Safety Evaluation of Corner Clearance at Signalized Intersections

EXECUTIVE SUMMARY

The Federal Highway Administration established the Development of Crash Modification Factors (DCMF) program in 2012 to address highway safety research needs for evaluating new and innovative safety improvement strategies by developing reliable quantitative estimates of their effectiveness in reducing crashes. The ultimate goal of the DCMF program is to save lives by identifying new strategies that effectively reduce crashes and to promote those strategies for nationwide implementation by providing measures of their safety effectiveness and benefit–cost (B/C) ratios through research. State transportation departments and other transportation agencies need to have objective measures of safety effectiveness before investing in broad applications of safety countermeasures. Forty State transportation departments provide technical feedback on safety improvements to the DCMF program and implement new safety improvements to facilitate evaluations. These States are members of the Evaluation of Low-Cost Safety Improvements Pooled Fund Study, which functions under the DCMF program.

This study investigates the safety effects of corner clearance on the mainline at four-leg, signalized intersections. Previous studies have explored various access management techniques and the effects of access points on safety at a corridor level. However, little quantitative information is available for the safety effects of driveways located near the corners of a signalized intersection and the effects of access management strategies on intersection crashes.

The research team obtained crash, geometric, and traffic data for four-leg, signalized intersections with various corner clearances in California and Charlotte, North Carolina, then conducted a cross-sectional analysis to estimate the effects of corner clearance while controlling for other factors. The team used propensity score matching to select reference intersections with similar characteristics to those with limited corner clearances. The analysis controlled for changes in safety due to differences in traffic volume and other differences among intersections with various corner clearances. The base condition for the evaluation was a four-leg, signalized intersection without limited clearance on all mainline corners.

The estimated crash modification factors (CMFs) indicated that more limited clearance on receiving corners (i.e., driveway(s) on receiving approaches within 50 ft of the signalized intersection) was associated with increases for all crash types, based on the data included in this analysis. The estimated CMFs indicated that more limited clearance on receiving corners was associated with increases for all crash types, based on the data included in this analysis. The following CMFs for one and two receiving corners, respectively, were statistically significant at the 90-percent level for these crash types:

• Total crash—1.33 (standard error (SE) = 0.11) and 1.76 (SE=0.30).
• Fatal and injury—1.29 (SE = 0.11) and 1.68 (SE = 0.29).
• Rear-end—1.36 (SE = 0.14) and 1.86 (SE = 0.38).
• Sideswipe—1.31 (SE = 0.14) and 1.71 (SE=0.38).
• Right-angle—1.42 (SE = 0.20) and 2.02 (SE = 0.56).
• Nighttime—1.29 (SE = 0.13) and 1.67 (SE = 0.35).

The CMFs for turning crashes were 1.22 (SE = 0.15) and 1.49 (SE = 0.36) for one and two receiving corners, respectively. These were the only results that were not statistically significant at the 90-percent level.

For limited corner clearance on the approach corners, the results indicated statistically significant reductions in total, fatal and injury, and rear-end crashes. The results also indicated reductions in sideswipe and nighttime crashes and increases in right-angle and turning crashes, but none of these results were statistically significant at the 90-percent level. In other words, each additional mainline approach corner with at least one driveway within 50 ft of the corner was statistically associated with decreases in these crash types. Although nonintuitive, this may be the result of localized congestion on the approach corners of an intersection. The total CMFs for one and two approach corners were 0.82 and 0.67 (SE = 0.08 and 0.13), respectively. Similarly, the CMFs for fatal and injury were 0.79 and 0.62 (SE = 0.08 and 0.13). The CMFs for rear-end crashes were 0.79 and 0.63 (SE = 0.09 and 0.15). The estimated CMFs for sideswipe, right-angle, turning, and nighttime crashes indicated a mix of no changes, a slight increase, or a slight decrease in crashes associated with limited clearance on the approach corners, and none of these results were statistically significant.

The disaggregate analysis sought to identify those conditions under which the strategy is most effective. Several variables were considered in the disaggregate analysis, including major and minor road traffic volume, number of lanes on the major and minor road, posted speed limit, driveway density, and presence of left- and right-turn lanes. The disaggregate analysis did not indicate any differential effect of corner clearance at the 80-percent confidence level.

The economic analysis, based on total crashes and assuming a 10-year service life, resulted in an average B/C ratio of at least 294 to 1 for most intersections when removing or relocating access at one or more mainline receiving corners with limited corner clearance. With the United States Department of Transportation–recommended sensitivity analysis, these values could range from 162 to 1 up to 405 to 1. While this research suggests the presence of driveways on mainline approach corners does not increase total, fatal and injury, rear-end, and sideswipe crashes, more research is required before agencies may consider this as a strategy for reducing crashes.

These results suggest that removing or relocating driveways on the mainline receiving corners can be cost effective in reducing crashes at signalized intersections.