right-­turn Traffic Volume Adjustment in Traffic Signal Warrant Analysis: Final Report
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Right-­turn Traffic Volume Adjustment in Traffic Signal Warrant Analysis: Final Report

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    Final report
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    This report was based on the research project, Right-Turn Traffic Volume Adjustment in Traffic Signal Warrants, sponsored by the Nevada Department of Transportation (NDOT) and SOLARIS. Right-turn traffic does not affect intersection performance in the same magnitude as through or left-turn traffic. Therefore, it is necessary to apply an adjustment to the right-turn volume when conducting signal warrant analysis. Without any reduction, an intersection with heavy right-turn volume might mislead the signal warrant analysis result, and could make a difference in whether a signal is deemed warranted or not. Firstly, a comprehensive literature review was conducted focusing on the state-of-the-practice handling of minor-street right-turn volumes while conducting signal warrant studies. Further, an agency survey through the Institute of Transportation Engineers (ITE) community discussion was performed to acquire valuable information from practicing engineers. It was found that the limited guidance in the Manual on Uniform Traffic Control Devices (MUTCD) does not provide a clear direction on determining whether or how much right turns impact the signal warrant analysis. In reality, most traffic engineers have done the reduction based on engineering judgments by incorporating key factors such as geometry and main street volume. Sometimes agencies develop and adopt internal procedures but do not necessarily publish them. Based on the lack of an adequate guideline, a new one is needed to estimate the reduction factor for right-turn traffic on the minor street when conducting a traffic signal warrant study. The proposed guideline is based on the delay equivalent relationship between right-turn and through traffic. The right-turn volume equals an equivalent number of through vehicles, which would produce the same control delay on the minor street. The equivalent factor is defined as the measurement of the reduction of right turns. Because equivalent factors are calculated based on delay, it incorporates major impact factors of the right-turn and through traffic inherently, such as flow rates, conflicting flow rates, capacity, critical headways, and follow-up headways. Especially, the volume ratio in the two directions of the main street is considered. The research found that uneven volume distribution has a greater impact on the right-turn movement on the minor street. Therefore, just considering the main street volume can cause over- or under-estimation of the impact of the main street traffic on the minor street. Further, regression models were developed for all the configurations with calibrated regression coefficients. The advantage of these models is that they could give an equivalent factor for a specific volume scenario. The proposed guidelines were tested at three intersections and the results indicated that they are convenient to use and easily help to determine right-turn volume equivalents. Lastly, pedestrian impact on right-turn traffic adjustment was discussed. Usually, pedestrians crossing the main street would block right-turn vehicles on the minor street, and on the other hand, the through vehicles on the minor street can use this gap to cross an intersection. A Monte Carlo model was built to simulate the real operation of two-way stop-control (TWSC) intersections, and further validated with field data that was collected at one intersection near the University of Nevada, Reno (UNR) campus. With this model, minor-street through capacity considering pedestrian crossings was estimated. Using this enlarged capacity allows more accurate calculation of equivalent factors by considering the counter impact of pedestrians on right turns. In summary, this research focused on the right-turn adjustment in the signal warrant. The decision on reducing the right turns on the minor street is somewhat subjective. Therefore, this study developed a practical guidance for determining the percentage of right turns to be considered in the signal warrant analysis. Based on the data analysis and case study results, statewide uniform guidelines were developed for implementation in the State of Nevada. However, the recommendations reached in this research could be applied in other states as well.
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