TechBrief:
Safety Effectiveness of Intersection Left- and Right-Turn Lanes
A new report called Safety Effectiveness of Intersection Left- and
Right-Turn Lanes (RD-02-089) from the Federal Highway Administration presents the results of research on the safety effectiveness of providing left- and right-turn lanes for at-grade intersections.(1) The research was performed as part of a pooledfund study; highway agencies in the District of Columbia and the States of Iowa, Illinois, Louisiana, Minnesota, Montana, Nebraska,
New Jersey, North Carolina, Oregon, and Virginia all contributed
a portion of the funding for the research.
Literature Review and Project Priorities
The research began with a review of published literature related
to the safety effectiveness of intersection design and traffic
control elements. This review summarized current knowledge on
the safety effects of a broad range of intersection features. Safety
effectiveness estimates exist for many features. In many cases,
however, the validity of those estimates is in question, because
the estimates were based on studies that were poorly designed
and executed. Based on the literature review, representatives
of the participating States decided that the research
should focus on quantifying the safety effectiveness of left- and
right-turn lanes through a well-designed before-after evaluation.
Selection of Evaluation Sites
Researchers identified and selected three types of sites for
the study: improved or treatment sites, comparison sites, and
reference sites. The database assembled for the study included
580 intersections of these three types, as explained below.
The improved or treatment sites are intersections at which a left or
right-turn lane was added and for which data on intersection
geometrics, traffic volumes, and traffic accidents were available for
time periods before and after the improvement. These improvements were made at
existing unsignalized intersections,
existing signalized intersections,
and newly signalized
intersections where both a turn
lane and a signal were installed
at the same time at a previously
unsignalized intersection. The
study evaluated a total of 280
intersection improvement projects
of the following types:
- Added left-turn lanes.
- Added right-turn lanes.
- Added left- and right-turn
lanes at the same intersection.
- Extension of the length of an
existing left- or right-turn lane.
The study was limited to projects
at three- and four-leg
intersections. All of the evaluation
sites had either two-way
STOP control or traffic signal
control. All of the improvement
projects evaluated were constructed
during the years 1989
through 1998; the vast majority
of the sites were improved
during the period from 1994
through 1997.
The intersections were located
in eight of the States that participated
in the study: Illinois,
Iowa, Louisiana, Minnesota,
Nebraska, North Carolina,
Oregon, and Virginia. The
States supplied traffic volume
and accident data for the study
and permitted the research
team to visit the sites and document
intersection features.
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Right-turn lane was installed at this signalized intersection located at
SR 159 and Center Grove Rd/Goshen Rd in Edwardsville, Illinois |
For 260 of the 280 improved
intersections (93 percent), researchers selected a matching
comparison site that was not
improved during the study period.
The matching improved
and comparison sites were
always in the same State and
were located geographically
close to each other whenever
practical. The matching sites
were as similar to each other as
possible in intersection configuration,
traffic control, geometric
design, and traffic volume.
The other 20 improved
sites were sufficiently unique
that researchers could not find
matching comparison sites.
In addition, researchers selected
40 reference sites. Reference
sites were unimproved intersections
that were not
matched to any particular improved
site.
Data Collection
Researchers collected extensive
data for the improved,
comparison, and reference sites.
Field visits to nearly every study
intersection provided geometric
design and traffic control data.
Traffic volume data for both
the major- and minor-road legs
were obtained from counts or
estimates for as many years as
possible for each intersection
from highway agency records.
Intersections were only included
in the study if average daily
traffic volumes for the major
and minor roads were available
for at least one year during the
study period. Traffic volume
estimates for each individual
year of the study period were
obtained through a careful
process of interpolation and extrapolation.
Researchers evaluated traffic
accident records for each participating
highway agency for periods
before and after each of
the improvement projects. The
evaluation generally included all
accidents within 75 m (250 ft)
of each intersection that were
related to the presence of the
intersection, as designated by the
investigating officer or accident
coder. The database assembled
for the 580 study intersections included
a total of 26,056 intersection-
related accidents (123
fatal accidents, 10,203 nonfatal
injury accidents, and 15,730
property-damage-only accidents).
Study Periods
The accident database used in the study included 9 to 13
years of data for each intersection. Study periods before and after each improvement project
were defined with durations as long as possible, consistent
with the availability of data. The data for the year in which
the project was constructed were not included in either the
before or after study period. The before study periods for
the treatment sites ranged from 1 to 10 years, with a mean duration
of 6.7 years. The after study periods also ranged from
1 to 10 years, with a mean duration of 3.9 years.
Evaluation Approaches
Researchers used three alternative statistical approaches to
evaluate the effectiveness of the intersection improvement
projects. These alternative approaches were:
- Before-after evaluation with yoked comparisons.
- Before-after evaluation with a comparison group.
- Before-after evaluation with the Empirical Bayes approach.
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Left-turn lane was installed at this signalized intersection located at
US 18 and Pierce Avenue in Mason City, Iowa
|
These approaches were developed from those recommended
by Griffin and Flowers(2) and by
Hauer.(3) The yoked-comparison
(YC) approach is a traditional
approach to the evaluation of
traffic accident countermeasures
and involves one-to-one
matching between improved
and comparison sites. The safety
performance of the matched
comparison site is used to estimate
what change in safety
would have occurred at the improved
site had the improvement
not been made. The comparison-
group (CG) approach is
similar to the YC approach but
replaces the single comparison
site matched to each improved
site with a group of similar sites
whose collective safety performance
serves the same purpose.
The Empirical Bayes (EB)
approach replaces the comparison
group with a negative binomial
regression model used to
predict the change in safety
performance of the improved
site that would have been expected
had the improvement
not been made.
When an intersection has relatively
high accident experience
during a particular time period,
its annual accident frequency is
likely to decrease even if it is
not improved; this phenomenon
is known as regression to the
mean. Thus, when an improvement
project is constructed at
an intersection with relatively
high accident experience, the
natural decrease in accident
frequency due to regression to
the mean may be mistaken for
an effect of the project. Thus,
regression to the mean is a major
threat to the validity of before-
after evaluations.
The EB approach is the only
known technique to account for
the effect of regression to the
mean on evaluation results. The
YC and CG approach can account
for the effects of
changes in traffic volume levels
and for general time trends in
accident frequency, but not for
regression to the mean. The CG
approach is generally preferable
to the YC approach, because
the CG approach uses multiple
comparison sites for each improved
site and because, as implemented
in this evaluation, it
has a more sophisticated
method to account for traffic
volume changes than the YC
approach. Thus, the ranking of
the three evaluation approaches,
in descending order of their
theoretical accuracy, is EB, CG,
and YC. The evaluation results
confirmed this expected relative
accuracy of the three methods.
Evaluation Results
Tables 1 and 2 summarize the
safety effectiveness of installing
left-turn lanes on the major-road
approaches to rural and urban
intersections, respectively. Table
3 presents comparable effectiveness
estimates for rightturn
lanes that are applicable to
both rural and urban intersections.
The safety effectiveness of
adding turn lanes is presented in
the tables as the expected percentage
reduction in total intersection
accidents. All of the results
in tables 1 through 3 were
derived in the current study except
where noted; the full research
report includes estimates
of the precision of each of these
results.(1) Effectiveness measures
for situations not addressed in
the current study were based on
the findings of an expert panel
convened to assess published
literature in another recent FHWA
study.(4) Furthermore, all of
the results from the current study
shown in tables 1 through 3 are
based on the EB approach, with
one exception noted in table 2.
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Right-turn lane was installed at this unsignalized intersection located at US
97 and Moore Lane in Sherman County, Oregon |
The effectiveness of projects
involving the addition of both
left- and right-turn lanes on the
major road at the same intersection
can be determined by
combining the relevant effectiveness
measures from tables
1 through 3. For example, at
an urban four-leg signalized intersection,
the addition of two
major-road left-turn lanes would
be expected to reduce total
intersection accidents by 19
percent, and the addition of
two major-road right-turn lanes
would be expected to reduce
accidents by 8 percent. The
combined effectiveness would
be computed as 1-(1-0.19)
(1-0.08) = 0.25, or a 25-percent
reduction in total intersection
accidents.
No reliable effectiveness measures
were found for extending
the length of an existing left- or
right-turn lane.
Economic evaluations of the installation
of left-turn lanes at intersections
of various types
were conducted. The primary
measure of the cost effectiveness
of improvement projects is
the benefit-cost ratio, which is
determined as the present value
of future accident costs reduced,
divided by the estimated
cost of constructing the leftturn
lanes. Conservative estimates
of accident costs were
used:
· Fatal and injury accidents —
$103,000.
Table 1. Expected Percentage Reduction in Total Accidents from Installation of Left-Turn Lanes
on the Major-Road Approaches to Rural Intersections
Intersection type
|
Intersection traffic
control
|
One approach
Number of major-road approaches on which
left-turn lanes are installed |
One approach
|
Both approaches
|
Three-leg intersection
|
STOP signa
Traffic signal
|
44b
15c
|
|
Four-leg intersection
|
STOP signa
Traffic signal |
28b
18c
|
48b
33c
|
a
STOP signs on minor-road approach(es)
b
based on EB evaluation in Reference 1
c
based on Reference 4
Table 2. Expected Percentage Reduction in Total Accidents from Installation of Left-Turn Lanes on
the Major-Road Approaches to Urban Intersections
Intersection type
|
Intersection traffic
control |
One approach
Number of major-road approaches on which
left-turn lanes are installed |
One approach
|
Both approaches
|
Three-leg intersection
|
STOP signa
Traffic signal
|
33b
7d
|
|
Four-leg intersection
|
STOP signa
Traffic signal |
27c
10b
|
47b
19b
|
a
STOP signs on minor-road approach(es)
b
based on EB evaluation in Reference 1
c
based on CG evaluation in Reference 1
d
estimated from EB results in Reference 1 and from results in Reference 4
Table 3. Expected Percentage Accident Reduction in Total Accidents from Installation of Right-
Turn Lanes on the Major-Road Approaches to Rural and Urban Intersections
Intersection traffic
control
|
Number of major-road approaches on which right-turn lanes are installed |
One approach |
Both approaches
|
STOP signa
|
14b
|
26b
|
Traffic signal
|
4c
|
8c
|
a
STOP signs on minor-road approach(es)
b
based on EB evaluation for rural intersections in Reference 1
c
based on EB evaluation for urban intersections in Reference 1
·Property-damage only accidents
— $2,300.
The average cost of installing a
single left-turn lane is $85,000
based on estimates from four of
the States that participated in
the study.
For rural three-leg unsignalized
intersections, the results indicate
that left-turn installation would
become cost-effective for a major-
road ADT of 4,000 vehicles/
day with 10 percent of the
major-road volume on the minor
road and at 2,000 vehicles/day
with 50 percent of the major-road volume on the minor road. For
rural four-leg unsignalized intersections,
left-turn lane installation
would become cost-effective
for a major-road ADT of
3,000 vehicles/day with 10
percent of the major-road volume
on the minor road. With a minor-
road volume equal to 50
percent of the major-road volume,
left-turn installation
would be cost effective at all of
the major-road volume levels
down to the lowest level considered
of 1,000 vehicles/day.
For urban four-leg unsignalized
intersections, left-turn installation
would become cost-effective
for a major-road ADT of
2,000 vehicles/day with both
10 and 50 percent of the majorroad
volume on the minor road.
For urban four-leg signalized intersections,
left-turn installation
was found to be cost-effective
for all combinations of majorand
minor-road ADTs considered.
The lowest combination
of major-road ADT of 10,000 vehicles/
day and minor-road ADT of
2,500 vehicles/day has a benefit-
cost ratio of 1.5.
References
- Harwood, D. W., K. M. Bauer, I. B. Potts, D. J. Torbic, K. R. Richard, E. R. Kohlman Rabbani, E. Hauer,
and L. Elefteriadou. Safety Effectiveness of Intersection Left- and Right-Turn Lanes, Report No. FHWARD-
02-089, Federal Highway Administration, July 2002.
- Griffin, L. I., and R. J. Flower. "A Discussion of Six Procedures for Evaluating Highway Safety Projects,"
Federal Highway Administration, December 1997.
- Hauer, E. Observational Before-After Studies in Road Safety, Pergamon/Elsevier Science, Inc., Tarrytown,
New York, 1997.
- Harwood, D. W., F. M. Council, E. Hauer, W. E. Hughes, and A. Vogt. Prediction of the Expected Safety
Performance of Rural Two-Lane Highways, Report No. FHWA-RD-99-207, Federal Highway Administration,
December 2000.
Research—This research study was performed as part of a pooled fund study; highway agencies in the
District of Columbia and the States of Iowa, Illinois, Louisiana, Minnesota, Montana, Nebraska, New Jersey,
North Carolina, Oregon, and Virginia all contributed a portion of the funding for the research.
Distribution—This TechBrief is being disseminated according to standard distribution. It will also be displayed
on the TFHRC Web site, www.tfhrc.gov.
Availability—The report on this study is available on the TFHRC Web site in the Research Library. Copies will be available from the National Technical Information
Service, 5285 Port Royal Road, Springfield, VA 22161.
Key Words—left-turn lanes, right-turn lanes, intersection, safety.
Notice—This TechBrief is disseminated under the sponsorship of the Department of Transportation in the
interest of information exchange. The TechBrief provides a synopsis of the study's final publication. The
TechBrief does not establish policies or regulations, nor does it imply FHWA endorsement of the conclusions
or recommendations. The U.S. Government assumes no liability for the contents or their use.
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NOVEMBER 2002
FHWA-RD-02-103