Safety Effectiveness of Intersection Left- and Right-Turn Lanes
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7. CONCLUSIONS AND RECOMMENDATIONS
This section presents the conclusions and recommendations of the research on
the effectiveness of left- and right-turn lane improvements for at-grade intersections.
The conclusions of the study are as follows:
Added left-turn lanes are effective in improving safety at signalized and unsignalized intersections in both rural and urban areas. Installation of a single left-turn lane on a major-road approach would be expected to reduce total intersection accidents at rural unsignalized
intersections by 28 percent for four-leg intersections and by 44 percent for three-leg intersections. At urban unsignalized intersections, installation of a left-turn lane on one
approach would be expected to reduce accidents by 27 percent for four-leg intersections and by 33 percent for three-leg intersections. At four-leg urban signalized intersections, installation of a
left-turn lane on one approach would be expected to reduce accidents by 10 percent. Installation of left-turn lanes on both major-road approaches to a four-leg intersection would be expected to increase, but not quite double, the resulting effectiveness measures for total intersection accidents; the increased effectiveness measure for adding left-turn lanes on both major-road approaches can be determined using Equation (50). The complete set of effectiveness measures for left-turn lane installation is presented in Tables 48 and 49.
Added right-turn lanes are effective in improving safety at signalized and unsignalized intersections in both rural and urban areas. Installation of a single right-turn lane on a major-road approach would be expected to reduce total intersection accidents at rural unsignalized
intersections by 14 percent and accidents at urban signalized intersections by 4 percent. Right-turn lane installation reduced accidents on individual approaches to four-leg intersections
by 27 percent at rural unsignalized intersections and by 18 percent at urban signalized intersections. Only limited results were found for right-turn lane installation at three-leg intersections. Installation of right-turn lanes on both major-road approaches to a four-leg intersections would be expected to increase, but not quite double, the resulting effectiveness measures for total intersection accidents; the increased effectiveness measure for adding
right-turn lanes on both major-road approaches can be determined using Equation (50). The complete
set of effectiveness measures for right-turn lane installation is presented in tables 50 and 51.
For both left- and right-turn lane improvements, the results obtained from this study are within the range of all previous studies reported in the literature, but are slightly higher than the best estimates from previous studies recently made by an expert panel.
Evaluation results for adding both left- and right-turn lanes at the same intersection are presented in table 52.
A small sample of projects involving extension of the length of existing turn lanes at rural unsignalized and urban signalized intersections was evaluated. However, no reliable effectiveness measures could be developed from this small sample.
In general, turn-lane improvements at rural intersections resulted in larger percentage reductions in accident frequency than comparable improvements at urban intersections.
In the various evaluations performed, the effectiveness of turn-lane improvements in reducing fatal and injury accidents was greater than for total accidents in some cases, and less than for total accidents in others. Overall, there is no indication that any type of turn-lane improvement is either more or less effective for different accident severity levels.
Tables 48 through 52 include estimates of the standard error of the mean improvement effectiveness. The standard error is a measure of the precision of the mean improvement effectiveness (i.e., smaller standard errors represent more precise estimates). The most precise effectiveness estimates were generally obtained for the project and intersection types with the largest sample sizes, particularly added left-turn lanes at rural four-leg unsignalized intersections and at urban four-leg signalized intersections.
The results of economic analyses for addition of left-turn lanes at typical rural and urban intersections, as a function of traffic volume, are presented in tables 58 through 65. These economic analyses are based on the effectiveness estimates derived in this study and illustrate the traffic volume levels at which installation of left-turn lanes becomes cost effective.
The EB approach to observational before-after evaluations of safety improvements appears to
perform effectively. Comparisons of the EB approach to the YC and CG approaches found that the EB approach was more likely to provide statistically significant effectiveness measures. Furthermore,
the effectiveness measures obtained from the EB approach were generally smaller than those from the other approaches; this may have resulted from reduced effect of the regression-to-the-mean
phenomenon; compensation for regression to the mean is highly desirable in providing accurate evaluation results.
The recommendations of the study are as follows:
The effectiveness measures for left-turn improvements in tables 48 and 49 and for right-turn improvements in tables 50 and 51 should be considered by highway agencies in evaluating potential improvements at intersections.
FHWA should consider incorporating these results in the AMFs used for safety prediction in the Interactive Highway Safety Design Model (IHSDM) and in other ongoing initiatives, such as the Comprehensive Highway Safety Improvement Model (CHSIM). Tables 55 through 57 present revised AMFs for use in these models.
The EB approach should be considered the most desirable approach for observational before-after evaluation of safety improvements. The EB approach is the only evaluation approach with the potential to compensate for regression to the mean. Where the EB approach cannot be applied, the CG and YC approaches should be considered as preferable to evaluation designs without
comparison sites. The CG approach should generally be considered as preferable to the YC approach,
because it incorporates a comparison group consisting of multiple sites. However, both the CG and YC approaches are likely to provide overly optimistic evaluation results.
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