TECHBRIEF Safety Evaluation of 

Offset Improvements for 

Left-Turn Lanes

FHWA Publication No.: FHWA-HRT-09-036

FHWA Contact: Roya Amjadi, HRDS-06, (202) 493-3383, 

roya.amjadi@dot.gov

This document is a technical summary of the Federal 

Highway Administration report, Safety Evaluation of Offset 

Improvements for Left-Turn Lanes (FHWA-HRT-09-035), which

will be available online at http://www.tfhrc.gov/safety.

Objective

The Federal Highway Administration (FHWA) organized 

26 States to participate in the FHWA Low Cost Safety 

Improvements Pooled Fund Study as part of its strategic highway safety plan support effort. The purpose of the pooled fund 

study is to estimate the safety effectiveness for several of 

the unproven low-cost safety strategies identified in the 

National Cooperative Highway Research Program (NCHRP) 

Report 500 Series. One of the strategies chosen to be evaluated for this study is offset improvements for left-turn lanes 

at signalized intersections. This strategy is intended to reduce 

the frequency of crashes by providing better visibility for 

drivers that are turning left. The safety effectiveness of this 

strategy has not been thoroughly documented, and this study 

is an attempt to provide a crash-based evaluation through scientifically rigorous procedures.

Introduction

Intersections account for a small portion of the total highway system, yet approximately 2.42 million intersection-related crashes occurred in 2006, representing 41 percent of all 

reported crashes and 21 percent (8,291) of all fatal crashes. 

Crashes at signalized intersections represent about 51 percent 

(1.23 million) of all intersection-related crashes, of which, 2,740 

involved a fatality in 2006.(1)

The typical geometry and operation of signalized intersections

can present several challenges. Sight distance is important 

for drivers to identify acceptable gaps in opposing traffic. 

Opposing left-turn lanes are typically aligned directly across 

from one another and immediately adjacent to the through 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.tfhrc.gov
2

lanes, as shown in figure 1(b). Thus, a left-turning vehicle in the left-turn lane can obstruct the 

view of oncoming vehicles, particularly those in 

the opposite left-turn lane.

To improve sight distance and safety for leftturning drivers at intersections, the use of offset 

left-turn lanes has been recommended, as discussed in the NCHRP Report 500 Series Volume 

12, “A Guide for Reducing Collisions at Signalized 

Intersections.”(2) Sight distance for left-turning 

vehicles is diminished by creating a negative 

offset (figure 1(a)). It can be improved by shifting 

left-turn lanes that are currently aligned with no 

offset (figure 1(b)) to the left to create a positive 

offset, as shown in figure 1(c). 

This strategy is most applicable at signalized intersections that operate as permissive 

or permissive/protective for left-turn movements.(2) Left-turn movements that only operate 

as protected do not have the same concerns 

for gap acceptance. The American Association 

of State Highway and Transportation Officials 

(AASHTO) provides guidelines for the design 

of positive offset left-turn lanes.(3) The installation or upgrade of any pavement markings 

should follow the guidelines in the Manual 

on Uniform Traffic Control Devices (MUTCD).

(4)

While the literature supports the use of offset 

left-turn lanes, there have been no rigorous 

evaluations of the safety effective ness of 

this strategy. An investigation is needed to 

thoroughly evaluate the effectiveness of offset 

improvements for left-turn lanes in reducing 

crash frequency and severity at signalized 

intersections. The safety effectiveness of offset 

improvements for left-turn lanes is explored 

empirically in this study to provide better 

support to the States when selecting safety 

improvements at signalized intersections.

Methodology

Data were collected from several States that 

have implemented offset improvements for 

left-turn lanes at signalized intersections. 

Study locations were selected from Florida, 

Nebraska, and Wisconsin based on the availability of installation data, including location 

and installation dates. Geometric, traffic, and 

crash data were obtained for a total before 

period sample of 851.6 site-years (120.6 

from Florida, 644 from Nebraska, and 87 

from Wisconsin) and 473.8 site-years in 

the after period (72.8 from Florida, 368 from 

Nebraska, and 33 from Wisconsin). Siteyears are the number of intersections where 

the strategy is applied multiplied by the 

number of years the strategy has been in 

place at each intersection. For example, if a 

strategy is applied at nine intersections and 

is in place for 3 years at all nine intersections, 

there are a total of 27 site-years available for 

the study. Geometric, traffic, and crash data 

were also obtained for reference intersections 

with characteristics similar to the strategy 

sites in all three Sates but without offset 

improvements for left-turn lanes.

During the data collection process, the project 

team identified a variation in the design of offset left-turn lanes among the three States. For 

example, nearly all installations in Wisconsin 

were similar to the positive offset left-turn 

lane concept identified in the NCHRP Report 

500 Series Volume 12 shown in figure 1(c).(2)

However, many of the installations in Florida 

and Nebraska did not result in a positive offset. 

Instead, the offset was improved by shifting 

the left-turn lane further away from the 

adjacent through lane, but the end result 

was a less negative offset or no offset (refer 

to figure 1 for definitions). Due to the variation in offset designs among the States, the 

Negative Offset 

(a) 

Positive Offset 

(c) 

No Offset 

(b) 

Figure 1. Illustration of negative, no, and positive 

offset left-turn lanes.
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project team adopted a classification scheme to 

define the installations as one of three types 

of offset improvements. The adopted classification scheme is presented below:

•	 Type 1—Positive offset: The left-turn 

lanes are shifted to the left to enhance 

sight distance for opposing left-turn drivers 

(figure 2). 

•	 Type 2—Lateral separation with no 

offset: The left-turn lanes are separated 

from the adjacent through lanes, but 

opposing left-turn lanes are directly 

aligned with no offset or a very slight 

positive offset (figure 3). 

•	 Type 3—Lateral separation with a 

negative offset: The left-turn lanes are 

separated from the adjacent through 

lanes, but opposing left-turn lanes are still 

negatively offset, although less negatively 

offset than in the before period (figure 4). 

Empirical Bayes (EB) methods were incorporated in a before-after analysis to determine 

the safety effectiveness of offset improvements 

for left-turn lanes at signalized intersections. 

The EB methodology for observational beforeafter studies was used for the evaluation.(5)

Safety performance functions (SPFs) were calibrated separately for each State for use in the 

EB methodology. Generalized linear modeling 

(GLM) was used to estimate the model coefficients using the software package SAS®.

(6) A negative binomial error distribution was assumed 

for the GLM, which was consistent with the 

state of research in developing these models.

SPFs were estimated for the following crash 

classifications:

•	 Total intersection-related crashes (within 

76.25 m (250 ft) of the target intersection).

•	 Injury crashes (fatal plus all injury crashes).

•	 Left-turn crashes.

•	 Rear-end crashes.

The full report includes a detailed explanation of 

the methodology, including a description of how 

the estimate of percent reduction was calculated.

Results

Two sets of results were calculated and are 

presented in the following sections. One set 

contains aggregate results for each of the 

three States. The other is based on a disaggregate analysis of the Nebraska sites, grouping sites by various characteristics to evaluate 

the impact of these variables on the safety 

Figure 2. Example of a type 1 installation in 

Lincoln, NE (positive offset).

Figure 3. Example of a type 2 installation in 

Lincoln, NE (lateral separation with no offset).

Figure 4. Example of a type 3 installation in 

Lincoln, NE (lateral separation with negative 

offset).
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effectiveness. A disaggregate analysis was not 

conducted for Florida or Wisconsin because of 

the small sample sizes.

Aggregate Analysis

The aggregate results are shown in table 1 

for Florida, Nebraska, and Wisconsin.

The following points summarize the results 

for the individual State analyses:

•	 For Nebraska and Florida, the results show 

little or no evidence that this strategy is 

effective overall (i.e., for total crashes). 

Installations in these States were mostly 

type 2 or type 3 (negative offset improved 

to no offset or less negative offset).

•	 For left-turn opposing crashes, a reduction 

in crashes was shown in Florida, although 

the reduction was not significant. The 

opposite, a highly significant increase 

in left-turn opposing crashes, was found 

in Nebraska.

•	 For rear-end crashes, there was a small 

and insignificant increase in Florida. In 

Nebraska, rear-end crashes increased 

slightly, and this increase was statistically 

significant.

•	 In Wisconsin, where the analysis focused 

on crashes involving mainline vehicles 

only and where the installations were 

predominantly type 1 (negative or no offset 

improved to positive offset), the results 

indicate substantial and highly significant 

crash reductions in all categories—total, 

injury, left-turn, and rear-end. 

Combined effects for the three States were 

not estimated because the treatments and 

their effects varied significantly among the 

jurisdictions. Results from the individual State 

analyses generally indicated a greater safety 

benefit for the more enhanced offset improvements (i.e., creating significant positive offsets), as was the case in Wisconsin. For Florida 

and Nebraska, which included primarily type 2 

and type 3 installations, the safety effects were 

negligible for total crashes. The disaggregate 

analysis is presented in the following section, 

indicating specific situations where the strategy 

may be more effective.

Disaggregate Analysis

A disaggregate analysis was completed for 

Nebraska, the only State with a sufficient sample 

size to facilitate this analysis. The disaggregate 

Table 1. Results for Florida, Nebraska, and Wisconsin strategy sites.

Crash Type by State

EB Estimate of 

Crashes in After 

Period Without 

Strategy

Observed 

Crashes in 

After Period

Estimate 

of Percent 

Reduction

Standard 

Deviation 

of Percent 

Reduction

Florida—Total 969.91 938 3.4 4.7

Florida—Injury 471.66 472 0.2 6.6

Florida—Left-turn opposing 118.78 106 11.4 11.2

Florida—Rear-end 257.89 273 -5.3 9.9

Nebraska—Total 2,795.81 2,811 -0.5 2.4

Nebraska—Injury 1,536.12 1,441 6.2 3.0

Nebraska—Left-turn opposing 478.96 695 -45.0 6.7

Nebraska—Rear-end 1,248.64 1,335 -6.9 3.6

Wisconsin—Total 233.77 155 33.8 6.0

Wisconsin—Injury 95.88 62 35.6 9.0

Wisconsin—Left-turn 94.85 59 38.0 8.9

Wisconsin—Rear-end 72.76 50 31.7 10.9

Note: A negative sign (-) indicates an increase in crashes. Bold denotes those safety effects that are significant at 

the 95-percent confidence level. Unlike Nebraska and Florida, left-turn opposing crashes could not be precisely 

identified in Wisconsin; thus, the analysis includes all non-rear-end crashes involving a left-turning vehicle.
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analysis attempted to discern factors that 

may impact the safety effectiveness of offset 

improvements for left-turn lanes at signalized 

intersections. The results of the disaggregate 

analysis are summarized below.

Offset Improvement Type

The most obvious variable to examine in the 

disaggregate analysis was the type of offset 

improvement (i.e., type 1, type 2, or type 3). 

However, the effects by group were neither 

statistically significant nor statistically different 

from each other.

Expected Number of Crashes

The expected crash frequency prior to improvement was found to be most related to the 

safety effectiveness. Analysis revealed that the 

safety effectiveness increased as the expected 

number of crashes increased. For example, the 

30 sites in Nebraska with 9 or more expected 

crashes per year in the before period had 

an 8-percent reduction in crashes compared 

to an insignificant 0.5-percent increase in 

crashes for all 92 Nebraska locations. This 

finding seems logical in that safety treatments 

are generally expected to be most effective 

where a safety problem is manifested in a 

high frequency of crashes.

There are advantages and disadvantages to

applying the results from the disaggregate 

analysis. The disaggregate analysis can shed 

light on specific conditions for which strategies may be most effective; however, disaggregate analyses are, by nature, based on smaller 

sample sizes than aggregate analyses. Smaller 

samples lead to larger confidence intervals 

and less precise results. A general rule cannot be applied for using either the aggregate 

or disaggregate analysis, but the decision of 

which analysis to use should be based on a 

case-by-case basis. Comparing the aggregate 

analyses among the three States, results generally indicate that safety benefits increase 

as the offset improvement increases (i.e., 

results appear more promising for type 1 than 

type 3). The disaggregate analysis indicates 

specific conditions that should be given 

priority due to the relative effectiveness of 

this strategy (i.e., where there are nine or 

more expected crashes per year in the period 

before installation).

Economic Analysis

The purpose of the economic analysis was 

to evaluate the economic feasibility of offset 

improvements for left-turn lanes at signalized intersections. The economic analysis was 

accomplished by estimating the life-cycle cost 

of the strategy and the discounted annual cost 

of the strategy. Crash cost savings were estimated from the most recent FHWA unit crash 

cost data.(7) The annual crash reductions, 

necessary to offset the cost, were estimated 

by comparing the cost of the strategy to the 

crash costs.

The most recent FHWA mean comprehensive 

cost per crash for signalized intersections with 

approach speeds less than 72.45 km/h (45 mi/h) 

were identified as $15,788 for head-on crashes 

and $23,872 for rear-end crashes.(7) These estimated costs include all severities combined. 

Comprehensive crash costs represent the present value computed at a discount rate of all 

costs over the victim’s expected life span that 

result from a crash. The major categories of 

costs used in the calculation of comprehensive crash costs include medical-related costs, 

emergency services, property damage, lost 

productivity, and monetized quality-adjusted 

life years.(7)

Wisconsin was the only State for which a 

safety benefit was detected overall and for 

which a benefit-cost ratio could be estimated. 

Wisconsin installations were almost all of the 

type 1 variety involving major reconstruction 

with capital costs that averaged $315,873 and 

an estimated service life of 20 years. Assuming 

a discount rate of 2.8 percent for a 20-year 

service life as suggested by the Office of 

Management and Budget (OMB), this translates 

into an annualized cost of $20,840.(8) The results 

in table 1 suggest a reduction of 2.39 total 

crashes per site-year. Using the lower comprehensive crash cost ($15,788) as a conservative 
6

estimate of the benefit, the annual savings due 

to the reduced crashes is $37,733. This conservative value compares favorably to the annualized 

installation cost of $20,840, giving (conservatively) a benefit-cost ratio of approximately 2:1.

The disaggregate analysis for Nebraska showed 

that the strategy may be effective if implemented at sites with high crash frequencies. Based on the disaggregate results, it is 

possible to estimate crash frequency levels for 

which implementation of the strategy would 

be economically justified. Restriping costs, 

similar to those in Nebraska, are minimal.

To provide conservative estimates, the more 

expensive type 2 and type 3 treatments, similar 

to those in Florida, were used in the cost estimate. Based on a discount rate of 2.7 percent 

as suggested by OMB, for a 10- to 15-year 

service life, the annualized cost is $5,067, assuming that four approaches are reconstructed.(8)

The cost requires an annual crash savings of 

$10,134 to justify an installation based on a 2:1 

benefit-cost ratio.

The required crash savings as a dollar value 

were converted into crash frequency using 

the recent FHWA comprehensive crash cost 

estimates.(7) Again, using the lower crash 

cost ($15,788) as a conservative estimate, the 

annual crash reduction needed to justify the

installation cost is 0.64 crashes per year (i.e., 

a 2:1 benefit-cost ratio is calculated as $10,134 

divided by $15,788 equals 0.64 crashes per 

year). The disaggregate analysis indicates 

that the crash benefits increased as the 

expected number of crashes increased. The 

required crash benefit of 0.64 crashes per year 

could be achieved at intersections with nine 

or more expected crashes per year. Thus, the 

installation of the type 2 and type 3 varieties of 

this strategy through reconstruction similar to 

the Florida installations appears to be costeffective at intersections with at least nine 

expected crashes per year and where left-turn 

lanes are justified by traffic volume warrants.

Even with the conservative assumptions made, 

a very modest reduction in crashes is required 

to justify this strategy economically. Based 

on the results of this study, it appears that 

type 1 improvements (i.e., providing positive 

offsets) are cost-effective, even when major 

reconstruction is involved similar to the installations in Wisconsin. The type 2 and type 3 installations appear to be less effective than providing a significant positive offset. However, 

these types of installations are still justified 

as a safety treatment for intersections with 

nine or more expected crashes per year in the 

period before installation. 

Summary

The objective of this study was to evaluate

the safety effectiveness of offset improvements for left-turn lanes at signalized 

intersections, as measured by crash frequency 

within 76.25 m (250 ft) of the intersection. The 

study examined the effects of this strategy on 

specific crash types, including total, injury, 

rear-end, and sideswipe crashes, in a rigorous 

crash-based analysis. 

The evaluation was based on 92 installations

in Nebraska, 13 in Florida, and 12 in Wisconsin. 

The offset improvements varied greatly in the 

three States and, as such, the results were 

not aggregated. Many of the installations in 

Florida and Nebraska did not result in positive offsets. Nebraska installations mostly 

involved a simple modification of an existing 

left-turn bay for which the striping was reconfigured to narrow the existing left-turn lane and 

shift left-turn vehicles further to the left. 

The Florida installations were similar in principle, but they were somewhat more elaborate in that they involved shifting the left-turn 

lanes further into the median (on divided highways). However, the end results in Florida 

and Nebraska were often less negative offsets 

or no offsets rather than positive offsets. 

Wisconsin installations involved major reconstruction to improve the offset, and all but 

two were conversions from negative or no 

offset to a significant positive offset.

From a practical standpoint, results presented

in table 2 support the conclusion that offset 

improvements for left-turn lanes have the 

potential to reduce total and target crashes. 

This conclusion is based on the installations 
7

in Wisconsin, which resemble the strategy 

identified in the NCHRP Report 500 Series

Volume 12 more closely than the sites in 

Florida and Nebraska.(2) The aggregate results 

for Florida and Nebraska show little or no evidence to suggest that this strategy is effective for reducing total crashes. However, the 

large variation in installation methods in the 

three States may explain the large difference 

in observed effects.

The disaggregate analysis provided further

insight into the circumstances where offset 

improvements for left-turn lanes may be more 

effective. Based on the disaggregate analysis 

of Nebraska installations, the percentage 

reduction in crashes increased as the expected 

number of crashes increased. Specifically, the 

30 sites in Nebraska with an expected frequency 

of 9 or more crashes per year in the before 

period had an 8.0-percent reduction in crashes 

(significant at the 5.0-percent level) compared 

to an insignificant 0.5-percent increase in total 

crashes for all 92 Nebraska locations. 

Conclusion

The general conclusion from this research is 

that offset improvements for left-turn lanes 

that result in a positive offset have the 

potential to reduce crashes and crash severity at signalized intersections. Based on the 

installations in Wisconsin, the results indicate 

substantial and highly significant crash reductions in all categories—total (34 percent), injury 

(36 percent), left-turn (38 percent), and rear-end 

(32 percent). While the aggregate results for 

Florida and Nebraska show little or no evidence 

that would suggest that this strategy is effective for reducing total crashes, many of the 

installations in these two States did not result 

in a positive offset. The disaggregate analysis 

indicated that the percent reduction in crashes 

tended to increase as the expected number 

of crashes increased. As such, type 2 or type 3 

installations, similar to those in Florida, were 

cost-effective at intersections with at least nine 

expected crashes per year before treatment. 

Type 1 installations, similar to those in 

Wisconsin, resulted in a conservative benefitcost ratio of approximately 2:1. Needless to 

say, the provision of left-turn lanes should be 

justified by traffic volume warrants. Given the 

potential to reduce crashes, the use of offset 

improvements for left-turn lanes is justified 

as a safety improvement, particularly at intersections with nine or more expected crashes 

per year.

References

1. National Highway Traffic Safety 

Administration National Center for 

Statistics and Analysis. (2006). Traffic Safety 

Facts. National Highway Traffic Safety 

Administration. Washington, DC.

2. Antonucci, N.D., Hardy, K.K., Slack,

K.L., Pfefer, R., and Neuman, T.R. (2004). 

“Guidance for Implementation of the 

AASHTO Strategic Highway Safety 

Plan: A Guide for Reducing Collisions at 

Signalized Intersections.” NCHRP Report 

500 Series, Vol. 12. Transportation Research

Board. Washington, DC. 

Table 2. Summary of crash reduction factors.

Installation Type

Data 

Source

Crashes Per 

Year Before 

Offset

Type of Offset 

Improvement

Level of 

Effort

Crash Reduction 

Factor

Standard 

Deviation

Type 1 Wisconsin 

12 sites

7.5 Original 

offset: none 

or negative

Final offset: 

positive

Major 

construction

Total 33.8 6.0

Injury 35.6 9.0

Left-turn 38.0 8.9

Rear-end 31.7 10.9

Note: Crashes in Wisconsin were identified as mainline crashes only. Left-turn crashes were defined as all non-rearend crashes involving a left-turning vehicle.
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Researchers—This study was performed by Bhagwant Persuad and Craig Lyon of Persaud Lyon, Inc. 

with support from Kimberly Eccles and Frank Gross of Vanasse Hangen Brustlin, Inc. For more 

information about this research, contact Roya Amjadi, FHWA Project Manager, HRDS by phone at 

(202) 493-3383 or through email at roya.amjadi@dot.gov.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct 

distribution is being made to the Divisions and Resource Center.

Availability—The TechBrief may be obtained from the FHWA Product Distribution Center by email to 

report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at http://www.tfhrc.gov/

safety.

Key Words—Left-turn lanes, low-cost, safety improvements, safety evaluations, Empirical Bayesian, 

signalized intersections.

Notice—This document is disseminated under the sponsorship of the U.S. Department of 

Transportation in the interest of information exchange. The U.S. Government assumes no liability for 

the use of the information contained in this document. The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this report only because they 

are considered essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration (FHWA) provides high-quality 

information to serve the Government, industry, and public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and 

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processes to ensure continuous quality improvement.

3. American Association of State Highway and 

Transportation Officials (AASHTO). (2004). 

A Policy on Geometric Design of Highways 

and Streets. 5th ed. Washington, DC.

4. Federal Highway Administration. (2003). 

The Manual on Uniform Traffic Control 

Devices. Washington, DC. 

5. Hauer, E. (1997). Observational Before-After 

Studies in Road Safety: Estimating the Effect 

of Highway and Traffic Engineering Measures 

on Road Safety. Pergamon Press, Elsevier 

Science Ltd. Oxford, U.K.

6. SAS®. SAS Institute Inc. Cary, NC. 

7. Council, F., Zaloshnja, E., Miller, T., 

and Persaud, B. (2005). Crash Cost 

Estimates by Maximum Police-Reported 

Injury Severity within Selected Crash 

Geometries. FHWA-HRT-05-051. Federal 

Highway Administration. McLean, VA.

8. Federal Register. (2008). Notice of Public

Hearing, Federal Register, Vol. 73, No. 20. 

Obtained from: http://www.whitehouse.gov/

omb/fedreg/2008/013008_discountrate.pdf. 

Site last accessed June 25, 2008. 

june 2009 FHWA-HRT-09-036

HRDS-06/06-09(1M)E