Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Safety Evaluation of Offset 

Improvements for Left-Turn Lanes

Publication No. FHWA-HRT-09-035 June 2009

Negative Offset No Offset Positive Offset
FOREWORD 

The goal of this research was to evaluate and estimate the safety effectiveness of offset 

improvements for left-turn lanes in the Evaluation of Low-Cost Safety Improvements Pooled 

Fund Study (ELCSI-PFS) Phase II. This strategy is intended to reduce the frequency of crashes 

by providing better visibility for drivers that are turning left. 

The estimate of effectiveness for offset improvements for left-turn lanes was determined by 

conducting scientifically rigorous before-after evaluations at sites where this strategy was 

implemented in the United States. The ELCSI-PFS provides crash reduction factor (CRF) and 

economic analysis for the targeted safety strategies where possible. 

This safety improvement and all other targeted strategies in the ELCSI-PFS are identified as 

low-cost strategies in the National Cooperative Highway Research Program (NCHRP) Report 

500 Series. Participating States in the ELCSI-PFS are Arizona, California, Connecticut, Florida, 

Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Maryland, Massachusetts, Minnesota, 

Mississippi, Montana, New York, North Carolina, North Dakota, Oklahoma, Pennsylvania, 

South Carolina, South Dakota, Tennessee, Texas, Utah, and Virginia. 

 Dr. Chou-Lin Chen 

Acting Director, Office of Safety 

Research and Development 

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 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement. 
TECHNICAL DOCUMENTATION PAGE 

1. Report No. 

FHWA-HRT-09-035 

2. Government Accession No. 3. Recipient’s Catalog No. 

5. Report Date 

June 2009 

4. Title and Subtitle 

Safety Evaluation of Offset Improvements for Left-Turn Lanes 

6. Performing Organization Code 

7. Author(s) 

Bhagwant Persaud, Craig Lyon, Kimberly Eccles, Nancy Lefler, 

and Frank Gross 

8. Performing Organization Report No. 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

Vanasse Hangen Brustlin, Inc. (VHB) Persaud Lyon, Inc. 

8300 Boone Blvd., Ste. 700 87 Elmcrest Road 

Vienna, VA 22182-2626 Toronto, ON M9C 3R7 

11. Contract or Grant No. 

DTFH61-05-D-00024 (VHB) 

13. Type of Report and Period 

Safety Evaluation 

12. Sponsoring Agency Name and Address 

U.S. Department of Transportation 

Federal Highway Administration 

1200 New Jersey Avenue, SE 

Washington, DC 20590 

14. Sponsoring Agency Code 

FHWA 

15. Supplementary Notes 

The Federal Highway Administration (Office of Safety Research and Development) managed this study. The project team 

members were Dr. Bhagwant Persaud, Craig Lyon, Kimberly Eccles, Dr. Frank Gross, Dr. Hugh McGee, Nancy Lefler, Michelle 

Scism, Joedel Zaballero, and Robert Hoyle. The FHWA Office of Safety Research and Development Contract Task Order 

Manager was Roya Amjadi. 

16. Abstract 

The Federal Highway Administration (FHWA) organized a pooled fund study of 26 States to evaluate low-cost safety strategies as 

part of its strategic highway safety effort. One of the strategies chosen to be evaluated for this study was offset improvements for 

left-turn lanes. 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 

an evaluation through scientifically rigorous procedures. 

Geometric, traffic, and crash data were obtained for 92 installations in Nebraska, 13 in Florida, 12 in Wisconsin, and for a number 

of untreated reference sites in each State. To account for potential selection bias and regression-to-the-mean, an Empirical Bayes 

(EB) before-after analysis was conducted to determine the safety effectiveness of improving the offset for left-turn lanes. There 

was a large difference in observed effects among the three States, which may be explained, in part, by the wide variation in offset 

improvements. Florida and Nebraska employed pavement marking adjustments or minor construction to improve the offset. While 

the offset was improved at each site, most improvements did not result in a positive offset. Wisconsin reconfigured left-turn lanes 

through major construction projects, resulting in significant positive offsets. Results in Florida and Nebraska showed little or no 

effect on total crashes. Wisconsin showed significant reductions in all crash types investigated—total (34 percent), injury 

(36 percent), left-turn (38 percent), and rear-end (32 percent). 

A disaggregate analysis was conducted for Nebraska, the only State with enough installations to disaggregate the results. The 

analysis revealed that the percent reduction in crashes increased as the expected number of crashes increased. An economic 

analysis was conducted to identify the level of expected crashes that would yield a crash benefit to justify the construction cost. 

Based on this analysis, offset improvement through reconstruction is cost-effective at intersections with at least nine expected 

crashes per year and where left-turn lanes are justified by traffic volume warrants. 

17. Key Words 

Left-turn Lanes, Low-cost, Safety improvements, Safety evaluations, Empirical 

Bayesian, Signalized intersections 

18. Distribution Statement 

No restrictions. 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No. of Pages 

 47 

22. Price 

Form DOT F 1700.7 (8-72) Reproduction of completed pages authorized
ii 
iii 

TABLE OF CONTENTS 

EXECUTIVE SUMMARY............................................................................................................1 

INTRODUCTION..........................................................................................................................3 

BACKGROUND ON STRATEGY.........................................................................................3 

BACKGROUND ON STUDY.................................................................................................4 

LITERATURE REVIEW........................................................................................................4 

OBJECTIVE...................................................................................................................................7 

STUDY DESIGN............................................................................................................................9 

SAMPLE SIZE ESTIMATION OVERVIEW ......................................................................9 

METHODOLOGY.......................................................................................................................13 

DATA COLLECTION ................................................................................................................15 

FLORIDA ...............................................................................................................................17 

Background ........................................................................................................................17 

Installation Data .................................................................................................................17 

Reference Sites...................................................................................................................18 

Roadway Data ....................................................................................................................18 

Traffic Data ........................................................................................................................18 

Crash Data..........................................................................................................................18 

NEBRASKA............................................................................................................................18 

Background ........................................................................................................................18 

Installation Data .................................................................................................................19 

Reference Sites...................................................................................................................19 

Roadway Data ....................................................................................................................19 

Traffic Data ........................................................................................................................19 

Crash Data..........................................................................................................................19 

WISCONSIN...........................................................................................................................20 

Background ........................................................................................................................20 

Installation Data .................................................................................................................20 

Reference Sites...................................................................................................................20 

Roadway Data ....................................................................................................................20 

Traffic Data ........................................................................................................................20 

Crash Data..........................................................................................................................20 

DATA CHARACTERISTICS AND SUMMARY...............................................................21 

DEVELOPMENT OF SAFETY PERFORMANCE FUNCTIONS ........................................23 

RESULTS......................................................................................................................................25 

AGGREGATE ANALYSIS...................................................................................................25 

DISAGGREGATE ANALYSIS............................................................................................27 

Offset Improvement Type ..................................................................................................27 

Expected Number of Crashes.............................................................................................27 

ECONOMIC ANALYSIS............................................................................................................29

SUMMARY AND CONCLUSIONS...........................................................................................31 
iv 

APPENDIX: SAFETY PERFORMANCE FUNCTIONS........................................................33 

ACKNOWLEDGEMENTS.........................................................................................................37 

REFERENCES.............................................................................................................................39 
v 

LIST OF FIGURES 

Figure 1. Chart. Illustration of negative, no, and positive offset left-turn lanes ..............................3 

Figure 2. Photo. Example of a type 1 installation in Lincoln, NE (positive offset).......................16 

Figure 3. Photo. Example of a type 2 installation in Lincoln, NE (lateral separation 

with no offset) ................................................................................................................................16 

Figure 4. Photo. Example of a type 3 installation in Lincoln, NE (lateral separation 

with negative offset).......................................................................................................................17 

LIST OF TABLES 

Table 1. Minimum and desirable offset distance by vehicle type and vehicle speed ......................5 

Table 2. Before period crash rate assumptions ................................................................................9 

Table 3. Minimum required before period site-years for treated sites...........................................11 

Table 4. Definitions of crash types.................................................................................................21 

Table 5. Data summary for treatment sites ....................................................................................22

Table 6. Results for Florida strategy sites......................................................................................25 

Table 7. Results for Nebraska strategy sites ..................................................................................26 

Table 8. Results for Wisconsin strategy sites.................................................................................26 

Table 9. Summary of crash reduction factors ................................................................................32 

Table 10. Florida total ....................................................................................................................33 

Table 11. Florida injury..................................................................................................................33 

Table 12. Florida rear-end..............................................................................................................33 

Table 13. Nebraska total.................................................................................................................34 

Table 14. Nebraska proportions to apply to total crash model for other crash types.....................34 

Table 15. Wisconsin total...............................................................................................................34 

Table 16. Wisconsin injury ............................................................................................................35 

Table 17. Wisconsin left-turn.........................................................................................................35 

Table 18. Wisconsin rear-end.........................................................................................................35 
vi 

ABBREVIATIONS AND SYMBOLS 

Abbreviations 

 A Injury, incapacitating 

 AADT Average annual daily traffic 

 AASHTO American Association of State Highway and Transportation Officials 

Avg Average 

B Injury, non-incapacitating 

 C Possible injury 

DOT Department of transportation 

EB Empirical Bayes 

FDOT Florida Department of Transportation 

FHWA Federal Highway Administration 

KABCO Scale used to represent injury severity in crash reporting 

 K Fatality 

Max Maximum 

Min Minimum 

MUTCD The Manual on Uniform Traffic Control Devices

NCHRP National Cooperative Highway Research Program 

NDOR Nebraska Department of Roads 

NHTSA National Highway Traffic Safety Administration 

 OMB Office of Management and Budget 

SE Standard error 

SPF Safety performance function 

 TRB Transportation Research Board 

WisDOT Wisconsin Department of Transportation 
vii 

Symbols 

α Greek letter alpha 

β Greek letter beta 

λ Greek letter lambda 

π Greek letter pi 

θ Greek letter theta 

k Dispersion parameter 

m Expected number of crashes 

P Sum of annual safety performance function (SPF) estimates 

w1 Weight for observed crash count used in Empirical Bayes calculation 

w2 Weight for annual SPF estimates used in Empirical Bayes calculation 

x Observed count of crashes in the before period 

1 

EXECUTIVE SUMMARY 

The Federal Highway Administration (FHWA) organized a pooled fund study of 26 States to 

evaluate low-cost safety strategies as part of its strategic highway safety effort. The purpose 

of the FHWA Low-Cost Safety Improvements Pooled Fund Study was to evaluate the safety 

effectiveness of several of the low-cost safety strategies through scientifically rigorous 

crash-based studies. One of the strategies chosen to be evaluated for this study was offset 

improvements for left-turn lanes. This strategy is intended to reduce the frequency of crashes by 

providing better visibility for drivers who are turning left. The safety effectiveness of this strategy 

has not been thoroughly documented. This study is an attempt to provide an evaluation through 

scientifically rigorous procedures. 

Geometric, traffic, and crash data were obtained for 92 installations in Nebraska, 13 in Florida, 

12 in Wisconsin, and for a number of untreated reference sites in each State. The configuration of 

offsets in Nebraska varied from site to site, but they all involved a simple modification of an 

existing left-turn bay. The striping was reconfigured with thermoplastic to narrow the existing 

left-turn lane, thereby shifting left-turn vehicles further to the left and improving the offset. The 

installations in Florida were somewhat more elaborate in that they involved shifting the left-turn 

lanes further into the median (on divided highways) either by restriping or by installing raised 

concrete channelizing islands. While the installations in Florida and Nebraska improved the 

offset, many of the installations did not result in a positive offset. The majority of the Wisconsin 

installations involved a combination of pavement markings and raised concrete channelizing 

islands to create a positive offset; three treatments involved pavement markings only. 

In Wisconsin, the results indicated substantial and highly significant crash reductions in all 

categories—total (34 percent), injury (36 percent), left-turn (38 percent), and rear-end 

(32 percent). Nebraska and Florida results showed little or no evidence that this strategy was 

effective for reducing total crashes. For the main target crashes (i.e., left-turn opposing crashes), a 

reduction in crashes was found in Florida, although it was not significant. While the results for 

Nebraska indicated a significant reduction in injury crashes, they also showed an increase in 

target crashes, including a 45-percent increase in left-turn opposing crashes and a 7-percent 

increase in rear-end crashes. Overall, the large variation in installation methods and resulting 

offset in the three States may help explain the large difference in observed effects. 

A disaggregate analysis of the Nebraska installations revealed that the percent reduction in 

crashes increased as the expected number of crashes increased. For intersections that had nine or 

more expected crashes per year in the before period, an 8-percent reduction in total crashes was 

found. An economic analysis was conducted to identify the level of the expected number of 

crashes that would yield a crash benefit that would justify the construction cost, with a benefitcost ratio of at least 2:1. Based on this analysis, the installation of this strategy through 

reconstruction was cost-effective at intersections with at least nine expected crashes per year and 

where left-turn lanes were justified by traffic volume warrants. The Wisconsin installations had 

on average 7.52 mainline crashes per year pre-installation and yielded benefits on the order of a 

30-percent reduction with a conservative benefit-cost ratio of approximately 2:1.

3 

INTRODUCTION 

BACKGROUND ON STRATEGY 

Intersections account for a small portion of the total highway system, yet, in 2006, approximately 

2.42 million intersection-related crashes occurred. Intersection crashes accounted for 

41 percent of all reported crashes and 21 percent (8,291) of all fatal crashes in 2006.(1) The 

disproportionately high percentage of intersection crashes is not surprising because intersections 

present more points of conflict than non-intersection locations. 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 of signalized intersections can present several challenges. Visibility of 

oncoming vehicles is important for drivers to identify acceptable gaps. The geometry at some 

intersections actually creates a negative offset, as shown in figure 1(a), which further reduces 

sight distance for left-turning vehicles. Typical intersection alignments have opposing left-turn 

lanes directly across from one another and immediately adjacent to the through lanes, as shown 

by the intersection without an offset in figure 1(b). Thus, a left-turning vehicle in the opposite 

left-turn lane can obstruct the view of oncoming vehicles. Sight distance for left-turning vehicles 

can be improved by shifting the left-turn lanes to the left to create a positive offset, as shown in 

figure 1(c).(2) When initial offsets are negative, a variation of the offset improvement strategy can 

be applied by increasing the lateral separation between the left-turn and adjacent through lane 

(i.e., modifying the left-turn lane from a negative to less negative offset). Hence, the offset is still 

negative, but the sight distance to oncoming vehicles is slightly improved. 

Figure 1. Chart. Illustration of negative, no, and positive offset left-turn lanes. 
4 

This strategy may be particularly helpful for older drivers, but it should be noted that the strategy 

is most applicable when permissive or permissive/protective phasing for left-turn movements 

are in effect.(2) Left-turn movements that only operate as protected do not have the visibility 

concerns for gap acceptance. As such, this strategy would provide little if any benefit only for 

protected left-turn movements. The installation or upgrade of any pavement markings should 

follow the guidelines in The Manual on Uniform Traffic Control Devices (MUTCD) in order to 

maintain consistency throughout the given roadway.(3) 

BACKGROUND ON STUDY 

In 1997, the American Association of State Highway and Transportation Officials (AASHTO) 

Standing Committee on Highway Traffic Safety, with the assistance of the Federal Highway 

Administration (FHWA), the National Highway Traffic Safety Administration (NHTSA), and the 

Transportation Research Board (TRB) Committee on Transportation Safety Management, met 

with safety experts in the field of driver, vehicle, and highway issues from various organizations 

to develop a strategic plan for highway safety. These participants developed 22 key areas that 

affect highway safety. 

The National Cooperative Highway Research Program (NCHRP) published a series of guides to 

advance the implementation of countermeasures targeted to reduce crashes and injuries. Each 

guide addresses 1 of the 22 emphasis areas and includes an introduction to the problem, a list of 

objectives for improving safety in that emphasis area, and strategies for each objective. Each 

strategy is designated as proven, tried, or experimental. Many of the strategies discussed in these 

guides have not been rigorously evaluated; about 80 percent of the strategies are considered 

tried or experimental. 

The FHWA organized a pooled fund study of 26 States to evaluate low-cost safety strategies as 

part of this strategic highway safety effort. The purpose of the pooled fund study is to evaluate 

the safety effectiveness of several tried and experimental low-cost safety strategies through 

scientifically rigorous crash-based studies. Improving the offset for left-turn lanes at signalized 

intersections was selected as a strategy to be evaluated as part of this effort. Offset left-turn lanes 

are identified in the NCHRP Report 500 Series Volume 12 as a strategy to reduce crashes at 

signalized intersections.(2) This strategy is particularly applicable where crashes related to 

left-turn movements are an issue. 

LITERATURE REVIEW 

Khattak et al. analyzed the treatment of providing positive offset left-turn lanes at urban 

signalized intersections in Lincoln, NE.(4) The positive offset left-turn lanes were installed by 

increasing the lateral separation between the left-turn lanes and the adjacent through lanes, 

thereby improving the sight distance for left-turning vehicles. Six intersections were treated, and 

two intersections without offset left-turn lanes were selected as a control group. All treated 

intersections operated with protected/permitted left-turn phasing. The characteristics of the 

intersections varied, including the lane width, median type, median width, speed limit, and 

amount of offset. Nine years of crash and traffic data were analyzed. There were 298 left-turn 

related crashes at the study locations in the 9-year period. The study included an analysis of crash 

trends, regression analysis of crash rates, naïve and comparison group before-after studies, and 
5 

ordered logit injury severity modeling. The comparison group method showed a reduction of 

27 percent and 40 percent for total and property damage only accidents at the treated sites, 

respectively. However, the variances of these estimates were not reported, so it is unknown if 

these results were statistically significant. Poisson regression modeling of the before-after data 

indicated a slight non-significant reduction in crashes at the treated sites. Similarly, the injury 

severity modeling indicated that injury severity decreased in the after period, but the results were 

not statistically significant. 

McCoy et al. developed guidelines for positive offset distances for left-turning vehicles 

positioned at the stop line and opposed by a left-turning vehicle within the intersection.(5) 

The minimum and desirable positive offsets are shown in table 1 for various design speeds. 

Table 1. Minimum and desirable offset distance by vehicle type and vehicle speed. 

Design Minimum Positive Offsets (ft) Desirable Positive Offsets (ft) 

Speed 

(mi/h) Passenger Cara

 Truckb Passenger Cara Truckb

40 1.0 2.5 2.0 3.5 

45 1.0 3.0 2.0 3.5 

50 1.5 3.0 2.0 3.5 

55 1.5 3.0 2.0 3.5 

60 1.5 3.0 2.0 3.5 

65 1.5 3.0 2.0 3.5 

70 1.5 3.0 2.0 3.5 a = Opposing left-turn vehicle is a passenger car 

b = Opposing left-turn vehicle is a truck 

1 ft = 0.305 m 

1 mi = 1.61 km 

This strategy may be particularly well suited for older drivers. Several studies have found that 

older drivers have more left-turn accidents at signalized intersections than younger drivers.(6)

Common older driver errors include misjudging the oncoming vehicle speed, misjudging the 

available gap, assuming that the oncoming vehicle is going to stop or turn, and driving without 

seeing the other vehicle. Furthermore, older drivers may experience greater difficulties at 

intersections as a result of diminished visual capabilities, such as depth and motion perception. 

These traits, which are associated with older drivers, can increase the potential for collisions 

between vehicles turning left from the major road and through vehicles on the opposing majorroad approach.(6) 

Staplin et al. studied the effects of both positive and negative offsetting left-turn lanes on the 

turning performance of drivers with respect to driver age and gender.(7) Left-turn performance of 

100 subjects within 3 age groups (ages 25–45, 65–74, and 75+ years old) was evaluated under 

normal driving conditions at 4 intersections of different left-turn offset configurations. The 

results indicate that driver performance can be adversely affected by left-turn lanes that are offset 

by more than a 0.915-m (3-ft) negative offset. Such large negative offsets significantly increase 
6 

the size of the critical gaps for drivers turning left and also seem to increase the likelihood of 

conflicts between left-turns and opposing through traffic. Surprisingly, driver perceptions of the 

level of comfort and degree of difficulty were not found to improve with the increased sight 

distance provided by larger (i.e., more positive) offsets. The 1.83-m (6-ft) positive offset was 

associated with a lower level of comfort and a higher degree of difficulty perceived by drivers 

making left-turns than the 0.915-m (3-ft) negative offset, which provided less sight distance. This 

may have been because the 0.915-m (3-ft) negative offset is much more common than the 1.83-m 

(6-ft) positive offset.
7 

OBJECTIVE 

This research examined the safety impacts of offset improvements for left-turn lanes at signalized 

intersections in Florida, Nebraska, and Wisconsin. The objective was to estimate the safety 

effectiveness of this strategy as measured by crash frequency. Target crash types included the 

following: 

• Total crashes (all types and severities combined). 

• Injury crashes (K, A, B, and C injuries on KABCO scale, which is used to represent 

injury severity in crash reporting). 

• Left-turn opposing crashes on roadway with offset improvements for left-turn lanes 

(all severities combined). 

• Rear-end crashes on roadway with offset improvements for left-turn lanes (all 

severities combined). 

A further objective addresses questions such as the following: 

• Do effects vary by level of traffic volumes? 

• Do effects vary by the level of crashes before treatment? 

• Do effects vary by the amount and type of offset? 

• Do effects vary by vehicle speeds? 

The evaluation of overall effectiveness included consideration of the installation costs and crash 

savings in terms of the benefit-cost ratio. Crash savings were computed by crash type and 

severity using crash costs recently developed by FHWA.(8)

Meeting these objectives placed some special requirements on the data collection and analysis 

tasks including the need to complete the following: 

• Select a large enough sample size to detect, with statistical significance, what may be 

small changes in safety for some crash types. 

• Identify appropriate reference sites. 

• Account for traffic volume changes properly. 

• Pool data from multiple jurisdictions to improve reliability of the results and facilitate 

broader applicability of the products of the research. 

9 

STUDY DESIGN 

The study design involved a sample size analysis and prescription of needed data elements. 

The sample size analysis assessed the sample size required to statistically detect an expected 

change in safety. It also determined what changes in safety can be detected with likely 

available sample sizes. 

SAMPLE SIZE ESTIMATION OVERVIEW 

Sample size estimations require assumptions of the expected treatment effect and the average 

crash rate at treatment sites in the before period. Minimum and desired sample sizes were 

calculated assuming a conventional before-after with reference group study design, as described 

in Hauer and a literature review of likely safety effects.(9) The sample size analysis undertaken 

for this study addressed the sample size required to statistically detect an expected change 

in safety. The sample size estimates were conservative because the Empirical Bayes (EB) 

methodology was incorporated in the before-after analysis rather than applying a conventional 

before-after analysis with a reference group. 

Sample sizes were estimated for various assumptions of the likely annual crash rate in the before 

period and likely safety effects of the strategy. Annual crash rates were assumed for four crash 

types (total, injury, left-turn, and rear-end crashes), as shown in table 2. The range of possible 

values for the expected safety effects was surmised from the literature review and knowledge of 

similar intersection treatments. These rates represent a range of mean crash rate. The large 

difference in the two rates is likely due to average annual daily traffic (AADT) differences. The 

study design assumed that the number of reference sites is equal to the number of strategy sites. 

Table 2. Before period crash rate assumptions. 

Crash Type 

Rate A (Crashes/ 

Intersection/Year) 

Rate B (Crashes/ 

Intersection/Year) 

All 9.0 3.4 

Injury 2.7 1.0 

Left-turn opposing 

on treated roadway 

1.4 0.5 

Rear-end crashes 

between through 

vehicles on opposing 

approach of 

treated roadway 

1.4 0.5 

Table 3 provides estimates of the required number of before and after period site-years for a 

90-percent confidence level. The minimum sample indicates the level for which a study seems 

worthwhile; that is, it is feasible to detect with 90-percent confidence the largest effect that may 

reasonably be expected based on what is currently known about the strategy. In this case, a 

10-percent reduction in total crashes was assumed as this upper limit on safety effectiveness. 
10 

The minimum sample size was based on the larger expected annual crash rate before treatment 

(rate A). The desirable sample also assumes a 10-percent reduction in total crashes, but this 

corresponds to the smallest benefit that would be interesting in detect with 90-percent confidence. 

The logic behind this approach is that safety managers may not wish to implement a measure that 

reduces crashes by less than 10 percent, and the required sample size to detect a reduction smaller 

than 10 percent would likely be prohibitively large. The desirable sample size was conservatively 

based on the smaller expected annual crash rate before treatment (rate B). 

These sample size calculations were based on specific assumptions regarding the number 

of crashes per intersection and years of available data. Site-years are the number of intersections 

where the strategy was implemented multiplied by the number of years the strategy was in 

place at each intersection. For example, if a strategy is implemented 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. 

The sample size values recommended in this study are highlighted in bold in table 3. The sample 

size estimates provided are conservative in that the state-of-the-art EB methodology proposed 

for the evaluations would require fewer sites than a conventional before-after study with a 

reference group. A minimum sample size of 100 site-years and a desirable sample size of at least 

263 site-years per period were calculated. Estimates that may be predicted with greater 

confidence or a smaller reduction in crashes will be detectable if there are more site-years of data 

available in the after period. The same holds true if there is a higher crash rate than expected in 

the before period. Note that much more data are necessary for analysis of injury, left-turn, or rearend crashes. For example, in order to detect a 10-percent change in left-turn crashes with 

90-percent confidence, 1,791 intersection-years of data would be desired. 
11 

Table 3. Minimum required before period site-years for treated sites. 

90-Percent Confidence

Crash Type

Expected Percent 

Crash Reduction 

to be Detected Rate A Rate B

5 437 1,157

10 100 263

All 

20 20 54

5 1,457 3,935

10 332 896

Injury 

20 68 183

5 2,811 7,870

10 640 1,791

Left-turn opposing on treated 

roadway 

20 131 366

5 2,811 7,870

10 640 1,791

Rear-end crashes on treated 

roadway 

20 131 366

Note: Bold denotes the calculated minimum and desirable sample size for intersection-years per 

period. Rate A and rate B correspond to the assumed crash rates in table 2.

13 

METHODOLOGY 

The EB methodology for observational before-after studies was used for the evaluation.(9) This 

methodology is rigorous in that it addresses the following: 

• It properly accounts for regression-to-the-mean. 

• It overcomes the difficulties of using crash rates in normalizing for volume differences 

between the before and after periods. 

• It reduces the level of uncertainty in the estimates of safety effect. 

• It provides a foundation for developing guidelines for estimating the likely safety 

consequences of a contemplated strategy. 

• It properly accounts for differences in crash experience and reporting practice in 

amalgamating data and results from diverse jurisdictions. 

In the EB approach, the change in safety for a given crash type at a site is given by the following: 

Δ Safety = λ – π (1)

Where: 

λ = The expected number of crashes that would have occurred in the after period without the 

strategy.

π = The number of reported crashes in the after period. 

In estimating λ, the effects of regression-to-the-mean and changes in traffic volume were 

explicitly accounted for using safety performance functions (SPFs), relating crashes of different 

types to traffic flow and other relevant factors for each jurisdiction based on untreated sites 

(reference sites). Annual SPF multipliers were calibrated to account for temporal effects on safety 

(e.g., variation in weather, demography, and crash reporting). 

In the EB procedure, the SPF was used to first estimate the number of crashes that would 

be expected in each year of the before period at locations with traffic volumes and other 

characteristics similar to the one being analyzed (i.e., reference sites). The sum of these 

annual SPF estimates (P) was then combined with the count of crashes (x) in the before period 

at a strategy site to obtain an estimate of the expected number of crashes (m) in the before 

strategy. This estimate of m is as follows: 

( ) ( ) m = w1 x + w2 P (2)
14 

Where w1 and w2 are estimated from the mean and variance of the SPF estimate as the following: 

k P

P

w 1 1

+ =

 (3)

) 1 (

1 2

k k P

w

+ =

 (4) 

Where: 

k = The constant for a given model. It is estimated from the SPF calibration process with the 

use of a maximum likelihood procedure. In that process, a negative binomial distributed 

error structure is assumed with k being the dispersion parameter of this distribution. 

A factor was then applied to m to account for the length of the after period and differences in 

traffic volumes between the before and after periods. This factor is the sum of the annual SPF 

predictions for the after period divided by P, the sum of these predictions for the before period. 

After applying this factor, the result was an estimate of λ. The procedure also produces an 

estimate of the variance of λ. 

The estimate of λ was then summed over all sites in a strategy group of interest (to obtain λsum) 

and compared with the count of crashes observed during the after period in that group (πsum). The 

variance of λ was also summed over all sites in the strategy group. 

The Index of Effectiveness (θ ) is estimated as the following: 

⎟

⎠

⎞ ⎜

⎝

⎛ +

=

2

( ) 1 sum

sum

sum

sum

Var

λ

λ

λ

π

θ

 (5) 

The standard deviation of θ is given by the following:

2

2

2 2

2

( ) 1

( ) ( )

( )

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

=

sum

sum

sum

sum

sum

sum

Var

Var Var

StDev

λ

λ

λ

λ

π

π θ

θ

 (6) 

The percent change in crashes was calculated as 100 (1−θ ). Thus, a value of θ = 0.7 with a 

standard deviation of 0.12 indicated a 30-percent reduction in crashes with a standard 

deviation of 12 percent. 
15 

DATA COLLECTION 

Florida, Nebraska, and Wisconsin provided installation data, including locations and dates for 

offset improvements to left-turn lanes. These States also provided roadway geometry, traffic 

volumes, and crash data for both installation and reference sites. This section provides a summary 

of the data assembled for the analysis. 

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 and shown previously 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 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, and 

examples of the three types of offset improvements are provided in figure 2 to figure 4. 

• 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 negative offset—The left-turn lanes are separated from 

the adjacent through lanes, but opposing left-turn lanes are still negatively offset, although 

they appear less negatively offset than in the before period (figure 4). 
16 

Figure 2. Photo. Example of a type 1 installation in Lincoln, NE (positive offset). 

Figure 3. Photo. Example of a type 2 installation in Lincoln, NE 

(lateral separation with no offset). 
17 

Figure 4. Photo. Example of a type 3 installation in Lincoln, NE 

(lateral separation with negative offset). 

FLORIDA 

Background

The Florida Department of Transportation (FDOT) implemented offset improvements for leftturn lanes to increase safety at specific signalized intersections throughout the State. This was not 

a blanket strategy, so several districts were contacted to identify potential locations with offset 

improvements for left-turn lanes. Several potential intersections were initially identified, but it 

was later determined that some of these intersections had protected left-turn signal phasing, 

and they were dropped from the study. Those sites with permissive, protected-permissive, or 

permissive-protected left-turn signal phasing were retained in the study, and additional 

information was obtained. 

Installation Data

District engineers provided locations and installation dates of offset improvements for left-turn 

lanes. In total, 13 locations were identified in 3 districts, including districts 2, 4, and 7. Of these, 

eight installations were defined as type 2, and five were defined as type 3 according to the 

classification scheme. When available, data on other strategies installed at the study locations 

were also collected. 
18 

Reference Sites

District engineers provided a list of signalized intersections which were to be used as reference 

sites. The reference sites were selected based on the availability of information (traffic counts, 

roadway geometry, etc.), and they were reflective of the study site characteristics. The difference 

was that the reference sites were not treated (i.e., no offset improvement). In total, 39 locations 

were identified in 3 districts including districts 2, 4, and 5. 

Roadway Data

Roadway data were collected for both strategy sites and reference sites. Most roadway data were 

obtained from the crash database provided by FDOT, including number of lanes, area type, 

roadway type, and speed limit. Lane configuration as well as the presence and type of median 

were identified using aerial images. For locations where the images were not clear, field visits 

were conducted to obtain the necessary data. 

Traffic Data

Traffic volume data were obtained from several sources, including the crash database provided 

by FDOT and county agencies. The major roads were all State-maintained routes, and as such, 

traffic volumes were available in the crash database. Minor road data were not available in the 

crash database unless the minor road was also a State-maintained route. In most cases, the minor 

road traffic volume had to be obtained from the corresponding county agency. When available, 

county Web sites were used to obtain the necessary traffic volume data. In other cases, county 

engineers were asked to provide estimates of the minor road traffic volume. 

Major road traffic volumes were available for each year of the study period. Minor road traffic 

volumes were sparser. In some cases, only the 8-hour traffic counts were available for minor 

roads. In these cases, a multiplier was applied to estimate the average daily traffic from the 

8-hour count. The multipliers were obtained from the county or district where the site was 

located, or they were computed using the 8-hour count and average daily traffic volume from the 

major road at the intersection. Growth factors were computed and applied to the traffic volumes 

to fill in years where data were not available. 

Crash Data

FDOT provided a crash database containing all crashes on state-maintained roads from 1983 to 

2005 inclusively. The crash data were matched to the strategy and reference sites using the 

section number and intersection milepost. A radius of 76.25 m (250 ft) was used to identify 

crashes at the strategy and reference intersections. 

NEBRASKA 

Background

The city of Lincoln, NE, improved offsets for left-turn lanes throughout the city to increase safety 

at signalized intersections. All three types of offset improvements for left-turn lanes, as defined 

by the project team, were implemented in Lincoln. The majority of offset improvements (44) 
19 

included the reconfiguration of existing left-turn lanes to type 2 offsets. There were 

9 intersections with type 1 offset left-turn lanes included in the study and 39 intersections 

with type 3 improvements. 

Installation Data

The Lincoln Department of Public Works provided a list of signalized intersections where offset 

left-turn lane improvements had been implemented. The list indicated the type of median and 

type of left-turn signal phasing (permissive, protected-permissive, permissive-protected, or fullyprotected), but it did not indicate the type of offset improvement. Those sites with fully-protected 

left-turn signal phasing were excluded from the analysis as were sites with inadequate traffic 

volume data, leaving a total of 92 intersections to be included in the analysis. When available, 

data on other strategies installed at the study locations were collected as well. 

Reference Sites

A list of potential reference locations in Lincoln was provided, which included signalized but 

untreated locations (i.e., no offset treatment). The final list of reference sites was selected based 

on data availability, resulting in a total of 64 reference sites. 

Roadway Data

Roadway data were collected for both the strategy sites and reference sites. The presence and 

type of median as well as left-turn signal phasing were provided by the city engineers. The lane 

configuration was identified using electronic intersection design files from the city as well as 

aerial images. For locations where electronic files were not available and the aerial images were 

not clear, field visits were conducted to obtain the necessary data. 

Traffic Data

Traffic volume data were obtained from Lincoln. Traffic volumes were available for 1998, 1999, 

2000, 2002, and 2006. When the subject intersection was located between two count locations, an 

average of the two closest counts was computed. Growth factors were computed to fill in the 

counts for years that were not available. 

Crash Data

The Nebraska Department of Roads (NDOR) helped provide crash data for each of the strategy 

and reference sites. Crash data were obtained for each location from 1994 to 2006. NDOR 

does not use a specific distance to determine whether a crash is intersection-related. Rather, 

intersection-related crashes are determined by an analyst after careful review of each case. 

They include any crashes where any of the vehicles involved were in the process of stopping, 

turning, slowing down, or making any other type of maneuver that was a result of the presence 

of an intersection. 
20 

WISCONSIN 

Background

The Wisconsin Department of Transportation (WisDOT) implemented offset improvements for 

left-turn lanes to increase safety at specific signalized intersections throughout the State. This is 

not a blanket strategy, but it is becoming a standard practice in some regions. For example, the 

southeast region indicated that they are beginning to install positive offset left-turn lanes 

whenever there is a sufficient median, particularly at signalized intersections. 

Installation Data

The University of Wisconsin provided a list of potential installation locations, including the 

region, project number, and intersection name. Regions were contacted to verify the installation 

and obtain more detailed information regarding the installation date and project cost. It was 

later determined that some of these intersections had protected left-turn signal phasing; those 

with fully protected phasing were dropped from the study. Those sites with permissive, 

protected-permissive, or permissive-protected left-turn signal phasing were retained in the 

study, and additional information was obtained. In total, 12 locations were identified in 2 regions. 

Of these, 10 were type 1 and only 2 were type 2. One region provided detailed cost information 

for each project. 

Reference Sites

WisDOT provided a database of all state-maintained signalized intersections. This database was 

used to identify possible reference sites that were similar to the intersections included in the 

treatment group, but these sites did not have offset improvements or other recent safety 

improvements. The final reference group was selected based on the availability of traffic volume 

and intersection data. 

Roadway Data

Roadway data were collected for both strategy sites and reference sites. WisDOT provided an 

electronic roadway inventory. The inventory provided the urban/rural designation, illustrated 

whether the roadway was divided or undivided, and indicated the presence of a shoulder. Specific 

intersection geometry, such as lane designations, was obtained from aerial photography. 

Traffic Data

Traffic volumes were obtained from county AADT maps. Traffic volumes were not available for 

every year during the study period, and the years of available traffic counts varied by site. Linear 

interpolation was used to fill in years where counts were not available. 

Crash Data

WisDOT provided electronic crash data from 1994 to 2006. The crash data were matched to the 

strategy and reference sites using the major and minor road names. A radius of 76.25 m (250 ft) 

was used to identify crashes at both the strategy and reference intersections. 
21 

DATA CHARACTERISTICS AND SUMMARY 

Table 4 provides the definitions of crash types used. For the Florida data, the vehicle direction 

and movement were used to identify specific target crashes occurring on the treated roadway. The 

crash data from Nebraska did not include information on each vehicle involved prior to 2002. As 

such, the left-turn and rear-end crashes could not be restricted to only those occurring on the 

treated roadways, and all crashes identified as rear-end and left-turn were included. The 

Wisconsin crash data only included crashes coded to the mainline (treated approaches) and did 

not include the initial vehicle direction. As such, crash data in Wisconsin were coded as left-turn 

related if a vehicle was turning left and if the crash was not coded as rear-end. 

Table 4. Definitions of crash types. 

State Total Injury Left-turn Opposing Rear-end 

Florida All within 76.25 m 

(250 ft) of 

intersection and 

identified as atintersection or 

intersection-related 

K,A,B, and C 

on KABCO 

scale 

Vehicles 

approaching from 

opposite directions 

on treated roadway; 

one turning left, the 

other going straight 

Both vehicles 

approaching from 

same direction on 

treated roadway; 

both going straight 

or one proceeding 

straight and one 

turning left 

Nebraska All crashes 

identified as 

intersection-related 

K,A,B, and C 

on KABCO 

scale 

Defined as left-turn 

opposing anywhere 

in the intersection 

Defined as rearend anywhere in 

the intersection 

Wisconsin Only includes 

crashes on the 

mainline within 

76.25 m (250 ft) of 

the intersection 

K,A,B, and C 

on KABCO 

scale 

Includes any 

mainline crash with 

a left-turning vehicle 

and not coded as 

rear-end; includes 

crashes where one is 

proceeding straight 

and one is turning 

left 

On the mainline 

and defined as 

rear-end 

1 ft = 0.305 m 

Table 5 provides summary information for the data collected. It is shown that the total three-State 

sample adequately satisfies the requirements for a minimum sample size of 100 site-years and a 

desirable sample size of 263 site-years per period. Nebraska alone satisfies both requirements; 

Florida comes close to satisfying the minimum requirement; and Wisconsin is close to the 

minimum for the before period. It is also evident from table 5 that the crash frequencies and 

major road volumes for the Nebraska and Wisconsin treatment sites are reasonably similar and 

substantially lower than the values for Florida. 

The information in table 5 should not be used to make simple before-after comparisons of crashes 

per site-year. Such an analysis would not account for factors, other than the strategy, that may 
22 

cause a change in safety between the before and after periods. Such comparisons are properly 

done with the EB analysis, as presented in subsequent sections. 

Table 5. Data summary for treatment sites. 

Variable Florida Nebraska Wisconsin 

Number of sites 13 92 12

Site-years before 120.6 644.0 87.0

Site-years after 72.8 368.0 33.0

Crashes/site/year before 11.1 7.1 7.7

Crashes/site/year after 12.9 7.6 4.7

Injury crashes/site/year before 6.5 3.9 3.5

Injury crashes/site/year after 6.5 3.9 1.9

Left-turn opposing 

crashes/site/year before 

1.8 1.2 3.3 

(all left-turn)

Left-turn opposing 

crashes/site/year after 

1.5 1.9 1.8 

(all left-turn)

Rear-end crashes/site/year before 2.3 3.1 2.2

Rear-end crashes/site/year after 3.7 3.6 1.5

Major road AADT before Avg 43,237

Min 28,587

Max 58,051

Avg 20,454

Min 3,067

Max 38,300

Avg 19,548

Min 8,175

Max 33,050

Major road AADT after Avg 45,960

Min 35,514

Max 56,509

Avg 22,878

Min 5,800

Max 37,300

Avg 18,892

Min 7,150

Max 29,200

Minor road AADT before Avg 10,506

Min 1,582

Max 36,731

Avg 10,605

Min 1,500

Max 20,267

Avg 7,028

Min 2,300

Max 12,575

Minor road AADT after Avg 11,145

Min 1,810

Max 34,999

Avg 11,836

Min 3,100

Max 24,800

Avg 6,668

Min 2,200

Max 13,350

Note: Thirty-seven intersections in Nebraska, one in Florida, and two in Wisconsin did not have minor road 

volumes available. 
23 

DEVELOPMENT OF SAFETY PERFORMANCE FUNCTIONS 

This section presents the SPFs developed for each State. The SPFs were used in the EB 

methodology to estimate the safety effectiveness of this strategy.(9) Generalized linear modeling 

was used to estimate model coefficients using the software package SAS® assuming a negative 

binomial error distribution, which was consistent with the state of research in developing these 

models.(10)

SPFs were calibrated separately for Florida, Nebraska, and Wisconsin using the corresponding 

reference sites from each State. The primary form of the SPFs is as follows: 

Crashes/year = α(MajAADT)β1

(MinAADT)β2 (7) 

Where minor road AADT data are unavailable, the primary form of the SPFs is as follows: 

Crashes/year = α(MajAADT)β1 (8) 

Where: 

MajAADT = The average daily traffic on the major roadway. 

MinAADT = The average daily traffic on the minor roadway. 

α ,β1 ,β2 = Parameters estimated in the SPF calibration process. 

The SPFs without minor road AADT (equation 8) were calibrated only for those sites without 

minor road AADT data. 

In specifying a negative binomial error structure, the dispersion parameter, k, was estimated 

iteratively from the model and the data. The dispersion parameter relates the mean and variance 

of the SPF estimate and is used in equations 3 and 4 of the EB procedure. For a given dataset, 

smaller values of k indicate relatively better models. 

The safety performance functions developed are presented in appendix A. Note the following in 

interpreting the output: 

• The value of α used in equations 7 and 8 is obtained as e (α)

, where α is from the SPF 

model output. 

• The value of the parameter k is used in the EB approach. 

• The P-value gives the level at which the estimate is significant. For example, 

P-value = 0.05 indicates that the parameter estimate is statistically significant at the 

5-percent level (or, alternatively, that the 95-percent confidence interval does not 

include 0). 

25 

RESULTS 

Results were obtained for several analyses. Each State was analyzed separately, and a composite 

effect was obtained for all sites in each State. The most pertinent and insightful results were 

obtained by conducting a disaggregate analysis of the Nebraska sites where sites were grouped by 

various characteristics to evaluate the impact of these variables on the safety effectiveness. A 

disaggregate analysis was not conducted for Florida or Wisconsin because of the small sample 

sizes. Florida and Wisconsin sites could not be combined with Nebraska for the disaggregate 

analysis because of the substantially higher AADTs and crash frequencies in Florida and the 

predominance of type 1 installations in Wisconsin compared to the other two States. 

AGGREGATE ANALYSIS 

The aggregate results for each State individually are shown in table 6 through table 8. Combined 

effects for the three analyses were not estimated because the treatments and their effects vary 

significantly among the jurisdictions. The tables show the EB estimate of the crashes expected in 

the after period if the treatment had not been installed, the actual number of crashes in the after 

period, and the change in safety. The change in safety is the estimated percent reduction in 

crashes due to the strategy along with the standard error (SE) of this estimate. A negative percent 

reduction indicates an increase in crashes. If the magnitude of the percent change is at least 1.96 

times greater than the SE, the change is statistically significant at the 95-percent confidence level. 

Similarly, if the percent change is at least 1.64 times greater than the SE, the change is 

statistically significant at a 90-percent confidence level. Safety effects that are significant at the 

95-percent confidence level are denoted by bold text, which begins starting with table 8. 

Table 6. Results for Florida strategy sites. 

Total Injury 

Left-turn 

Opposing Rear-end 

EB estimate of crashes expected 

in the after period without 

strategy 

969.91 471.66 118.78 257.89

Count of crashes observed in the 

after period 

938 472 106 273

Estimate of percent reduction 3.4 0.2 11.4 -5.3

SE of estimate of percent 

reduction 

4.7 6.6 11.2 9.9

Note: A negative sign (-) indicates an increase in crashes. 
26 

Table 7. Results for Nebraska strategy sites. 

Total Injury 

Left-turn 

Opposing Rear-end 

EB estimate of crashes expected 

in the after period without 

strategy 

2,795.81 1,536.12 478.96 1,248.64

Count of crashes observed in the 

after period 

2,811 1,441 695 1,335

Estimate of percent reduction -0.5 6.2 -45.0 -6.9

SE of estimate of percent 

reduction 

2.4 3.0 6.7 3.6

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

significant at the 95-percent confidence level.

Table 8. Results for Wisconsin strategy sites. 

Total Injury Left-turn Rear-end 

EB estimate of crashes expected 

in the after period without 

strategy 

233.77 95.88 94.85 72.76

Count of crashes observed in the 

after period 

155 62.0 59 50

Estimate of percent reduction 33.8 35.6 38.0 31.7

SE of estimate of percent 

reduction 

6.0 9.0 8.9 10.9

Note: Bold denotes those safety effects that are significant at the 95-percent confidence level. Unlike the 

case for the Nebraska and Florida, left-turn opposing crashes could not be precisely identified; thus, the 

analysis included all non rear-end crashes involving a left-turning vehicle. 

Left-turn and rear-end crashes were selected as target crash types for this strategy. As presented 

in table 4, varying definitions of left-turn crashes were selected as the primary target crash types 

for this strategy. A strict definition of left-turn opposing crashes, in which one vehicle is turning 

left while the second is proceeding straight on the treated roadway, was not possible in Nebraska 

or Wisconsin due to constraints with the data. Rear-end crashes were identified as a possible 

target crash with the belief that they may be affected by improved left-turning vehicle operation. 

Similarly, the definitions of rear-end crashes varied as shown in table 4. Separate analyses were 

completed for left-turn and rear-end crashes to determine potential differential effects. A separate 

analysis was also completed for injury crashes (i.e., fatal plus all injury crashes) to determine the 

effects of this strategy on crash severity. 
27 

The results among the three States are relatively different. The following points summarize the 

results for the individual State analyses: 

• In Wisconsin, where the analysis focused on crashes involving mainline vehicles only and 

where the installations were predominantly type 1 (positive offset), the results indicated 

substantial and highly significant crash reductions in all categories—total, injury, leftturn, and rear-end. 

• The Nebraska and Florida results show little or no evidence that this strategy was 

effective overall (i.e., for total crashes) in these jurisdictions. Installations in these States 

were mostly type 2 or type 3 (no offset or negative offset). 

• For left-turn opposing crashes, a reduction in crashes was found in Florida, although it 

was not significant. The opposite, a highly significant increase in left-turn opposing 

crashes, was found in Nebraska. This is likely because the positive effects for sites where 

the strategy may have been worthwhile and warranted were diluted by effects at those 

where the strategy may not be justified by a specific safety concern. In particular, a 

majority of all signalized intersections in the Nebraska study area were treated, raising the 

possibility that at some intersections, this strategy may not have been justified. In 

addition, the surprisingly negative effects for left-turn opposing crashes may have been 

due to the inability to control possible increases in left-turn traffic because turning 

movement counts were unavailable. 

• For rear-end crashes, there was a small, insignificant increase in Florida. In Nebraska, 

rear-end crashes also increased slightly, and this increase was statistically significant. 

DISAGGREGATE ANALYSIS 

A disaggregate analysis was completed for Nebraska, the only State with a sufficient sample size 

to facilitate this analysis. The objective was to determine if safety effects were more or less 

pronounced for specific conditions. Total crashes were the only crash type included in the 

disaggregate analysis because of the counterintuitive results for left-turn opposing crashes. Also, 

there were fewer target crashes than total crashes, which reduced the certainty of a disaggregate 

analysis. While the disaggregate analyses indicated significant crash reductions for specific 

circumstances, these estimates were based on limited sample sizes and were not intended to be 

used as individual crash reduction factors. The results of the disaggregate analysis are 

summarized below. 

Offset Improvement Type

The most obvious variable to examine in the disaggregate analysis is the type of offset 

improvement (i.e., type 1, type 2, or type 3). However, the effects by group were not statistically 

significant. Also, the differential effects were not statistically different from each other. 

Expected Number of Crashes

The variable found to be most related to the safety effectiveness was the expected number of 

crashes in the before period. Analysis revealed that the percent reduction in crashes increased as 
28 

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. 
29 

ECONOMIC ANALYSIS 

The economic analysis was conducted from two perspectives. First, a benefit-cost ratio was 

estimated for Wisconsin, the only State for which an overall safety benefit was detected. Second, 

the disaggregate analysis for Nebraska was used to identify the level of expected number of 

crashes that would yield a crash reduction to justify the construction costs. 

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.(11) Wisconsin was the only 

State for which a safety benefit was detected overall and for which a benefit-cost ratio could 

be estimated. Using recent FHWA comprehensive crash cost estimates for signalized 

intersections with approach speeds less than 72.45 km/h (45 mi/h), the unit crash costs were 

identified as $15,788 for head-on crashes and $23,872 for rear-end crashes.(8) These estimated 

costs include all severities combined. 

The results in table 8 suggest a reduction of 2.39 total crashes per site-year (subtracting the 

observed crashes in the after period from the expected crashes and dividing by the total site-years 

from table 5). Using the lower comprehensive crash cost of $15,788 as a conservative estimate 

of the benefit, the 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. 

Nebraska installations were mostly of the type 2 and type 3 variety, typically involving a simple 

modification of an existing left-turn bay that was 45.75 m (150 ft) long, for which the striping 

was reconfigured with thermoplastic to improve the offset. The estimated cost was $200 per 

approach with an 8- to 10-year service life depending on traffic volumes. Florida installations 

were more elaborate versions of the type 2 and type 3 installations, involving the shifting of the 

left-turn lanes further into the median (on divided highways). The estimated cost is $15,000 per 

approach, assuming a left-turn bay length of 45.75 m (150 ft) and a lane width of 3.66 m (12 ft). 

The service life is estimated to be 14 years. 

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 so minimal that it can be assumed that this 

measure can be implemented at any signalized intersection with sufficient left-turn traffic to 

justify a left-turn lane and permissive or protected-permissive phasing. Therefore, the focus of the 

economic analysis was to determine the crash reduction factor and crash frequency levels that 

would justify the more expensive type 2 and type 3 treatments, similar to the Florida installations, 

which would involve minor construction. Based on a discount rate of 2.7 percent as suggested by 

OMB for a 10- to 15-year service life and assuming that four approaches are reconstructed, the 

annualized cost is $5,067.(11) 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, can be converted into crash frequency using the 

recent FHWA comprehensive crash cost estimates.(8) As before, for signalized intersections with 

approach speeds less than 72.45 km/h (45 mi/h), the unit costs for crashes of all severities 
30 

combined range from $15,788 for head-on crashes to $23,872 for rear-end crashes. Again, using 

the lower crash cost 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/$15,788 = 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, for which the crash reduction factor is at least 8 percent. Thus, the installation of the type 2 

and type 3 varieties of this strategy through reconstruction similar to the Florida installations 

appears to be cost-effective at intersections with at least nine expected crashes per year and where 

left-turn lanes are justified by traffic volume warrants. 
31 

SUMMARY AND CONCLUSIONS 

This study used the EB methodology to evaluate the safety effectiveness of implementing 

offset improvements for left-turn lanes at signalized intersections. 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. Many of the installations in Florida and Nebraska did not 

result in a positive offset. Nebraska installations mostly involved a simple modification of 

an existing left-turn bay for which the striping which was reconfigured with thermoplastic was 

used to narrow the existing left-turn lane, thereby shifting left-turn vehicles further to the left and 

improving the offset. 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 positive offset. 

In Wisconsin, the results indicated substantial and highly significant crash reductions in all 

categories—total (34 percent), injury (36 percent), left-turn (38 percent), and rear-end 

(32 percent). The aggregate results for Florida and Nebraska showed little or no evidence that 

would suggest that this strategy is effective for reducing total crashes. While the results for 

Nebraska indicated a significant reduction in injury crashes, they also showed an increase in 

target crashes, including a 45-percent increase in left-turn opposing crashes and a 7-percent 

increase in rear-end crashes. The minimal effect on total crashes in Nebraska may be due to 

the fact that the strategy was implemented at a majority of signalized intersections in the 

Nebraska study area, including sites where an improvement may not be justified based on a 

specific safety concern. Thus, the positive effects where the strategy is worthwhile and 

warranted may be diluted by the effects at sites where the strategy is not justified. Potential 

increases in left-turn volumes could also explain the significant increase in left-turn crashes 

in Nebraska. Overall, the large variation in installation methods in the three States may explain 

the large difference in observed effects. 

The disaggregate analysis revealed that the percentage reduction in crashes increased as the 

expected number of crashes increased. For example, the 30 sites in Nebraska with an expected 

frequency of 9 or more crashes per year in the before period had an 8-percent reduction in crashes 

(significant at the 5-percent level) compared to an insignificant 0.5-percent increase in total 

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. 

On the basis of this disaggregate analysis, the economic analysis sought to identify the level of 

expected number of crashes that would yield a crash benefit that would justify the construction 

cost. Based on this analysis, type 2 or type 3 installations through reconstruction, as was 

undertaken in Florida, are cost-effective at intersections with at least nine expected crashes per 

year, for which the expected reduction in crashes is at least 8 percent. Needless to say, the 

provision of left turn lanes should be justified by traffic volume warrants. The crash reduction 

factors found in this study are summarized in table 9. 
32 

Table 9. Summary of crash reduction factors. 

Installation 

Type 

Data 

Source 

Crashes 

per Year 

Before 

Offset 

Type of Offset 

Improvement 

Level of 

Effort 

Crash 

Reduction 

Factor SE 

Total 33.8 6.0

Injury 35.6 9.0

Left-turn 38.0 8.9

Type 1 Wisconsin

12 sites

7.5 Original offset: 

none or negative 

Final offset: 

positive 

Major 

construction

Rear-end 31.7 10.9

Note: Crashes in Wisconsin were identified as mainline crashes only. Left-turn crashes were defined as all crashes 

involving a left-turning vehicle that were not coded as rear-end. 

This study did not address offset improvements at unsignalized intersections. While there are 

examples of offset improvements at unsignalized intersections in the United States, these results 

should not be extrapolated to that situation. Rather, it is more appropriate to conduct a separate 

evaluation once there are sufficient installations at unsignalized intersections.
33 

APPENDIX: SAFETY PERFORMANCE FUNCTIONS 

Table 10. Florida total. 

Crashes/year = α(MajAADT)β1

(MinAADT)β2 Crashes/year = 

α (MajAADT)β1

Parameter Estimate SE P-value Estimate SE P-value 

ln(α ) -12.7694 3.5721 0.0004 -9.9989 3.1435 0.0015

β1 0.8275 0.3220 0.0102 1.1192 0.2973 0.0002

β2 0.6865 0.2270 0.0025 

k 0.2596 0.0777 0.3087 0.0721 

Table 11. Florida injury. 

Crashes/year = α(MajAADT)β1

(MinAADT)β2 Crashes/year = 

α (MajAADT)β1

Parameter Estimate SE P-value Estimate SE P-value 

ln(α ) -10.8768 3.5227 0.0020 -7.9837 3.0636 0.0092

β1 0.6296 0.3126 0.0440 0.8800 0.2897 0.0024

β2 0.6499 0.2205 0.0032 

k 0.2504 0.0785 0.2970 0.0717 

Florida left-turn opposing: apply model for total crashes with a proportion of 11 percent. 

Table 12. Florida rear-end. 

Crashes/year = α(MajAADT)β1

(MinAADT)β2 Crashes/year = 

α (MajAADT)β1

Parameter Estimate SE P-value Estimate SE P-value 

ln(α ) -12.9905 5.0389 0.0099 -11.5082 3.8283 0.0026

β1 0.8555 0.4567 0.0611 1.1438 0.3619 0.0016

β2 0.5259 0.3098 0.0896 

k 0.5175 0.1567 0.4460 
34 

Table 13. Nebraska total. 

Crashes/year = α(MajAADT)β1

(MinAADT)β2 Crashes/year = 

α (MajAADT)β1

Parameter Estimate SE P-value Estimate SE P-value 

ln(α ) -14.9649 1.7405 < 0.0001 -8.7588 2.0685 <0.0001

β1 1.1117 0.1903 < 0.0001 1.0804 0.2143 <0.0001

β2 0.6689 0.0997 < 0.0001 

k 0.0950 0.2972 

Table 14. Nebraska proportions to apply to total crash model for other crash types. 

Crash Type Proportion 

Injury 0.54

Left-turn opposing 0.15

Rear-end 0.47

Table 15. Wisconsin total. 

Crashes/year = 

α(MajAADT)β1(exp)β2(totalthrumaj)

Parameter Estimate SE P-value 

ln(α ) -5.8564 1.5239 0.0001 

β1 0.7208 0.1678 < 0.0001 

β2 0.1421 0.0748 0.0575 

k 0.2446 0.0455

Note: totalthrumaj = total thru lanes on the major road. 
35 

Table 16. Wisconsin injury. 

Crashes/year = 

α(MajAADT)β1(exp)β2(totalthrumaj)

Parameter Estimate SE P-value 

ln(α ) -5.516 1.6023 0.0006 

β1 0.5711 0.1754 0.0011 

β2 0.2016 0.0757 0.0077 

k 0.2455 0.051

Note: totalthrumaj = total thru lanes on the major road.

Table 17. Wisconsin left-turn. 

Crashes/year = 

α(MajAADT)β1(exp)β2(totalthrumaj)

Parameter Estimate SE P-value

ln(α ) -4.7532 2.1052 0.024 

β1 0.4406 0.2309 0.0563 

β2 0.2538 0.1034 0.0141 

k 0.425 0.0842 

Note: totalthrumaj = total thru lanes on the major road.

Table 18. Wisconsin rear-end. 

Crashes/year = α (MajAADT)β1

Parameter Estimate SE P-value 

ln(α ) -11.111 1.8213 < 0.0001 

β1 1.217 0.1841 < 0.0001 

k 0.3931 0.0763

Note: totalthrumaj = total thru lanes on the major road.

37 

ACKNOWLEDGEMENTS 

The project team gratefully acknowledges the participation of the following organizations and 

individuals for their assistance in this study: 

• Broward County, FL, particularly John Kleinedler and Marilda Hoover. 

• Dr. Aemal J. Khattak, Associate Professor from the University of Nebraska-Lincoln. 

• Hillsborough County, FL, particularly John Patrick. 

• The city of Daytona Beach, FL, particularly Will Landon. 

• The city of Lincoln, NE, particularly Virendra Singh. 

• The city of Omaha, NE, particularly Glenn Hansen. 

• The city of Tampa, FL, particularly Mike Scanlon. 

• The FDOT, particularly Pat Brady, Joe Santos, Jonathan Overton, Dwayne Kile, Bijan 

Behzadi, Jose Guerrero, Holly Walker, Tony Nosse, Fawzi Bitar, Scott Hardee, Mary 

Sheets, and Mark Chianese. 

• The NDOR, particularly Bob Grant and Sean Owings. 

• The University of Wisconsin, particularly Dr. David Noyce. 

• The WisDOT, particularly Scott Nelson, Joyce Dimmer, Chris Quesnell, Greg Helgeson, 

Scott Erdman, and Mary McFarlane. 

39 

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. 

3. Federal Highway Administration. (2003). The Manual on Uniform Traffic Control Devices

(MUTCD). Washington, DC. Obtained from: 

http://mutcd.fhwa.dot.gov/pdfs/2003r1r2/pdf_index.htm. Site last accessed June 19, 2008. 

4. Khattak, A.J., Naik, B., and Kannan, V. (2004). Safety Evaluation of Left-Turn Lane Line 

Width at Intersections with Opposing Left-Turn Lanes. NDOR Research Project Number 

SPR-P1(03) P554. Nebraska Department of Roads. 

5. McCoy, P.T., Navarro, U.R., and Witt, W.E. (1992). Guidelines for Offsetting Opposing LeftTurn Lanes on Four-Lane Divided Roadways. Transportation Research Record 1356. 

Transportation Research Board. Washington, DC. 

6. Potts, I., Stutts, J., Pfefer, R., Neuman, T.R., Slack, K.L., and Hardy, K.K. (2004). “Guidance 

for Implementation of the AASHTO Strategic Highway Safety Plan: A Guide for Reducing 

Collisions Involving Older Drivers.” NCHRP Report 500 Series, Vol. 9. Transportation 

Research Board. Washington, DC. 

7. Staplin, L., Harkey, D.L., Lococo, K.H., and Tarawneh, M.S. (1997). Intersection Geometry 

Design and Operational Guidelines for Older Drivers and Pedestrians, Volume I: Final 

Report. FHWA-RD-96-132. Federal Highway Administration. Washington, DC. 

8. 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. 

9. 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. 

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

11. Office of Management and Budget. “Discount Rates for Cost-Effectiveness, Lease Purchase, 

and Related Analyses.” Obtained from: 

http://www.whitehouse.gov/omb/circulars/a094/a94_appx-c.html. Site last accessed 

December 3, 2008. 
HRDS-06/06-09(WEB)E