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This report is an archived publication and may contain dated technical, contact, and link information
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Publication Number:  FHWA-HRT-16-036    Date:  April 2016
Publication Number: FHWA-HRT-16-036
Date: April 2016

 

Safety Evaluation of Continuous Green T Intersections

CHAPTER 2. LITERATURE REVIEW

SAFETY

CGT intersections have been used for several decades in Florida.(5) It has been reported that Florida citizens do not feel that CGT intersections are safe, especially for unfamiliar drivers.(5) For this reason, Sando et al. completed a safety evaluation of the CGT intersection using only data from Florida.(5) The authors used a paired t-test and an ordered probit model to analyze crash type proportions and severity, respectively. The analysis compared crashes that were reported on the continuous through lane on the major CGT roadway with crashes that were reported on the major road turning lane (i.e., the lane that must stop at the signal). Data consisted of crashes at nine Florida intersections from the years 2003 through 2008. There were a total of 398 crashes in the study sample.

The paired t-tests compared the proportions of total crashes in each direction that were lane-changing (sideswipe), rear-end, and angle. No differences were found in the proportion of rear-end and angle crashes when comparing the two travel lanes (i.e., continuous flow versus signal-controlled travel lane). The continuous flow lanes had a statistically significant higher proportion of sideswipe crashes than the lanes that had to stop. This was likely due to turning vehicles merging onto the continuous flow lanes. This analysis did not account for the total crash frequency or any potential confounding factors. Because the analysis results were based on a simple comparison of crash proportions between lane groups, rather than rigorous statistical methods, the results of the study have limited practical value.

The ordered probit was used to analyze crash severity outcomes. The severity levels considered in the model included no injury, non-incapacitating injury, incapacitating injury, and fatal. Two ordered probit models were estimated—one model that controlled for crash type and one that controlled for geometric elements, lighting, weather, time of day, speed limit, and driver age. The findings indicated that the continuous flow lanes on the CGT major road had lower severity outcomes when controlling for geometrics, lighting, weather, time of day, speed limit, and driver age when compared with the turning lane on the major road. The opposite finding occurred when only crash type was considered in the model. Neither of these findings, however, were statistically significant.

In a second study using Florida CGT intersection information to evaluate safety, Jarem compiled crash data from five intersections to compare crash rates at each CGT intersection with a critical crash rate.(6) The method to determine the critical crash rate was not provided by the author; however, this method often involves determining the average crash rate for similar roadway types plus an adjustment for the desired level of statistical confidence. The study included reported crashes at the CGT intersections from 2000 through 2003, which included a total of 117 crashes (10 of which were rear-end collisions caused by drivers inadvertently stopping in the continuous flow lanes). The crash rate analysis assumes a linear relationship between crash frequency and traffic volumes, which is rarely found in traffic-safety relationships.(7) The findings from the analysis suggested that the reported crash rates for each of the CGT intersections were lower than the critical crash rates, likely indicating that the CGT intersections did not produce crash rates that exceeded average rates at similar intersections without the continuous green movement. A diagnostic review of the reported crashes at CGT intersections in Florida found that rear-end, sideswipe, and angle crashes were the most common types. The rear-end crashes were often caused by drivers who unexpectedly stopped in the continuous flow lane. Sideswipe and angle crashes occurred when drivers turning left from the minor leg of the intersection were turning or merging with the through traffic on the major road.

TRAFFIC OPERATIONS

Jarem also completed an analysis of the operational effectiveness of CGT intersections.(6) The analysis considered the five CGT intersections used in the safety analysis and subsequently performed traffic simulations using traffic analysis software to estimate the total delay savings (per vehicle) and total fuel savings achieved by the continuous flow lanes (compared with a standard signalized T intersection). The findings indicated that the CGT intersections resulted in savings of 3.7 to 28.4 s of delay per vehicle (1,601 and 4,786 vehicles/h, respectively) and 0.005 to 0.015 gal of fuel saved per vehicle (5,275 and 1,622 vehicles/h, respectively). The traffic volumes for the movements other than for the through lanes were not provided.

Litsas and Rakha provided a more comprehensive operational analysis of CGT intersections.(8) Simulation software was used to run 2,445 unique intersection condition combinations to compare CGT with traditional signalized T intersections.(8) The analysis estimated the reduction in vehicle delay, fuel usage, hydrocarbon emissions, carbon monoxide emissions, nitrogen oxide emissions, and carbon dioxide emissions. The simulation results indicated that CGT intersections resulted in a 10.29 percent reduction in vehicle delay, 2.78 percent fuel savings, 12.47 percent fewer hydrocarbon emissions, 14.44 percent fewer carbon monoxide emissions, 4.38 percent fewer nitrogen oxide emissions, and 2.29 percent fewer carbon dioxide emissions than traditional signalized T intersections.

SUMMARY OF CGT INTERSECTIONS

Safety and operational evaluations of the CGT intersection are relatively limited in the literature. With regards to safety, crash type proportion analyses have indicated that continuous flow movements at CGT intersections do not differ from the through lanes in the opposing direction. There are preliminary findings to suggest that the proportion of sideswipe crashes on the continuous flow lanes on the major road were higher relative to the opposing through lanes, but there were not significant differences in other crash types. No statistically significant differences among severity outcomes have been reported when comparing the CGT continuous flow lanes to the lanes in the opposing direction. With regard to operations, published research indicates that the vehicle delay, emissions, and fuel consumption are lower at CGT intersections relative to traditional signalized T intersections.

 

 

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