Right-\0xADturn traffic volume adjustment in traffic signal warrant analysis : final report.
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Published Date:2016-05-06
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Language:English
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Edition:Final report
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Abstract: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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