Safety Evaluation of Continuous Green T Intersections

Donnell, Eric T.; Wood, Jonathan; Eccles, Kimberly; Donnell, Eric T. · 2016 · Unknown

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Summary

This study evaluates the safety performance of continuous green T (CGT) intersections compared to conventional signalized T intersections. Motivated by the Federal Highway Administration’s Evaluation of Low-Cost Safety Improvements Pooled Fund Study, the research addresses a gap in existing literature, which had relied on limited samples and simple statistical comparisons that failed to account for confounding factors. While previous studies indicated operational benefits for CGTs, such as reduced delay and emissions, their safety effects remained unclear. The primary objective was to determine if CGTs reduce crash frequencies using a rigorous propensity scores-potential outcomes framework, which mimics randomized experiments to control for selection bias and covariate differences. The methodology utilized data from 30 CGT (treatment) and 38 conventional (comparison) intersections in Florida, as well as 16 treatment and 21 comparison sites in South Carolina. The study employed a binary logistic regression model to estimate propensity scores based on safety-influencing features, including average annual daily traffic, posted speed limits, cross-sectional widths, and channelization types. These scores were used to match treatment and comparison sites using nearest-neighbor, Mahalanobis, and genetic matching algorithms. Potential outcomes were then estimated using mixed effects negative binomial or Poisson count regression models. The dependent variables included total crashes, fatal and injury crashes, and target crashes (rear-end, angle, and sideswipe) occurring within 250 feet of the intersection. The results indicated that CGT intersections had lower expected crash frequencies than conventional signalized T intersections, though the differences were not statistically significant. Crash modification factors (CMFs) were estimated at 0.958 for total crashes, 0.846 for fatal and injury crashes, and 0.920 for target crashes. The lack of statistical significance was attributed to the small sample size of treatment sites. Cross-sectional regression models using all available data yielded similar, non-significant results. Despite the inconclusive safety findings, a benefit-cost analysis confirmed that CGTs are a cost-effective alternative to traditional signalized intersections. The study concludes that while CGTs do not demonstrate a statistically significant safety advantage over conventional designs in this sample, they remain a viable alternative when combined with their established operational and environmental benefits. However, the authors note anecdotal feedback from Florida and South Carolina indicating that pedestrians and bicyclists may find it challenging to cross the continuous flow lanes, particularly when traffic volumes limit gap availability. The research provides practitioners with evidence-based crash modification factors and demonstrates the utility of the propensity scores-potential outcomes framework for evaluating safety strategies in non-randomized observational studies.

Key finding

Continuous green T intersections demonstrated lower expected crash frequencies for total, fatal and injury, and target crashes compared to conventional signalized T intersections, but the safety improvements were not statistically significant.

Methodology

naturalistic

Sample size: 105

Provenance

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Topics

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• intersection design