Although 18% of all crashes are single vehicle run-off-the-road crashes they account for 30% of all the fatal crashes on Washington State highways. In an effort to reduce these fatalities, shoulder rumble strips are used to alert drivers that they have left their intended lane of travel.

What are Rumble Strips?
Rumble strips are grooves or rows of raised pavement markers placed perpendicular to the direction of travel to alert inattentive drivers. As a vehicle passes over the rumble strips, noise and vibration are produced, alerting the driver they are approaching a hazard.

The Washington State Department of Transportation (WSDOT) currently uses Roadway Rumble Strips and Shoulder Rumble Strips (SRS) as a matter of policy. We are also experimenting with Centerline Rumble Strips in a few locations.

• Roadway rumble strips are placed across the traveled way to alert drivers approaching a change of roadway condition or a hazard that requires substantial speed reduction or other maneuvering.

• Shoulder rumble strips are placed on the shoulders just beyond the traveled way to warn drivers they are entering a part of the roadway not intended for routine traffic use. WSDOT uses milled-in SRS as their standard design. 

• Centerline Rumble Strips are placed on centerline of undivided highways to warn drivers they are leaving their intended lane of travel.

Where does WSDOT use rumble strips?
The Washington State Department of Transportation (WSDOT) uses Roadway Rumble Strips and Shoulder Rumble Strips as a matter of policy. We are also evaluating Centerline Rumble Strips in a few locations around the state.

Roadway Rumble Strips
May be used at locations such as:

• Stop controlled intersections
• Port of entry/customs stations
• Lane reductions with a history of accidents where driver inattention is a contributing cause.
• Other locations where the character of the roadway changes, such as the end of a freeway.

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Shoulder Rumble Strips (SRS) – Divided Highways
WSDOT has a policy for use of SRS on divided highways:

• SRS are required on both the left and the right shoulders of rural interstate highways.
• Consider SRS on both shoulders of other rural divided highways (non-interstate).

Design exceptions may be granted when:

• Another project scheduled within 2 years of the proposed project will overlay or reconstruct the shoulders, or will use the shoulders as a detour.
• Pavement analysis determines that the pavement structure is inadequate for installation of SRS.
• Shoulders are less than 4’ wide on the left and 6’ wide on the right.

Shoulder Rumble Strips (SRS) – Undivided Highways
WSDOT's policy on the use of SRS on undivided highways is based on criteria driven evaluation, of the following characteristics:

• For use on rural highways with speeds of 45 mph or higher.
• Adequate pavement structure exists on the shoulder.
• Run-off-the-road accident experience is high (guidance provided on evaluation).
• Shoulders are wide enough to provide a minimum of 4’ of shoulder between the SRS and the edge of paved shoulder. Increase the dimension to 5’ if barrier or guardrail is present at the edge of the shoulder.
• An Engineering analysis indicates the run-off-the-road experience is correctable by installation of rumble strips.
• Must be coordinated with WSDOT’s Bicycle and Pedestrian Advisory Committee. (BPAC).

Centerline Rumble Strips
WSDOT has developed policy for use of rumble strips as a countermeasure for centerline crossover accidents on state highways. Sections of US 12, US 395, and state highway 522 have centerline rumble strips installed that are being monitored for their effectiveness.

What benefits has WSDOT realized from the use of rumble strips?
Shoulder rumble strips were installed on a 44 mile test section of Interstate 82 in 1992. An evaluation of run-off-the-road accidents before and after rumble strip installation reveled a 40% reduction in run-off-the-road crashes.

The Interstate 82 experience led to system-wide application on the rural interstate system. After wide spread installation, another before and after comparison was conducted for segments of Interstate 5, Interstate 90 and US 395. That comparison evaluated 56 miles and revealed a 35% reduction in run-off-the-road crashes.

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What are the trade-offs associated with rumble strips?
While the purpose of Shoulder Rumble Strips is to reduce run-off the road accidents, they can also produce less desirable impacts.

Vibration levels produced by rumble strips can create control problems for bicyclists. WSDOT’s goal is to provide the optimum level of safety for all users, therefore, the use of SRS as a counter-measure for run-off-the-road accidents must be balanced with the safety of bicyclists. This requires compromise in the design and/or placement of the SRS.

It has been reported that residents near highways where rumble strips are installed find the noise levels objectionable.

Milled-in rumble strips may cause the pavement to ravel, when used improperly. They should not be placed on pavements with inadequate structure, nor should they be placed too close to the pavement edge.

What is in the future for rumble strips in Washington State? Because of the reduction in the number of run-off-the-road accidents on the divided highway system, WSDOT has evaluated their role on undivided highways. That evaluation has revealed that shoulder rumble strips on the undivided highway system presents additional concerns that are not as prevalent on the divided highway system. In particular, accommodating bicyclists on the shoulders of the undivided highways emerged as a concern. This leads us to a criteria-driven approach for consideration of shoulder rumble strips on the undivided highway system. The criteria targets only those routes with a high run-off-the-road accident experience, where rumble strips can be accommodated while providing for the needs of the bicyclists. The WSDOT Bicycle and Pedestrian Advisory Committee has a pivotal role in the evaluation process.

The policy for use of Centerline Rumble Strips is now been placed in Chapter 1600 of the Design Manual.

  Evaluating Shoulder Rumble Strips on Rural Undivided Highways
Shoulder Rumble Strips (SRS) are not required on undivided highways, but may be used where run-off-the-road accident experience is high. SRS usage on the shoulders of undivided highways demands strategic application because bicycle usage is more prevalent along the shoulder of the undivided highway system. Rumble strips affect the comfort and control of bicycle riders; consequently, their use is to be limited to highway corridors that experience high levels of run-off-the-road accidents. Apply the following criteria in evaluating the appropriateness of rumble strips on the shoulders of undivided highways.

• Use on rural roads only
• Ensure shoulder pavement is structurally adequate to support milled rumble strips
• Posted speed is 45 mph or greater
• Ensure that at least 4 feet of useable shoulder remains between the rumble strip and the outside edge of shoulder. If guardrail or barrier is present, increase the dimension to 5 feet of useable shoulder.
• Preliminary evaluation indicates a run-off-the-road accident experience of approximately 0.6 crashes per mile per year, or approximately 34 crashes per 100 million miles of travel.
• An engineering analysis indicates a run-off-the-road accident experience considered correctable by shoulder rumble strips.
• Coordination with WSDOT's regional Bicycle and Pedestrian Coordinator.
• Project team coordinates with local bicycle clubs, sponsors of scheduled bicycling events, and other bicyclists known to use the route.

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Rural Roads
The Highway Road Log provides a desktop reference for which highway sections are considered rural.

Pavement Structure
Consult the regional Materials Engineer for assistance with this determination.

Posted Speed of 45 mph or greater
The Highway Road Log provides a listing of posted speed limits for highway sections.

Shoulder Width
Check the Highway Road Log, and field verify. Identify areas with barrier along the shoulder edge and consider discontinuing rumble strips at these locations if clearance is tight.

Crash Experience
Preliminary evaluation – Collect crash history for the most current three year period (or longer) along the route segment. Look at all single vehicle crashes that are not at intersections and are not intersection related, occurring on the shoulder or off the road, both to the left and to the right.

1. Determine the average number of crashes per year on the segment. Divide the annual number of crashes by the segment length, in miles. Compare that number against 0.6 run-off-the-road crashes per mile for rural undivided highways with shoulder widths of 6 feet or greater and speeds of 45 mph or greater.
2. Determine the current average annual traffic volume (AADT*365) and multiply by the section length to determine the annual miles of vehicle travel for the segment. Divide the annual miles of vehicle travel by one hundred million. Divide the average number of crashes per year on the segment by the annual vehicle miles traveled (expressed in units of one hundred million). This will yield the annual number of crashes per one hundred million miles of vehicle travel. Compare that number against a statewide average of 34 run-off-the-road crashes per one hundred million miles of vehicle travel for rural undivided highways with shoulder widths of 6 feet or greater and speeds of 45 mph or greater.

These calculations will give you a general idea of whether your project is experiencing a high number of run-off-the-road crashes, but this preliminary evaluation process does not yield conclusive evidence regarding the need for shoulder rumble strips. When the run-off-the-road crash experience on your project approaches or exceeds either of the statewide average values, it is an indication that your project warrants additional evaluation for shoulder rumble strips. Routes with higher volumes should give a little more weight to the crashes per one hundred million miles of vehicle travel value than to the crash per mile value.

Engineering Study
Detailed evaluation - Continue your analysis with the accident data from the preliminary evaluation.

1. Rumble strips have been demonstrated to be effective in reducing run-off-the-road crashes where the drivers are inattentive, asleep, or under the influence of alcohol (statistically, about 30% of run off-the-road-crashes on rural undivided highways). Evaluate the crashes on the segment to determine if these contributing circumstances exist.
2. Analyze the location of the run-off-the-road crashes. Clusters of run-off-the-road crashes at isolated locations within a corridor segment may indicate that geometric conditions are more the issue than the correctable types of crashes being evaluated for rumble strips.
3. Analyze the direction of the travel and whether the crash occurred with the vehicle leaving the roadway to the left or to the right (we find that 60-65% of our run off the road accidents are off-the-road to the right). Crashes leaving the roadway to the left on an undivided highway have crossed the opposing lane(s) of travel and may or may not be correctable by shoulder rumble strips. Significantly higher percentages of run-off-the-road to the left crashes should lead to an analysis of centerline rumble strips.
4. Perform a benefit/cost analysis for rumble strips on the segment, assuming that the accident reduction will be 30% of the run-off-the-road crashes that were determined in step one. The BC analysis normally shows a very favorable investment, aided by the relatively low cost of installing shoulder rumble strips.
5. If rumble strips are selected as a counter measure to reduce run-off-the-road crashes, evaluate and select a rumble strip pattern appropriate for the project. Coordinate with WSDOT’s Bicycle and Pedestrian Advisory Committee (BPAC) in this process.
6. Remember to discontinue rumble strips at locations where clearance is tight (such as bridges or locations with traffic barriers).

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Coordinate with WSDOT's Bicycle & Pedestrian Coordinator Consult your regional Bicycle and Pedestrian Coordinator to determine whether your project is along a rural bicycle touring route or accommodates regularly scheduled bicycle touring events. Ask if the route is developing into a bicycle route.

Work with the regional Bicycle and Pedestrian Coordinator to evaluate the impacts of rumble strips, balancing numbers of bicycle riders with run-off-the-road crash experience. Try to determine numbers of riders using the segment and what their peak volume and duration is. Compare crash experience against the average for similar routes.

If you select a rumble strip solution, work together with the regional Bicycle and Pedestrian Coordinator to determine which pattern is appropriate for your project. Standard Plan H-4a provides guidance for rumble strip patterns appropriate for use on undivided highways. Consider all of the following in your selection of a pattern:

1. Routes with minimal bicycle usage afford opportunities to use a 16" wide rumble strip where plenty of shoulder width is available. As remaining shoulder width becomes marginal, give more consideration of the narrower patterns.
2. For routes with moderate to heavy bicycle usage restrict your selection to a 12" wide pattern.
3. Where moderate to heavy bicycle usage occurs in locations where shoulders are known to collect considerable amounts of debris, consider a pattern with the more frequently spaced gaps in the pattern.
4. Routes with higher run-off-the-road accident rates should consider the wider patterns with longer spaces between gaps, if those decisions are consistent with guidance on bicycle usage.
5. For those locations where run-off-the-road accident rates are high, bicycle usage is high, and shoulder widths are narrow, discuss options such as profiled edge stripes. Although much more costly than milled-in ruble strips, they may present an appropriate solution for a specific site.

If a rumble strip solution is chosen, discuss the riding conditions on the shoulder. While no guarantees can be offered, identify maintenance personnel who may discuss the potential for sweeping. Budget limitations require that any such activity be strategic with respect to both location and timing. This discussion should focus on making wise decisions about where to spend our sweeping dollars. Who benefits (and how many users), when will they be there, and what specific locations have the greatest need, should all be part of the discussion with maintenance staff.

Consult your Bicycle and Pedestrian Coordinator for all conditions where shoulder rumble strips are being considered for locations that do not comply with the criteria previously outlined. In general, the use of rumble strips that do not comply with the criteria is discouraged. However, as exceptions are considered they must involve the Bicycle and Pedestrian Coordinator.

Links to other information sources on rumble strips
FHWA  http://safety.fhwa.dot.gov/roadway_dept/rumble/index.htm

Share your thoughts on rumble strips with WSDOT 
We are presenting this information to aid the knowledge and discussion of the use of Rumble Strips in Washington State. Please use this form to provide us comment or ask us a question.

Rod Erickson, PE
Roadside Safety Engineer
Ph: 360-705-7246
Fax: 360-705-6815

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