Research, Development, and Technology Turner-Fairbank Highway Research Center 300 Georgetown Pike, McLean, VA 22101-2296

www.tfhrc.gov

Crash Impact of Smooth Lane Narrowing 

with Rumble Strips at Two-Lane Rural 

Stop-Controlled Intersections

Introduction

According to the Fatality Analysis Reporting System, the 

proportion of fatal crashes at unsignalized rural intersections constitutes approximately 37 percent of all fatal 

crashes at intersections nationwide.(1) About 90 percent 

of these rural unsignalized intersection crashes occur on 

two-lane roads. As a low-cost remedy to address crashes 

at unsignalized intersections on two-lane rural roads, the

Federal Highway Administration (FHWA) developed and 

evaluated a treatment to reduce approach speeds by 

narrowing lanes using rumble strips in the median and 

on the right-lane edge. This narrowing was applied for 

about 150 ft on the major road approach of two-way 

stop-controlled (TWSC) intersections on high-speed rural 

roads. Eight experimental sites were retrofitted between 

2007 and 2008 in Missouri, Kentucky, Pennsylvania, 

Florida, and Maryland. Following the acquisition of at least

2 years of post-implementation crash data, pre- and postimplementation crash analysis was conducted to compare the performance of the new treatments. Results 

showed a 32 percent reduction in total crashes and a

34 percent reduction in fatal/injury crashes.

Background

In 2008, 22 percent of the 34,017 total fatal crashes in the 

United States (7,421) were intersection or intersectionrelated. Considering the 1,630,000 injury crashes that 

occurred in 2008, 45 percent of these (733,000) were intersection and intersection-related crashes. From the fatal 

injury crashes in 2008, 37 percent occurred at unsignalized rural intersections.

As part of an effort to reduce crashes at unsignalzed

rural intersections, this study evaluates a safety treatment for TWSC intersections. The intersection treatment 

was constructed at a limited number of high-speed twolane TWSC intersections in five States. Most of these 

intersections are in rural areas and have low traffic volumes and low levels of enforcement. The treatment 

was designed to induce drivers to reduce speeds when 

approaching such intersections. The rumble strips were 

applied on the edge of shoulders and also within a 

painted yellow median island on major road approaches. 

This treatment narrowed the smooth lane surface in 

both directions (see figure 1). It should be noted that 

this study is a continuation of an FHWA summary report 

(FHWA-HRT-08-063) that includes most of the same 

sites and already presents speed reduction analysis 

and raw crash data findings.(2)

Crash data for this study were collected from intersections where the treatment was introduced several 

years before implementation and at least 2 years after 

implementation (the number of pre- and post-implementation years differed according to site). These preand post-treatment crash data were then compared to 

TECHBRIEF

FHWA Publication No.: FHWA-HRT-10-047

FHWA Contact: Joe Bared, HRDS-05, (202) 493-3314, joe.bared@dot.gov

Figure 1. Lane narrowing using rumble strips at a rural 

Pennsylvania site.
2

determine whether there was a significant difference 

and to estimate the magnitude of any treatment effects. 

The data were analyzed using an empirical Bayes (EB) 

analysis, which is the preferred method for examining 

these types of comparisons.

The preliminary study indicated that the treatment sites

experienced, on average, an 85 percent speed reduction

of 4.5 mi/h for all vehicles and 4.8 mi/h for trucks. In

figure 2, speed reductions were measured at points 2 and 4.

Typical Field Design Features

The main feature of this safety treatment is to narrow 

lanes by applying rumble strips. A design template 

that was used in field applications is shown in figure 3. 

A median island is formed by pavement markings in 

conjunction with rumble strips placed between the two 

travel lanes of the major road. Rumble strips are also 

applied on the right side within the existing pavement 

width in the shoulder, as shown in figure 4. Rumble 

strips reduce the smooth travel lane widths on the 

major approaches for about 150 ft before and after the 

intersection and encourage drivers to slow down 

as they reach the intersection. Field applications in 

Pennsylvania cost between $50,000 and $70,000, 

while other States are much less expensive. Data from 

other States show that the implementation of the treatment costs between $10,000 and $30,000, excluding 

construction costs unrelated to the treatment.

Figure 3 shows that the major road lane is reduced 

from 12 to 9 ft measured from the inside edge of the 

pavement markings. Edge markings could be placed on 

the edge of the rumble strips or directly on the strips. 

In most applications, the effective lane width is 10 ft 

when measured from the inside edges of the rumble 

strip. Three distinct regions are created when this 

treatment is applied—labeled as sections A, B, and C in 

figure 3. It is important to note that drivers are warned 

of the upcoming narrow lane area through signs placed 

at the beginning of section A. Section B is called the 

taper section, which is where the lane narrowing 

begins. The median width in section B starts at 0 ft and 

increases to the full width, which is typically 4–6 ft. 

Table 1 provides an estimate of the length of each section 

based on speed limit.

In figure 3, the median and shoulders are installed 

with milled rumble strips. The strips are present throughout section B and end in section C 50 ft before the intersection. Two rows of rumble strips may be necessary 

to cover the median area (see figure 5). Rumble strips 

are not applied for the last 50 ft so that vehicles will not 

be forced to travel on the rumble strips while turning. 

The edge rumble strips start 50 ft before section B and 

continue throughout sections B and C. The application 

of rumble strips is shown in figure 3.

Prior to acquiring field data, various local and State 

agencies were approached to determine their interest in

carrying out a study on intersection treatments. The 

selected sites were required to meet one or more of the 

following criteria:(2)

• Detection of the upcoming intersection was difficult 

for approaching drivers.

• Speeding was identified as a problem at the intersection (measured speeds exceeded the established 

criteria, and crash patterns indicated speed-related 

causes).

• Noncompliance at stop signs was frequently 

observed at the intersection.

Most agencies responded favorably, and the treatments 

were applied at eight sites in five States. The intersection 

sites were chosen by the participating agencies based on 

typical stop control on the minor road with high crash 

frequencies and on the availability of data. Of the eight 

sites selected, seven are TWSC intersections, and one is 

a T-intersection. The site details are shown in table 2.

Table 1. Section lengths for various speeds.

Speed 

(mi/h)

Section A 

(ft)

Section B 

(ft)

Section C 

(ft)

45–55 100 200 150

60 150 200 150

Figure 2. Speed data collection points.
3

Analytical Methods and Findings

One way to estimate the safety impact of a treatment 

is to compare pre- and post-treatment crash frequencies and also to compare these data with historical data 

from reference groups that received no treatment. For 

the purpose of this evaluation, pre-treatment crash 

data were collected for 3–5 years, and post-treatment 

data were collected for 2–3 years depending on the site. 

Due to the short data collection periods and the small 

sample size (eight sites), the safety of the treated intersections could not be determined with sufficient 

certainty based on site-specific crash data alone. However, to increase the precision of the safety estimates, 

the EB method was used. The method increased the 

precision of estimates by correcting for the regressionto-the-mean bias common to certain other inferential 

methods. The EB method compared the selected intersections with other similar intersections using a safety 

performance function (SPF). The SPFs were regression 

models for estimating the predicted average crash frequency of individual roadway sections or intersections.(3)

The SPFs used in this study were developed from 

observed crash data of a set of similar sites.

Data Handling Procedures

Crash data from 2001 to 2009 were collected for most of 

the intersections. At least 3 years of pre-treatment data 

and 2 years of post-treatment data were collected for all 

intersections. For several intersection sites, 5 years of 

pre-treatment data were collected. Average annual daily 

traffic (AADT) was also collected for application of the 

EB method. 

The crash data were provided by the participating State 

highway agencies (see table 3). Note that for all eight 

intersections, there were less post-treatment period 

crashes than pre-treatment period crashes per year.

The AADT for the pre- and post-treatment periods were 

obtained from each participating State transportation 

department Web site. In some cases, the AADTs were 

not available for every year. Missing AADTs were estimated using the recommendations given by the Highway 

Safety Manual (HSM).(3) The following rules from the 

HSM were followed for estimating missing AADT values:

• If AADT data were available for only a single year, 

that same value was applied to all years of the 

pre-treatment period.

• If 2+ years of AADT data were available, the AADTs 

for the intervening years were computed using 

interpolation.

• The AADTs for years prior to the first year for which 

data were available were assumed to be equivalent 

to the AADT for that first year.

• The AADTs for years following the last year for 

which data were available were assumed to be 

equivalent to the last year.

Table 2. Site details.

Site Major Road

Type of 

Intersection

PA 1 SR 989 TWSC

PA 2 SR 85 TWSC

MO 1 MO 114 TWSC

MO 2 US 67 TWSC

KY 1 KY 9 TWSC

KY 2 US 31E TWSC

FL SR 31 TWSC

MD MD 35 T-intersection

Figure 3. Typical design used in the field.
4

Intersection and Lane Characteristics

The intersection angle for all intersections is 90 degrees, 

and all intersections are on two-lane roads. Also, the 

same number of lanes and the same intersection angles 

are common to most intersections. Only one intersection, which is located in Kentucky, has a left-turn 

lane that is accounted for in the calculations. The lane 

width varies from State to State because of the different 

application techniques that are used. Furthermore, lane 

widths differ from one point to the next due to limited 

accuracy in pavement marking and milling techniques. 

Table 4 shows the lane widths in the States where the 

treatment was applied.

Application of the EB Method

A description of the EB method used to account for the 

regression-to-the-mean bias while normalizing for differences in traffic volume and crash frequency between 

the pre- and post-treatment periods is found in Hauer.(4,5)

The expected accident frequency of similar intersections

is determined using SPFs, which yield the predicted 

number of crashes at similar intersections. 

The change in safety of an intersection (DNp) is measured 

as the difference between the predicted average crash 

frequency for the pre-treatment period and the predicted 

average crash frequency for the post-treatment period. 

The following steps were used to calculate the change 

in safety of the intersections due to the treatment: 

1. Apply the SPFs. 

The value of the SPF (Nspf) is calculated for all sites 

for all study years. For this study, the period of interest 

was based on the crash data availability. On average, 

5 years of pre-treatment crash data and more than 

2 years of post-treatment crash data were acquired. The 

SPF was used to generate a prediction of the crash 

frequency for a site with the base conditions. All sites 

were at the base condition, and no modification factors 

were applied.

Nspf was calculated for three- and four-legged stopcontrolled intersections using equations 1 and 2. The 

SPF equations for total crashes were taken from the 

HSM.(3) In the case of fatal and injury crashes, the 

equations from Vogt and Bared were used.(6) The SPFs 

used in this study have not been calibrated to each of 

the five States because of a lack of data. 

The equation for an SPF for three-legged stop-controlled 

intersections is as follows:

Nspf = exp(-9.86 + 0.79 ln(AADTmaj) + 0.49 ln(AADTmin)) (1)

Where:

Nspf = Estimate of intersection-related predicted

 average crash frequency for base conditions

 for three-legged stop-controlled intersections.

AADTmaj = Entering AADT (vehicles per day) on the major 

 road.

AADTmin = Entering AADT (vehicles per day) on the minor

road.

Table 3. Site data distributed over collection years.

Site

Year

Total Crashes/

Year

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Before After

PA 1 7 8 1 4 3 4 4 4 4.7 3.56

PA 2 3 2 3 5 1 3 0 1 0 0 0 2.43 0.31

MO 1 1 2 2 2 4 1 0 0 3 0 1 1.74 1.36

MO 2 5 10 6 9 7 15 10 4 5 7 9 9 8.49

KY 1 3 10 3 5 1 4 2 2 5.6 3.6

KY 2 3 3 3 0 3 1 3 1.2

FL 0 0 0 2 0 2 0 0 1 0.76 0.44

MD 8 6 4 4 2 6 1 3 0 5.37 2.82

Note: The gray shading indicates pre-treatment data, and the red shading indicates post-treatment data. Blank cells denote years 

for which crash data was not available. The split cells represent a more accurate date of retrofitting.

Figure 4. Typical rumble strip design.

SHOULDER

EDGELINE

Approx. 

5 inches

TRAVEL LANE

Approx. 

7 inches

Approx. 

16 inches
5

The over dispersion factor (k) for this SPF is 0.54.(3) It 

provides an indication of the statistical reliability of 

the SPF. The closer the over dispersion factor is to zero, 

the more statistically reliable the SPF.

The equation for an SPF for four-legged stop-controlled 

intersections is as follows: 

Nspf = exp(-8.56 + 0.60 ln(AADTmaj) + 0.61 ln(AADTmin)) (2)

Where:

Nspf = Estimate of intersection-related predicted

 average crash frequency for base conditions

 for four-legged stop-controlled intersections.

AADTmaj = Entering AADT (vehicles per day) on the major 

 road.

AADTmin = Entering AADT (vehicles per day) on the minor

road.

The over dispersion factor for this SPF is 0.24.(3)

2. Calculate Np using equation 3, which is the weighted 

average between the estimate of Nspf and the 

observed crash frequencies (Nobs). Np is calculated 

for all years for every site as follows:

Np = wNspf + (1 - w)Nobs (3)

The EB method makes a weighted adjustment to Nspf

to account for Nobs. The weight (w) can be calculated 

using equation 4, where k is an over dispersion factor, 

as follows: 

w = 1 (4)

 1+ k x SallstudyyearsN

spf

3. Calculate the pre-treatment and post-treatment period 

crash frequencies, Np(before) and Np(after).

Np(before) and Np(after) are calculated by averaging the Np 

values for the before and after periods over time, as shown 

in equations 5 and 6 as follows:

Np(before) = 

Sum of Np for before period/before period in years (5)

Np(after) = 

Sum of Np for after period/after period in years (6)

4. Calculate DNp and the percent reduction in crashes, as 

provided in equations 7 and 8 as follows:

DNp = Np(after) - Np(before) (7)

 

Percent crash reduction = 

Np(after) - Np(before) x 100% (8) Np(before) 

A similar method was used to calculate a safety 

estimate for fatal/injury crashes with changes in the 

SPFs, which were taken from Vogt and Bared.(6) The 

reduction in crash rate for each site as a percentage is 

shown in table 5.

Summary and Conclusions

This study evaluated the effectiveness of a low-cost 

safety treatment for stop-controlled intersections. The 

treatment was applied to eight intersections in five 

States, and crash data were collected for the pre- and 

post-treatment periods. The EB method was used to 

estimate effectiveness of the treatment in enhancing safety. The methods showed that, on average, the 

total number of crashes was reduced by 32 percent, 

while fatal/injury crashes were reduced by 34 percent. 

Although the reductions in total and fatal/injury crashes 

in the post-treatment were consistent at all sites, the 

reductions were not statistically significant within the 

95 percent confidence level. This may be due to inconsistencies in treatment application at the sites as well 

as the small sample size. However, this treatment 

resulted in a high overall reduction in crashes, and it can 

be applied to more sites for further data collection and 

analysis. Based on this study and a study by Bared et al. 

(FHWA-HRT-08-063), some suggestions for future deployments are provided below.(2)

Figure 5. Two rows of rumble strips in the median.

Table 4. Lane widths at various sites.

Site

Lane Width 

Prior to

 Taper (ft)

Lane Width 

Within

Narrowed 

Section (ft)

Painted 

Median

Width (ft)

PA 2 12 10 4

KY 1 12 9.5 3.5

MO 1 11 9 3

MO 2 11–12 9–10 4–6
6

Recommendations for Future 

Deployments

The following ideas may enhance future deployments of 

these treatments as presented by Bared et al.(2) Figure 6 

and figure 7 illustrate the potential enhancements to the 

lane-narrowing concept for both rumble strip and rumble 

stripe designs, respectively.

Lane-Narrowing Concept

The following section illustrates the lane-narrowing 

concept. The length of the narrowed section should be 

increased for the lane-narrowing concept. Currently, 

the design template shows a length of 150 ft for the 

narrowed section on the major approach. This length 

may be too short to achieve the desired effect. If the 

length is increased to 200 or 250 ft, then drivers will travel 

a greater distance in the narrowed section, which may 

induce lower speeds. While current speed reductions on 

the major approaches are statistically significant, there 

is an opportunity to further reduce driver speeds. The 

nearest area 50 ft from the intersection should not have 

rumble strips.

In addition, a different rumble strip pattern should be 

used in the median. There are concerns that drivers 

may react similarly (i.e., steer to the left) when encountering both shoulder and centerline rumble strips with 

the same pattern. A recent study verified this concern and 

concluded that some drivers initially steered to the left 

when encountering centerline rumble strips.(7) Therefore, 

future deployments may consider the use of centerline rumble strips that produce a distinct sensation and 

noise to avoid confusion with shoulder rumble strips.

Some States were concerned that 9-ft lanes were too 

narrow for the treatment section of the lane-narrowing 

concept. The American

Association of State

Highway and Transportation Officials publication, A Policy on Geometric Design of Highways and Streets, indicates that lane widths

of 9–12 ft are generally 

used, and 12-ft lanes are 

predominant on most 

high-type highways.(8)

One State indicated that

it was required to submit a design exception

to implement the lanenarrowing concept, while

three other States indicated that they were

not required to submit

a design exception to

Table 5. Predicted average annual crash frequencies.

Site Pretreatment Post-Treatment

Percent 

Reduction Pretreatment Post-Treatment

Percent 

Reduction

PA 1 4.85 3.55 27 2.54 1.29 49

PA 2 2.66 0.99 63 1.13 0.45 60

MO 1 1.45 1.01 30 0.35 0.22 38

MO 2 8.71 8.11 7 2.57 2.46 4

KY 1 5.92 3.47 41 1.34 1.20 11

KY 2 2.91 1.88 35 1.31 0.49 63

FL 0.83 0.75 10 0.20 0.19 4

MD 5.77 3.19 45 3.47 1.93 44

Average 32 Average 34

Standard deviation 18 Standard deviation 24

Figure 6. Enhancements to section C of figure 3 with rumble strips.
7

install 9- or 10-ft lanes as 

part of the deployment 

of the lane-narrowing

concept. For two-way, 

two-lane rural highways, 

wider lanes provide 

desirable clearance between heavy vehicles in 

opposite directions.(8)

For the lane-narrowing 

concept, the presence of

a painted median should 

provide adequate clearance for opposing vehicles, even with 9-ft 

lanes. For those States 

that would still prefer 

wider lanes, the pavement markings (i.e., 

center and edge lines) 

could be placed in the 

rumble strips to increase 

the lane width without changing the placement of 

the rumble strip. An alternative method for increasing 

the effective lane width is to use a wider edge line and 

place the shoulder rumble strips closer to the shoulder. 

Alternatively, rumble stripes create a vertical surface 

that provides enhanced visibility during nighttime and 

wet weather conditions.(9) However, the operational and

safety benefits of rumble strips (pavement markings 

exist on the side of rumble strips) versus rumble stripes 

(pavement markings exist within the rumble strips) has 

yet to be determined. At this time, States could deploy 

either rumble strips or rumble stripes based on their 

typical applications. 

Furthermore, cross hatching in the median should also be 

provided for the lane-narrowing concept. Cross hatching 

will better define the presence and width of the median. 

In some of the deployments of the lane-narrowing 

concept, cross hatching was not used in the median. 

The treatment is much more conspicuous when cross 

hatching is used in the median (see figure 8 and figure 9).

To further assist in the lane-narrowing concept, rumble strips along both sides of the median should be 

installed, and the installation of rumble strips across the 

entire width for narrow medians should be considered. 

Providing rumble strips across the entire median

enhances the conspicuity of the treatment.

A speed advisory plaque should be added to the 

warning sign (i.e., intersection ahead or lane narrowing) located prior to point 1 in figure 2. Data from 

North Carolina indicate that vehicles can travel up to 

45 mi/h on highways with 9-ft lanes without crossing 

Figure 7. Enhancements to section C of figure 3 with rumble stripes.

Figure 8. Cross hatching in median for better visibility 

of lane narrowing.

Figure 9. Cross hatching in median at an intersection 

with left-turn bay.
8

Researchers—Anand Asokan and Joe Bared authored this report. Ram Jagannathan and Warren Hughes were 

involved in creating the low-cost experimental designs, providing field implementations, and acquiring statistical

crash data. The peer reviewers for this report were Ed Rice and Fred Ranck with FHWA, Dr. Shinya Kikuchi and 

Nopadon Kronprasert, and Mary Carolyn Thies of the Federal Aviation Administration. For information about this 

research contact Joe Bared, FHWA Project Manager, HRDS-05, (202) 493-3314, joe.bared@dot.gov.

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

is being made to the Divisions and Resource Center.

Availability—The Summary Report may be obtained from the FHWA Product Distribution Center by e-mail to 

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

Key Words—Speed reduction treatment, Intersection safety, and Lane narrowing.

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

interest of information exchange. The U.S. Government assumes no liability for the use of the information contained 

in this document. The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ 

names appear in this report only because they are considered essential to the objective of the document.

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

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

are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically 

reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.

june 2010 FHWA-HRT-10-047

HRDS-05/06-10(1.5M)E

into the opposing lane. In areas where the posted 

speed is greater than 45 mi/h, it may be appropriate to 

provide a supplemental speed advisory plaque of 45 mi/h 

prior to the deployment of the lane-narrowing concept.

It is also necessary to add signs to warn drivers of a 

speed limit reduction. The crash data from Kentucky 

and Missouri indicate that rear-end crashes increased 

after the implementation of the lane-narrowing concept. 

The intent of the lane-narrowing concept is to reduce 

speeds on the major road; however, this may create 

greater speed differentials and may increase the chance 

of rear-end crashes. Advance signing could help to 

mitigate this issue.

It would also be helpful to install both W5-1 and W2-1 

warning signs prior to the treatment. W5-1 signs indicate that the lane narrows, and W2-1 signs indicate that 

there is an intersection ahead. The use of one or the 

other does not convey both messages, so both signs 

are necessary.

References

1. National Highway Traffic Safety Administration. 

(2009). Fatality Analysis Reporting System (FARS), 

Washington, DC. Obtained from: ftp://ftp.nhtsa.dot.

gov/FARS/. Site last accessed September 2009.

2. Hughes, W., Jagannathan, R., and Gross, F. (2008). 

Two Low Cost Safety Concepts for Two Way Stop 

Controlled, Rural Intersections on High Speed 

Two Lane, Two Way Roadways, Report No. FHWAHRT-08-063, Federal Highway Administration, 

Washington, DC.

3. AASHTO. (2009). Highway Safety Manual, 1st ed.,

American Association of State Highway and Transportation Officials, Washington, DC. 

4. Hauer, E. (1997). Observational Before-After Studies 

in Road Safety, Pergamon Press, Oxford, England.

5. Abbess, C., Jarret, D., and Wright, C.C. (1981). 

“Accidents at Blackspots: Estimating the Effectiveness 

of Remedial Treatment, With Special Reference to the 

“Regression-to-the-Mean” Effect,” Traffic Engineering 

and Control, 22(10), 535–542.

6. Vogt, A. and Bared, J. (1998). Accident Models for 

Two-Lane Rural Roads: Segments and Intersections, 

Report No. FHWA-RD-98-133, Federal Highway 

Administration, Washington, DC. 

7. Noyce, D. and Elango, V.V. (2004). “Safety Evaluation of Centerline Rumble Strips: Crash and Driver 

Behavior Analysis,” Transportation Research Record 

1862, 44–53, Transportation Research Board, 

Washington, DC. 

8. AASHTO. (2004). A Policy on Geometric Design of 

Highways and Streets, American Association of 

State Highway and Transportation Officials, 

Washington, DC.

9. Transportation Research Board. (2009). “Guidance 

for the Design and Application of Shoulder and 

Centerline Rumble Strips,” NCHRP Report 641, 22, 

National Cooperative Highway Research Program, 

Washington, DC.