TECHBRIEF

FHWA is initiating a new research 

focus area on intersection safety. More

than 50 percent of the combined fatal

and injury crashes occur at intersections, and AASHTO's Strategic Highway Safety Plan identifies “improving

the design and operation of highway intersections" as one of its 22 strategies

reducing highway deaths and injuries.

FHWA is identifying the most common

and severe problems and compiling information on the applications and design of innovative infrastructure configurations and treatments at both signalized and non-signalized intersections

and at interchanges in order to reveal

strategic operational and research opportunities.

Report No. FHWA-RD-02-103

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 leftor 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 

Research, Development, and

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.tfhrc.gov

Safety Effectiveness of 

Intersection Left- and 

Right-Turn Lanes
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.

For 260 of the 280 improved

intersections (93 percent), re -

searchers 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

Right-turn lane was installed at this signalized intersection located at 

SR 159 and Center Grove Rd/Goshen Rd in Edwardsville, Illinois
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.

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

Left-turn lane was installed at this signalized intersection located at 

US 18 and Pierce Avenue in Mason City, Iowa
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.

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.

Right-turn lane was installed at this unsignalized intersection located at US

97 and Moore Lane in Sherman County, Oregon
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 Number of major-road approaches on which 

control left-turn lanes are installed

One approach Both approaches

Three-leg intersection STOP signa 44b

Traffic signal 15c

Four-leg intersection STOP signa 28b 48b

Traffic signal 18c 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 Number of major-road approaches on which 

control left-turn lanes are installed

One approach Both approaches

Three-leg intersection STOP signa 33b

Traffic signal 7

d

Four-leg intersection STOP signa 27c 47b

Traffic signal 10b 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 RightTurn Lanes on the Major-Road Approaches to Rural and Urban Intersections

Intersection traffic Number of major-road approaches on which 

control right-turn lanes are installed

One approach Both approaches

STOP signa 14b 26b

Traffic signal 4

c 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.

Contact:

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 (go to

www.tfhrc.gov and click on “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.

NOVEMBER 2002 FHWA-RD-02-103

References 

1. 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.

2. Griffin, L. I., and R. J. Flower. “A Discussion of Six Procedures for Evaluating Highway Safety Projects,”

Federal Highway Administration, December 1997.

3. Hauer, E. Observational Before-After Studies in Road Safety, Pergamon/Elsevier Science, Inc., Tarrytown,

New York, 1997.

4. 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.