TECHBRIEF Safety Evaluation of 

Lane and Shoulder 

Width Combinations on 

Rural, Two-Lane, 

Undivided Roads 

FHWA Publication No.: FHWA-HRT-09-032

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 Lane and Shoulder 

Width Combinations on Rural, Two-Lane, Undivided Roads 

(FHWA-HRT-09-031), which will be available online at http://

www.tfhrc.gov/safety.

Objective

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 is 

to evaluate the safety effectiveness of several low-cost safety 

strategies presented in the National Cooperative Highway 

Research Program (NCHRP) Report 500 Series. Although not 

identified in the NCHRP Report 500 Series, one of the strategies 

selected for evaluation in the pooled fund study was the 

allocation of lane and shoulder width for fixed pavement widths 

on rural, two-lane, undivided roads (i.e., given a fixed roadway 

width for a rural, two-lane, undivided road, is it safer to provide 

wider shoulders or wider lanes?). The safety effectiveness of 

various allocations of total paved width had not previously been 

thoroughly documented, and this study is an attempt to provide 

an evaluation through scientifically rigorous procedures.

Introduction

State and local agencies are often faced with a decision of how 

to enhance safety on rural, two-lane roads when the total paved 

width is to remain the same. More than 42,000 fatalities occur 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.tfhrc.gov
2

annually on roadways in the United States. 

Nearly 60 percent of these fatalities are related to 

roadway departure crashes, 50 percent of which 

occur on rural, two-lane roads. (1) One option for 

addressing roadway departure crashes without 

adjusting the total paved width is to reconfigure 

the combination of lane and shoulder width. 

As an example of the question at hand, given 

a 10.97-m (30-ft) total paved width, is it safer to 

provide 3.66-m (12-ft) lanes with 0.91-m (3-ft) 

shoulders or 3.35-m (11-ft) lanes with 1.22-m 

(4-ft) shoulders? The application of this strategy 

is most likely to be incorporated during new 

construction or during a resurfacing project. It is 

not likely that an agency would restripe an existing roadway as a standalone treatment.

Research regarding the safety implications of 

lane and shoulder width has been conducted 

for over 20 years and remains of interest today. 

Among the more recent activities in this area is 

the work of an expert panel to review previous 

research results for inclusion in the Interactive 

Highway Safety Design Model (IHSDM). Several 

studies were reviewed to develop crash modification factors (CMFs)1

 for lane width and shoulder width.(2,3,4) The studies used a variety of data 

analysis techniques and statistical tools in an 

attempt to derive the effect of lane and shoulder 

width on safety. The CMFs developed by the 

expert panel were later adopted by the Highway 

Safety Manual (HSM).

 1 A crash reduction factor (CRF) indicates the 

percent crash reduction that might be expected 

after implementing a given countermeasure. 

A crash or accident modification factor (CMF 

or AMF, respectively) is a multiplier to adjust 

the number of expected crashes based on the 

estimated safety benefit for a particular countermeasure. The CMF or AMF is calculated as 

1-(CRF/100).

The literature review indicates that there is 

fairly substantial evidence of the benefits of 

adding shoulders to nearly all lane widths. 

Work by the expert panel for the HSM and 

IHSDM has established CMFs for lane width 

and shoulder width individually; however, they 

do not address the issue of the optimal lane 

and shoulder width combination given a fixed 

total paved width. The safety effectiveness of 

various lane-shoulder width combinations for a 

fixed paved width is explored empirically in this 

study to provide better support to the States 

when selecting a lane-shoulder combination 

for a given paved width on rural, two-lane, 

undivided roads.

Methodology

Geometric, traffic, and crash data were obtained 

for the entire population of rural, two-lane, 

undivided road segments in Pennsylvania 

(1997–2001 and 2003–2006) and Washington 

(1993–1996 and 2002–2003). Several crash 

types were investigated, emphasizing target 

crash types likely affected by lane and shoulder 

width. Analyses included the following: 

•	 All crashes on road segments (excluding 

intersection crashes).

•	 Target crashes on road segments, 

determined as the combination of the 

following:

•	 Run-off road.

•	 Head on.

•	 Sideswipe same direction.

•	 Sideswipe opposite direction.

A matched case-control analysis was conducted 

to determine the safety effectiveness of lane 

and shoulder width configurations for total 

paved widths from 7.92 to 10.97 m (26 to 36 ft) 

on rural, two-lane, undivided roads. Matching 
3

was used to control for the effects of average 

annual daily traffic (AADT) and segment length 

while other variables were included in the 

model as covariates to account for potential 

confounding effects. Segment lengths are 

not equal for all segments included in the 

analysis, but case-control pairs were matched 

on similar values of segment length.(5) The 

average segment lengths in Pennsylvania 

and Washington were approximately 0.80 

and 0.64 km (0.5 and 0.4 mi), respectively. For 

Pennsylvania, additional covariates included 

speed limit, unpaved right shoulder width, and 

regional indicators. For Washington, additional 

covariates included speed limit and indicators 

for horizontal and vertical curvature. For narrow 

pavement widths (less than 7.92 m (26 ft)), a 

supplemental analysis was conducted using 

data from Pennsylvania only.

One advantage of the case-control method 

is the careful adjustment for confounding 

variables such as AADT. For this study, an added 

benefit is the format and interpretation of the 

analysis. Conditional logistic regression models 

were applied to the matched case-control 

design, producing an odds ratio. The odds ratio 

represents the expected percent change in 

crashes compared with the baseline. The baseline is indicated by an odds ratio of 1.0. An 

odds ratio less than 1.0 represents an expected 

reduction in crashes while an odds ratio greater 

than 1.0 represents an expected increase in 

crashes (e.g., odds ratio of 1.10 indicates a 

10 percent increase in crashes compared with 

the baseline). The odds ratio can be interpreted 

as a CMF. This greatly facilitates the interpretation of complex modeling and produces 

results that are readily applied by practitioners. 

The full report includes a detailed explanation 

of the methodology, including a description of 

how to estimate the odds ratio.

Results

Wide Pavement Widths

A series of models were estimated for pavement 

widths from 7.92 to 10.97 m (26 to 36 ft). Separate 

models were developed for total crashes and 

target crashes; the process was repeated for 

both Pennsylvania and Washington datasets. 

The sample included relatively few segments 

with AADT less than 1,000 vehicles per day, 

and posted speeds were 40 km/h (25 mi/h) or 

greater. Therefore, results generally apply to 

rural, two-lane roads with AADTs greater than 

1,000 vehicles per day and posted speeds of 

40 km/h (25 mi/h) or greater. 

The objective of this study was to examine the 

results of specific lane-shoulder combinations. 

Considering Pennsylvania and Washington 

individually, some configurations show that 

additional lane width is favorable to additional 

shoulder width while others indicate the 

opposite trend. However, small sample sizes 

make it difficult to show a definitive trend for 

some configurations. As the results for each 

fixed pavement width are based on three 

lane-shoulder combinations, it is important to 

develop three reliable points to determine if a 

clear trend exists.

An additional analysis was undertaken to 

compare the study findings to the literature. 

Specifically, results were compared with the 

draft chapter on rural, two-lane roads from the 

HSM and a report on design decisions by the 

Texas DOT (TxDOT) that includes an analysis of 

lane and shoulder width.(6,7) The HSM considers 

lane and shoulder width separately and does 

not consider the interaction between the two 

variables.(6) The HSM implicitly assumes that 

a given shoulder width would have the same 

effect on safety for roadways with different lane 

widths (e.g., a 1.22-m (4-ft) shoulder would have 

the same effect on safety for roadways with a 
4

3.05-m (10-ft) lane as roadways with a 3.66-m 

(12-ft) lane). The TxDOT report explicitly considers interactions between lane width and shoulder width in the model formulation, allowing the 

effects of shoulder width to vary across different 

lane widths.(7) The approach adopted for this 

research also explicitly considers the interaction 

between lane and shoulder width by obtaining 

separate estimates of safety effectiveness for 

each lane-shoulder pair.

The findings from the three studies are compared in table 1, and a CMF value is selected for 

each lane-shoulder configuration. The selected 

values represent a compromise that emphasizes findings from this study. In cases where 

the CMF from this study had an insufficient sample size (i.e., number of segments), the TxDOT 

CMF was used to provide an estimated value. If 

there was a choice between using the CMF from 

Pennsylvania or Washington, the one with the 

larger sample size was chosen; this was always 

the Pennsylvania estimate. 

The last column in table 1 presents the selected 

CMFs for various lane-shoulder configurations 

compared with a baseline configuration with 

3.66-m (12-ft) lanes and 1.83-m (6-ft) shoulders. 

As a result of combining the research findings, 

there is a more apparent trend with respect to 

the optimal configuration of lane and shoulder 

width for a given pavement width. For total 

paved widths from 7.92 to 9.14 m (26 to 30 ft), 

the CMF decreases only slightly as lane width 

increases from 3.05 to 3.35 m (10 to 11 ft). 

The CMF then decreases more substantially as 

Table 1. Comparison of study results with CMFs from literature.

Total Width

(m)

Lane 

Width

(m)

Shoulder 

Width

(m)

Sample 

Size

(PA)

PA

CMF

Sample 

Size

(WA)

WA

CMF

TxDOT 

CMF

HSM

CMF

Selected 

CMF from 

Research

and 

Literature

7.92

3.05 0.91 3,510 1.13 60 1.95 1.22 1.53 1.13

3.35 0.61 2,649 1.12 449 1.79 1.13 1.36 1.12

3.66 0.30 135 1.85 33 1.71 1.09 1.40 1.09*

8.53

3.05 1.22 2,799 1.20 20 0.75 1.18 1.46 1.20

3.35 0.91 4,188 1.19 673 1.43 1.10 1.29 1.19

3.66 0.61 882 1.16 120 1.15 1.07 1.30 1.16

9.14

3.05 1.52 382 1.22 17 1.25 1.15 1.38 1.15*

3.35 1.22 6,403 1.14 501 1.58 1.07 1.22 1.14

3.66 0.91 1,411 1.11 272 1.27 1.05 1.23 1.11

9.75

3.05 1.83 246 0.75 12 1.49 1.11 1.30 1.11*

3.35 1.52 1,893 1.06 178 1.22 1.05 1.14 1.06

3.66 1.22 2,986 1.04 529 1.07 1.03 1.16 1.04

10.36

3.05 2.13 54 1.96 1 0.00 1.08 1.24 1.08*

3.35 1.83 1,597 0.84 256 1.15 1.02 1.06 0.84

3.66 1.52 702 0.87 191 0.89 1.02 1.08 0.87

10.97

3.05 2.44 42 0.38 14 0.47 1.05 1.17 1.05*

3.35 2.13 188 0.90 115 1.08 1.00 1.00 1.00*

3.66 1.83 932 1.00 382 1.00 1.00 1.00 1.00

Note: Values in italics for Pennsylvania and Washington imply insufficient sample size for a reliable CMF. 

* Selected CMF was based on the TxDOT research. 

1 ft = 0.305 m 
5

lane width increases from 3.35 to 3.66 m (11 

to 12 ft). For total paved widths from 9.75 to 

10.97 m (32 to 36 ft), the most substantial reduction in the CMF occurs as lane width increases 

from 3.05 to 3.35 m (10 to 11 ft). Results indicate 

a light benefit to adding lane width compared 

with shoulder width for a given paved width.

Comparing the magnitude of CMFs across 

studies in table 1, it is clear that the HSM values are generally high, while the values from 

the Pennsylvania analysis and the TxDOT 

study are relatively similar in magnitude. The 

Washington CMFs are a bit more inconsistent, 

owing largely to lower sample sizes for many 

lane-shoulder combinations. Considering the 

differing methodologies and data employed, 

the CMFs from TxDOT and Pennsylvania are 

remarkably similar. 

Selected CMFs for Comparison within a Given 

Paved Width

The CMFs in table 1 represent the expected 

change in target crashes compared with a baseline scenario of 3.66-m (12-ft) lanes and 1.83-m 

(6-ft) shoulders. When comparing lane-shoulder 

configurations for a total paved width other than 

10.97 m (36 ft), it is necessary to adjust the CMFs 

by creating a new baseline. The CMFs in table 2 

have been adjusted to reflect a baseline scenario with 3.05-m (10-ft) lanes for each total paved 

width (i.e., CMF = 1.00). For fixed total paved 

widths from 7.92 to 10.97 m (26 to 36 ft), CMFs 

are shown to decrease as lane width increases 

(i.e., wider lanes and narrower shoulders appear 

to be the optimal configuration with respect to 

safety). Specifically, configurations with 3.66-m 

(12-ft) lanes appear to be the optimal configuration with respect to safety for total paved widths 

from 7.92 to 9.75 m (26 to 32 ft). For 10.36-m 

(34-ft) total paved widths, configurations with 

3.35-m (11-ft) lanes are associated with the lowest CMF. For 10.97-m (36-ft) total paved widths, 

the CMF is identical for both 3.35- and 3.66-m 

(11- and 12-ft) lanes.

Narrow Pavement Widths

This section presents results from Pennsylvania 

for narrow paved widths (i.e., 7.32 m (24 ft)). 

The odds ratios developed for the narrow pavement widths were not significantly different 

from the baseline. However, a clear relationship 

was exhibited between low-volume roadways 

and narrow pavement widths. The odds ratio 

was shown to increase nonlinearly with AADT. 

Also, the rate of increase differed for each laneshoulder configuration, validating the use of 

interaction terms in addition to the main effects 

in the model.

Results were estimated in relation to a baseline configuration with 3.66-m (12-ft) lanes and 

0-m (0-ft) shoulders. At low AADTs (less than 

1,000 vehicles per day), odds ratios were generally less than 1.0. When AADT was greater 

than approximately 1,000 vehicles per day, the 

odds ratios exceeded 1.0. These results are not 

statistically significant but are intended to show 

Table 2. Selected CMFs from table 1 for comparison within a given paved width.

Pavement Width (m) 3.05-m Lanes 3.35-m Lanes 3.66-m Lanes

7.92 1.00 0.99 0.96

8.53 1.00 0.99 0.97

9.14 1.00 0.99 0.97

9.75 1.00 0.95 0.94

10.36 1.00 0.78 0.81

10.97 1.00 0.95 0.95
6

the importance of considering AADT when estimating the odds ratio for low-volume, narrow 

roadways. Taken as a whole, these results indicate some reduction in the odds ratio by adding 

shoulder width compared with lane width, but, 

in general, only when AADT is very low.

Economic Analysis

Completion of the economic analysis requires 

careful consideration of the project scope. The 

objective of this study, Given a fixed roadway 

width for a rural, two-lane, undivided road, is it 

safer to provide wider shoulders or wider lanes?, 

implies the activity of interest is not a restriping 

project or a change in total paved width. Rather, 

for a given total paved width, there is a decision 

to be made regarding the allocation of lane and 

shoulder width. Given this description, the cost 

of alternatives is essentially equal; therefore, it 

is reasonable to just consider the benefits.

The CMFs presented in table 2 represent the 

expected percent change in target crashes 

compared with a configuration with 3.05-m 

(10-ft) lanes for given total paved widths. The 

most safety-effective configuration for a given 

paved width is indicated by the lowest CMF. 

Using the 9.75-m (32-ft) paved width as an 

example, the adjusted CMFs are 1.00, 0.95, and 

0.94 for the 3.05-, 3.35-, and 3.66-m (10-, 11-, 

and 12-ft) lane configurations, respectively. The 

3.66-m (12-ft) lane configuration is associated 

with the lowest CMF; therefore, a 3.66-m (12-ft) 

lane with a 1.22-m (4-ft) shoulder is the most 

safety-effective configuration within the 9.75-m 

(32-ft) paved width group. 

Crash cost savings are based on the expected 

reduction in crash frequency. To compute the 

expected reduction in crash frequency, the 

CMFs must be applied to actual crash data for 

a given location. Again, using the example of 

a 9.75-m (32-ft) paved width and the adjusted 

CMFs (table 2), the expected change in target 

crashes by reallocating the lane and shoulder 

width from a configuration with 3.05-m 

(10-ft) lanes and 1.83-m (6-ft) shoulders to a 

configuration with 3.66-m (12-ft) lanes and 

1.22-m (4-ft) shoulders would be a 6 percent 

decrease. Assuming a long-term expected

crash experience of 100 target crashes per year 

for the base condition, the configuration with 

3.66-m (12-ft) lanes and 1.22-m (4-ft) shoulders 

would yield a reduction of 6 crashes per year. 

Estimated crash costs are then applied to the 

expected change in crashes to estimate the 

annual dollar savings. Crash costs typically vary 

by State but can be estimated from the recent 

FHWA crash cost guide when State-specific 

crash cost data are not available.(8)

Summary and Conclusions

The objective of this study was to determine the 

safety effectiveness of specific combinations 

of lane and shoulder width on rural, two-lane, 

undivided roads. This strategy is intended to 

reduce the frequency of roadway departure 

crashes. Matched case-control analyses were 

applied to geometric, traffic, and crash data for 

road segments in Pennsylvania and Washington 

to estimate CMFs.

In general, results were consistent with previous 

research efforts, showing crash reductions for 

wider paved widths, wider lanes, and wider 

shoulders, all else being equal. More specific 

to the research objective, CMFs were provided 

for various lane-shoulder configurations. 

Individual State analyses did not indicate a 

clear preference for lane or shoulder width 

given a fixed paved width. However, the individual State analyses were supplemented with 

previous research, producing a more apparent 

trend as follows: 

•	 For 7.92- to 9.75-m (26- to 32-ft) total paved 

widths, 3.66-m (12-ft) lanes provide the 

optimal safety benefit. The CMF ranges 
7

from 0.94 to 0.97, indicating a 3–6 percent 

crash reduction for 3.66-m (12-ft) lanes 

compared with 3.05-m (10-ft) lanes. 

•	 For 10.36-m (34-ft) total paved width, 

3.35-m (11-ft) lanes provide the optimal 

safety benefit. The CMF for 3.35-m (11-ft) 

lanes is 0.78 compared with the 3.05-m 

(10-ft) baseline. 

•	 For 10.97-m (36-ft) total paved width, 3.35- 

or 3.66-m (11- or 12-ft) lanes provide the 

optimal safety benefit. The CMF is 0.95 

for 3.35- and 3.66-m (11- and 12-ft) lanes 

compared with the 3.05-m (10-ft) baseline. 

These results apply, in general, to rural, twolane roads with traffic volumes greater than 

1,000 vehicles per day and posted speeds of 

40 km/h (25 mi/h) or greater. While 3.66-m (12-ft) 

lanes appear to be the optimal design for 7.92- to 

9.75-m (26- to 32-ft) total paved widths, 3.35-m 

(11-ft) lanes perform equally well or better than 

3.66-m (12-ft) lanes for 10.36- to 10.97-m (34- to 

36-ft) total paved widths. There may be additional 

benefits to providing a 3.35-m (11-ft) lane width 

compared with a 3.66-m (12-ft) lane width for 

specific scenarios. For example, the American 

Association of State Highway and Transportation Officials recommends, as a minimum, 

that 0.61 m (2 ft) of the shoulder width should 

be paved to provide for pavement support, wide 

vehicles, collision avoidance, and additional 

pavement width for bicyclists.(9) The 3.35-m 

(11-ft) lane will also provide an extra 0.30 m (1 ft) 

of shoulder, given a fixed paved width, which 

may provide the necessary additional shoulder 

width to accommodate disabled vehicles. From 

a maintenance perspective, it may be desirable 

to provide 3.35-m (11-ft) lanes compared with 

3.66-m (12-ft) lanes to help keep drivers off of 

the edge of the pavement, particularly for very 

narrow total paved widths.

For narrow paved widths (i.e., 7.32 m (24 ft)), 

there is a slight reduction in crash odds by 

adding shoulder width compared with lane 

width, but only when traffic volume is very 

low (i.e., less than 1,000 vehicles per day). 

However, configurations with relatively narrow 

lanes may not be appropriate for roadways 

with notable truck traffic. This is due to issues 

related to off-tracking, where the rear wheels of 

trucks generally track inside the front wheels on 

horizontal curves. Therefore, the design vehicle 

should be considered when identifying potential 

lane-shoulder configurations.

This study updates earlier research on the safety 

effectiveness of lane and shoulder width. While 

there has been fairly substantial evidence of the 

benefits of adding shoulders to nearly all lane 

widths, previous work does not address the issue of the optimal lane-shoulder configuration 

for a given total paved width. Based on the 

results of this study, reallocating lane and 

shoulder width for a fixed total paved width 

can be a cost-effective treatment for reducing 

crashes on rural, two-lane, undivided roadways. 

Comparing the results of this study with previous research, it is apparent that the effects of 

lane and shoulder width should be considered 

in the context of each other (i.e., the CMF for 

a given shoulder width may not be applicable 

across various lane widths).

References

1. Traffic Safety Facts. National Highway Traffic 

Safety Administration National Center for 

Statistics and Analysis, Washington, DC, 

2006.

2. Griffin, L.I. and Mak, K.K. Benefits to be 

Achieved from Widening Rural, Two-Lane, 

Farm-to-Market Roads in Texas. Prepared for 

presentation at the Transportation Research 

Board 67th Annual Meeting, Washington, 

DC, 1987.
8

Researchers—This study was performed by Frank Gross and Kimberly Eccles of Vanasse Hangen Brustlin, 

Inc., with support from Paul P. Jovanis and Ko-Yu Chen of The Pennsylvania State University. For more 

information about this research, contact Roya Amjadi, FHWA Project Manager, HRDS, at (202) 493-3383, 

roya.amjadi@fhwa.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 e-mail 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—Safety, lane width, shoulder width, crash modification factor, geometric design, two-lane, 

case-control.

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 

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processes to ensure continuous quality improvement.

3. Zegeer, C.V., Deen, R.C., and Mayes, J.G. 

“Effect of Lane and Shoulder Widths on 

Accident Reduction on Rural, Two-Lane 

Roads.” Transportation Research Record, 

806, 33–42, Washington, DC, 1981.

4. Zegeer, C.V., Hummer, J., Reinfurt, D., 

Herf, L., and Hunter, W. “Safety Effects of 

Cross-Section Design for Two-Lane Roads.” 

Transportation Research Record, 1195, 20–32, 

Washington, DC, 1988.

5. Gross, F. Alternative Methods for Estimating 

Safety Effectiveness on Rural, Two-Lane 

Highways: Case-Control and Cohort Methods. Dissertation (PhD). The Pennsylvania 

State University, 2006.

6. Highway Safety Manual (draft), Prototype 

chapter, “Two-Lane Rural Highways,” 2008.

7. Bonneson, J., Lord, D., Zimmerman, K., 

Fitzpatrick, K., and Pratt, M. Development of 

Tools for Evaluating the Safety Implications 

of Highway Design Decisions. Report 

0-4703-4, prepared for Texas Department 

of Transportation, 2007.

8. Council, F., Zaloshnja, E., Miller, T., and 

Persaud, B. Crash Cost Estimates by 

Maximum Police-Reported Injury Severity 

within Selected Crash Geometries. Federal 

Highway Administration, McLean, VA, 2005.

9. American Association of State Highway and 

Transportation Officials (AASHTO). A Policy 

on Geometric Design of Highways and 

Streets, Washington, DC, 2004.

MAY 2009 FHWA-HRT-09-032

HRDS-06/05-09(1M)E