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The Highway Safety Information System 

(HSIS) is a multi-State safety database that 

contains crash, roadway inventory, and traffic

volume data for a select group of States. The 

participating States, California, Illinois, Maine, 

Michigan, Minnesota, North Carolina, Ohio, 

Utah, and Washington, were selected based on 

the quality of their data, the range of data available,

and their ability to merge the data from the various

files. HSIS is used by FHWA staff, contractors,

university researchers, and others to study current 

highway safety issues, direct research efforts, and 

evaluate the effectiveness of accident countermeasures.

Safety Evaluation of Transverse Rumble 

Strips on Approaches to Stop-Controlled 

Intersections in Rural Areas

Transverse rumble strips (TRSs) (also called in-lane rumble strips) have been 

used by some agencies to warn drivers in rural areas that they are approaching 

a stop sign. The strips typically consist of grooves crossing the roadway surface 

to provide a tactile and audible warning for drivers. Figure 1 shows an example 

of TRSs on a rural road in Minnesota.

Many studies have focused on the effect of TRSs on driver behavior, and 

there is some evidence that TRSs are effective in reducing the intersection 

approach speeds.(1,2) However, the results from these crash-based studies are 

not reliable due to the lack of rigor in the accident evaluation designs.(3)

The objective of this effort was to examine the impact of TRSs on crashes, 

specifically total crashes, injury crashes, and specific crash types, such 

as right-angle and run stop sign crashes. The effort also included an 

economic analysis to investigate the tradeoffs between different crash 

types. It could be hypothesized that the major effect of TRSs would 

be to reduce instances of drivers failing to stop at an intersection 

because they are unaware of the intersection (or stop sign) presence 

(i.e., reduce run stop sign crashes). However, by increasing the 

driver’s awareness of the upcoming intersection, the TRSs might also 

affect crashes where the driver stops at the sign but then pulls out into 

the path of an oncoming vehicle (i.e., reducing right-angle crashes). 

Thus, analysis of both is warranted.

Summary Report

Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike . McLean, VA 22101-2296

Figure 1. TRSs on a rural road in Minnesota.

Source: Minnesota Department of Transportation (MnDOT)
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Empirical Bayes

To control for possible regression to the mean and other biases, an empirical

Bayes (EB) methodology was used.(4) With this methodology, a prediction 

of what would have happened at the treatment sites in the after period 

without treatment is based on a weighted combination of two factors: (1) the 

frequency of crashes on the treated sites in the before period, and (2) crashfrequency predictions from regression models developed with data from 

similar but untreated reference sites. The prediction of what would have 

happened without treatment is then compared to what actually happened 

with treatment to estimate the safety effect of the treatment. Annual factors 

are estimated for each year to correct for changes in factors such as weather, 

crash reporting practices, and demography over time. This methodology 

corrects for the regression bias, changes in traffic volume at the treatment 

sites, and other possible confounding factors. 

Data Collection

The Iowa Department of Transportation (Iowa DOT) and Minnesota 

Department of Transportation (MnDOT) provided data on rural inter-

sections with minor-leg stop control where TRSs were introduced. The 

following sections summarize the design of TRSs and the data that were 

collected in these two States. 

Initially, Minnesota used a full-lane width pattern for their TRSs. 

The pattern was changed to a wheel path pattern so that motorists 

who are aware of TRSs (i.e., local drivers) and bicyclists could avoid 

the pattern. An approach to a stop-controlled intersection can have up 

to five sets of TRSs, but a minimum of three sets are recommended. 

If three sets are used, the first set of TRSs encountered by the driver 

is located 250 ft (75 m) before the “Stop Ahead” sign. TRSs closest to 

the intersection are usually located about 500 ft (150 m) from the 

stop sign. The set of TRSs in the middle are typically located 15 ft 

(4.5 m) before the TRSs that are closest to the intersection. The 

length of each TRS panel is about 5 ft (1.5 m). Figure 2 shows 

an illustration plan view of TRSs in Minnesota.

In Iowa, until 2006, three sets of TRSs were required, with the 

first TRS encountered by the driver 200 ft (60 m) in advance of 

the “Stop Ahead” sign. The TRS closest to the intersection was 

300 ft (90 m) in advance of the stop line, and the center one 

was midway between these two. This standard was altered in 

April 2006 and again in May 2007 to require only two sets 

of TRSs, removing the TRS closest to the intersection (see 

figure 3). Currently, each TRS panel is 24 ft (7.2 m) long and 

consists of 25 grooves placed at 1-ft (0.305-m) intervals per-

pendicular to the centerline. An 18-inch (457.2-mm) width 

of pavement at the outside edge of the lane is left uncut to 

accommodate bicycles.

MnDOT provided data on 20 intersections where TRSs ad 

been implemented between 1990 and 2000. From those 

Methodology

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20 intersections, 11 were three-leg intersections and 9 were four-leg intersections. Iowa DOT provided data on 

134 intersections where TRSs had been installed between 1992 and 2005. From those 134 intersections, 49 were 

three-leg intersections and 85 were four-leg intersections. Table 1 provides a summary of the data collected for the 

treatment sites from the two States. Table 1 shows the statistics for the major and minor road average annual 

daily traffic (AADT) and the average number of total crashes and injury and fatal crashes per year in the 

before and after periods. In both States, crash severity was documented using the KABCO scale, where “K” 

Figure 3. TRS placement in Iowa.

Source: Iowa DOT Traffic and Safety Manual(6)

Figure 2. Plan view of TRSs in Minnesota. 

Source: MnDOT Traffic Engineering Manual(5)

Plan View

1.5 m

Roadway

Roadway

4.5 m 1.5 m 4.5 m 1.5 m

In place In place

75 m 1.5 m 4.5 m 1.5 m

150 m

STOP

Roadway

Enlarged Plan View

Section 0.2 m

Depressed Area

1.0 m

0.9 m

1.0 m

0.5 m

1.5 m 4.5 m

150 mm ±6 mm 300 mm c. to c. 10 mm ±3 mm

Wearing course

Varies with sight distance 

and vehicle speed

Site conditions as determined by the engineer should warrant the 

number of rumble strips used. A minimum of three sets is desired.

1 ft = 0.305 mm

1 inch = 25.4 mm
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Results

This section provides a discussion of results, focusing on the results that were statistically significant. Results are 

expressed in terms of a crash modification factor (CMF). A CMF of 1.0 indicates that the treatment had no effect. 

A CMF greater than 1.0 indicates that there was an increase in crashes due to the treatment, whereas a CMF less 

than 1.0 indicates that there was a decrease in crashes due to the treatment. 

Iowa

In Iowa, total and PDO crashes seem to have increased at the TRS sites. However, none of the increases were 

statistically significant within a single intersection type. For PDO crashes, the increase for the three- and fourleg intersections combined was statistically significant at the 0.10 significance level. To determine the possible 

Table 1. Data summary of treatment sites in Minnesota and Iowa.

Variable

Minnesota 

(20 intersections)

Iowa 

(134 intersections)

before after before after

Intersection-years 205 135 1,775 867

Major road AADT (minimum) 245 225 119 89

Major road AADT (average) 2,480 4,023 3,528 3,152

Major road AADT (maximum) 9,475 14,400 33,600 37,400

Minor road AADT (minimum) 270 286 18 25

Minor road AADT (average) 917 1,276 707 730

Minor road AADT (maximum) 2,550 3,185 6,810 6,810

Total intersection crashes per intersection per year (average) 0.517 0.815 0.343 0.346

Total intersection injury crashes per intersection per year 

(KABC) (average) 0.200 0.393 0.167 0.148

Analysis

Safety performance functions (SPFs) are used in the EB methodology to estimate the safety effectiveness of 

this strategy. SPFs relate crashes of different types to traffic flow and other relevant factors for each jurisdiction 

based on similar untreated sites. SPFs were estimated for different levels of crash severity including total, KABC, 

KAB, and KA crashes using data from reference groups in Minnesota and Iowa. Generalized linear modeling was 

used to estimate model coefficients using SAS® and assuming a negative binomial error distribution, which is 

consistent with the state of research in developing these models.(7) While estimating the SPFs, if a variable did not 

significantly improve the model, it was removed.

represents fatal crashes, “A” represents incapacitating injury crashes, “B” represents non-incapacitating injury 

crashes, “C” represents possible injury crashes, and “O” represents non-injury or property damage only (PDO) 

crashes. (Note that “KABC” refers to a combination of all crashes resulting in injury or death.)

In order to account for biases in treatment site selection, reference sites were identified (i.e., intersections 

that were similar to the treatment sites but which did not receive TRSs). In Iowa, reference sites were 

identified by Iowa DOT. In Minnesota, data from the Highway Safety Information System were used to 

identify stop-controlled intersections on rural roads similar to the treatment sites with respect to traffic 

volume, presence of lighting, and the number of lanes on the major approaches.
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reasons for this increase, each site was examined to identify potential outliers, 

which may have caused significant changes to the results; however, no such 

outliers were found.

KABC crashes (i.e., all crashes involving injury) seem to have decreased by 

about 7 percent at four-leg intersections, but this change was not statistically 

significant. Severe injury (KAB) crashes decreased at both three- and four-leg 

intersections. The reduction at four-leg intersections was about 25 percent. In 

the combined sample of three- and four-leg intersections, the reduction was 

about 20 percent. Both were statistically significant at the 0.10 level. Similarly, 

KA crashes decreased at both three- and four-leg intersections. The reduction 

at three-leg intersections was quite large (about 67 percent); however, this 

was based on an extremely low number of expected crashes (six crashes) in 

the after period. As a result, it was not very reliable, even if it was statistically 

significant. For the combined sample of three- and four-leg intersections, 

there was about a 30 percent reduction in KA crashes, which was statisti-

cally significant at the 0.10 significance level. These results indicate that 

the TRSs may be effective in reducing more severe injury crashes.

Iowa DOT provided data on crashes that involved drivers running stop 

signs. Results indicate that these crashes seem to have decreased by 18 to 

20 percent, but none of these reductions were statistically significant. 

Right-angle crashes were not examined in Iowa because less than 

10 crashes were reported in the after period for three- and four-leg 

intersections combined.

Minnesota

Similar to the results from Iowa, total and PDO crashes in Minnesota

seem to have increased following implementation at three-leg 

intersections and at three- and four-leg intersections combined. 

However, none of the increases were statistically significant. 

The only results that were statistically significant were the reduc-

tions in KA crashes at four-leg intersections and at three- and fourleg intersections combined. However, these reductions were based 

on a very limited sample and, as a result, were not reliable even if 

they were statistically significant. 

Right-angle crashes seem to have decreased at three-leg intersections, but this reduction was not statistically significant. While 

right-angle crashes seem to have increased at four-leg intersections, this increase was also not statistically significant. Run 

stop sign crashes could not be examined since such crashes 

could not be identified based on the variables in the Minnesota 

crash file.

To determine the possible reasons for the apparent increase 

in total and PDO crashes at the TRS sites in Minnesota, 

each site was examined to identify potential outliers which 

may have caused significant changes to the results. While 

this examination revealed one possible outlier, removing it 

did not provide additional insight into the apparent 

inconsistencies in the results.

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Combined Results from Iowa and Minnesota

Table 2 shows the combined results from Iowa and Minnesota. All intersections 

from Iowa and Minnesota were considered while combining the results. The table 

includes the observed crashes in the after period, the EB estimate of the crashes 

expected in the after period had there been no treatment, CMF, and the standard 

error of CMF. CMFs that are statistically significant (i.e., statistically different 

from 1.0) at the 0.10 and 0.05 significance levels are shown, as well.

For three-leg intersections, the only statistically significant result was a 

reduction in KA crashes; however, this was based on a very small sample 

size and, hence, is not reliable. For four-leg intersections, there was a statistically significant reduction in KA and KAB crashes. For three- and four-leg

intersections combined, there was a statistically significant increase in 

PDO crashes (about 19 percent) but a statistically significant reduction in 

KAB crashes (about 21 percent) and KA crashes (about 39 percent).

The fact that the results indicate decreases in severe crashes coupled with 

increases in PDO crashes is of interest. Such increases in PDOs could 

result from either shifts from more severe to less severe crash types (e.g., 

right-angle crashes decrease while rear-end crashes increase) or from a 

shift from more severe to less severe crashes within the same crash type 

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Table 2. Combined results of before-after evaluation from Iowa and Minnesota.

intersection type crash type observed crashes 

in after period

eb expected 

crashes in after 

period without 

treatment

cmf standard error 

of cmf

Three-leg (60 sites)

Total 139 112.9 1.223 0.142

PDO 84 64.8 1.284 0.185

Fatal and injury (KABC) 55 45.6 1.192 0.207

Fatal and injury (KAB) 23 25.2 0.903 0.211

Fatal and injury (KA) 3 7.2 0.410** 0.238

Four-leg (94 sites)

Total 271 252.7 1.066 0.104

PDO 144 126.0 1.138 0.121

Fatal and injury (KABC) 126 136.6 0.913 0.124

Fatal and injury (KAB) 63 83.6 0.745** 0.121

Fatal and injury (KA) 20 30.2 0.652** 0.165

Three- and four-leg 

combined (154 sites)

Total 410 365.6 1.118 0.086

PDO 228 190.8 1.191* 0.102

Fatal and injury (KABC) 181 182.2 0.987 0.109

Fatal and injury (KAB) 86 108.8 0.785** 0.107

Fatal and injury (KA) 23 37.4 0.608** 0.140

*CMF is statistically significant at the 0.10 level.

**CMF is statistically significant at the 0.05 level.
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(e.g., angle crashes are not eliminated but are made less severe). An attempt was made to examine shifts in crash 

types, but it was not possible due to small sample sizes.

Economic Analysis

In an attempt to draw conclusions given the tradeoff, a simplistic economic analysis was conducted using 

only the combined three- and four-leg results from table 2. The results are shown in table 3. Based on a report 

from the Federal Highway Administration, at the time of this study, the mean comprehensive cost of a PDO 

crash in a rural area in the United States was $7,800, the mean cost for a C injury crash was $49,549, the 

mean cost for a KAB crash was $353,359, and the mean cost of a KA crash was $662,817.(8) The differences in 

observed and expected crashes (shown in the table under the column “Crash Increase or Decrease”) was multiplied 

by the appropriate crash costs for each severity category to estimate the increase or decrease in crash harm 

associated with the changes in the three different crash types. By comparing the crash harm estimates for KAB 

crashes with PDO and C injury crashes, a benefit of $6,683 per intersection per year was estimated. Similarly,

by comparing the crash harm estimates for KA crashes with PDO and C injury crashes, a benefit of $8,168 

per intersection per year was estimated. Based on either approach, it is clear that there was a significant 

reduction in crash harm due to the installation of TRSs.

Table 3. Economic cost of crashes for selected crash severity levels.

crash type observed crashes in 

the after period

eb expected crashes 

in after period 

without treatment

crash increase 

or decrease

crash increase or 

decrease (crash 

harm)

crash cost increase 

or decrease per 

intersection per 

year

PDO* 228 190.8 37.2 $290,160 $290

C Injury 95 73.4 21.6 $1,070,258 $1,068

Fatal and injury (KAB)** 86 108.8 -22.8 -$8,056,585 -$8,041

Fatal and injury (KA)** 23 37.4 -14.4 -$9,544,565 -$9,526

*CMF is statistically significant at the 0.10 level.

**CMF is statistically significant at the 0.05 level.

Conclusions and Recommendations

This study investigated the safety effect of TRSs on approaches to stop-controlled intersections using the 

state-of-the-art EB methodology. Results indicate that TRSs may be effective in reducing severe injury crashes

(KAB and KA) at minor road stop-controlled intersections. Considering that many previous studies showed 

a reduction in speed following the implementation of TRSs, the decrease in KAB and KA crashes could be a 

result of reduced speeds.(1,2) However, it is important to recognize that coupled with the reduction in KA 

and KAB, there was an increase in PDO crashes. While it was not possible to determine the reasons for this 

tradeoff, a limited economic analysis indicated a reduction in crash harm of about $6,600 per intersection 

per year due to the installation of TRSs.

Since most TRSs are installed at rural intersections with relatively low volumes and hence relatively few 

crashes, a large sample of intersections with long before and after periods are required to find statistically 

significant results, especially since the reduction appears to be for severe injury crashes only. Further research 

on this topic should attempt to collect data from other States that have implemented TRSs and also examine 

the effect on other crash types such as run-off-road and sideswipe crashes at or near intersections. Additional

research should also investigate the effectiveness of this treatment under a variety of conditions, including 

the number of driveways, nature of the surrounding development and roadside hazards, and sight distance.
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1. Fitzpatrick, K., Brewer, M.A., and Parham, A.H. (2003). Left-Turn 

and In-Lane Rumble Strip Treatments for Rural Intersections, Report 

No. FHWA-TX-04-0-4278-2, Texas Department of Transportation, 

Austin, TX.

2. Harder, K.A., Bloomfield, J.R., and Chihak, B.J. (2006). Stopping 

Behavior at Real-World Stop-Controlled Intersections With and 

Without In-Lane Rumble Strips, Report No MN/RC-2006-42, 

Submitted to Minnesota Department of Transportation, St. Paul, MN.

3. Harwood, D. (1993). “Use of Rumble Strips to Enhance Safety,” 

NCHRP Synthesis of Highway Practice 191, Transportation Research 

Board, Washington, DC.

4. 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, United Kingdom.

5. MnDOT. (1999). Traffic Engineering Manual, Minnesota Department of Transportation, St. Paul, MN.

6. Iowa DOT. (2006). Traffic and Safety Manual, Iowa Department 

of Transportation, Ames, IA.

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

8. Council, F., Zaloshnja, E., Miller, T., and Persaud, B. (2005). 

Crash Cost Estimates by Maximum Police-Reported Injury 

Severity Within Selected Crash Geometries, Report No. 

FHWA-HRT-05-051, Federal Highway Administration, 

Washington, DC.

9. Srinivasan, R., Baek, J., and Council, F.M. (2010). Safety

Evaluation of Transverse Rumble Strips on Approaches 

to Stop-Controlled Intersections in Rural Areas, 2010 

Annual Meeting of the Transportation Research Board, 

Washington, DC.

10. Srinivasan, R., Baek, J., and Council, F.M. (2010). “Safety 

Evaluation of Transverse Rumble Strips on Approaches to 

Stop-Controlled Intersections in Rural Areas,” Journal of 

Transportation Safety & Security, 3(2), 261–278.

for more information

The research was conducted by Raghavan Srinivasan, 

Jongdae Baek, and Forrest Council of the University 

of North Carolina Highway Safety Research Center. 

Further details about the evaluation are available 

in Safety Evaluation of Transverse Rumble Strips on 

Approaches to Stop-Controlled Intersections in Rural 

Areas, which was presented at the 2010 Annual Meeting of 

the Transportation Research Board.(9) A slightly revised 

version of the paper was published in the Journal of Transportation Safety & Security.(10) For more information 

about HSIS, contact Carol Tan, HSIS Program Manager, 

HRDS, (202) 493-3315, carol.tan@dot.gov or Ana Maria 

Eigen, HRDS, (202) 493-3168, ana.eigen@dot.gov. 

Visit us on the Web 

at www.tfhrc.gov/research/

FHWA-HRT-12-047

HRDS-20/05-12 (900)E

References