Evaluation of j-turn intersection design performance in Missouri, final report.
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2013-12-01
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Abstract:Research shows that a high percentage of crashes that take place on high-speed rural expressways occur at intersections
with minor roads. One low-cost alternative design for improving the safety of at-grade intersections on such expressways is
the J-turn. In the last few years, the Missouri Department of Transportation has converted some two-way stop controlled
(TWSC) intersections into J-turns. This study evaluated the effectiveness of the J-turn intersection design in Missouri
utilizing field studies, a public survey, crash analysis, and traffic conflict analysis. The field studies collected detailed video
data at a J-turn site and a control site. The crash analysis included a statistically rigorous empirical Bayes before-after
safety evaluation of five J-turn sites in Missouri. The J-turn design resulted in a 34.8% reduction in crash frequency for all
crashes and a 53.7% reduction in crash frequency for all injury and fatal crashes. Both reductions were significant at the
95% confidence level. Annual disabling injury crashes and minor injury crashes decreased by 86% and 50%, respectively.
None of the five sites exhibited a fatal crash following J-turn implementation. This five-site analysis showed that annual
right angle crashes decreased from 6.3 to 1.3, a 80% reduction. One of the most severe crash types, the left turn, right angle
crash, was completely eliminated by the J-turn. One conflict measure, average time to collision, was found to be four times
higher at the J-turn site compared to the control TWSC site among minor road turning vehicles, indicating greater safety at
the J-turn site. The average wait time at the J-turn site was half the wait time at the control site, while the average travel
time at the J-turn site was approximately one minute greater than at the TWSC site. When the public was surveyed
regarding trip time perceptions resulting from the J-turn, the majority said there was no adverse effect. A high percentage
of minor road left turning and through movements at the J-turn site merged into the travel lanes within the first 400 feet of
the acceleration lane. Public opinion regarding the J-turn at US 63 and Deer Park Rd was mixed. Frequent concerns raised
by respondents included difficulty merging following the U-turn, improper use of acceleration and deceleration lanes,
insufficient U-turn radius to accommodate large vehicles, and driver confusion.
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