Safety Evaluation of Corner Clearance at Signalized Intersections

CHAPTER 9. SUMMARY AND CONCLUSIONS

The objective of this study was to undertake a rigorous cross-sectional evaluation of the safety effects, as measured by crash frequency, of mainline corner clearance at four-leg, signalized intersections. The study compared signalized intersections with various corner clearance using data from the State of California and the City of Charlotte, North Carolina, to examine the effects on specific crash types: total, fatal and injury, rear-end, sideswipe, right-angle, turning, and nighttime crashes. The study did not investigate the effects of corner clearance on the cross-street approaches, or intersections with three legs or more than four legs.

Table 19 and table 20 present the recommended CMFs for numbers of approach and receiving corners with limited clearance, respectively, at signalized intersections based on the aggregate analysis results. The disaggregate analyses by traffic volumes, number of lanes, posted speeds, driveway density, and the presence of exclusive right- and left-turn lanes indicated no differential effects of corner clearance on the specific crash types. However, the sample size may have been too small to detect differential effects at the desired level of confidence.

Table 19. Recommended CMFs for limited approach corner clearance.

Table 20. Recommended CMFs for limited receiving corner clearance.

The introduction of access points in proximity to the intersection area increases the number of potential conflict points on the approaches. Logically, this is expected to increase crashes. 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. These increases were statistically significant at the 90-percent level or greater for total, fatal and injury, rear-end, sideswipe, right-angle, and nighttime crashes. Only the results for turning crashes 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. The next section discusses these results in more detail.

The economic analysis resulted in an average B/C ratio of at least 294 to 1 for most intersections when removing access at one mainline receiving corner with limited corner clearance. The analysis assumed another access to the property is available beyond 50 ft from the intersection or on the cross street. With the USDOT recommended sensitivity analysis, these values could range from 162 to 1 up to 405 to 1. Removing access at both mainline receiving corners provided a higher B/C ratio. This study based the economic analysis on total crashes only. Including other crash types would change the resulting estimate of the project benefit, and may have different effects for intersections with different crash type distributions than represented by the sample in this study. Further, the economic analysis did not include the effects of adding or removing driveways on the approach corners. While the results suggest that adding driveways on the approach corners may reduce specific crash types, these results require further study. As such, the economic analysis assumed there are no disbenefits—with respect to total crashes—to leaving existing driveways in place on the approach corners.

These results suggest that removing or relocating driveways on mainline receiving corners can be highly cost effective in reducing total crashes at four-leg, signalized intersections.