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.
Conclusions
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.
Recommendations
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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