WSDOT began installing rumble strips on undivided highways in 1999 as a countermeasure for roadway departure crashes. Installations on the shoulders were intended to reduce run-off-the-road crashes, while centerline rumble strips targeted reductions in cross-centerline crashes.
A March 2011 report details the performance of centerline rumble strips on Washington State highways. An April 2013 report details performance of centerline and shoulder rumble strips installed in combination.
What are rumble strips?
Rumble strips are grooves in the roadway or rows of raised pavement markers placed on the roadway in such a manner that, as the tires of a vehicle contact them, they produce sound (noise) and vibration. The noise and vibration produced by rumble strips is intended to alert inattentive drivers that they have departed from their lane, or to give advance notice of a change in the roadway ahead.
Where and how does WSDOT use rumble strips?
The Washington State Department of Transportation (WSDOT) currently uses shoulder rumble strips (SRS), centerline rumble strips (CLRS), and roadway rumble strips (RRS) as a matter of policy. In Washington, rumble strips are most commonly a pattern of grooves milled into the pavement surface.
Shoulder rumble strips are placed on the shoulders just beyond the lane edge to warn drivers that they are entering a part of the roadway not intended for routine traffic use.
Centerline rumble strips are placed on the centerline of undivided highways to warn drivers that they are leaving their intended lane of travel.
Roadway rumble strips are placed across the traveled way to alert drivers who are approaching a change of roadway condition or an object that requires substantial speed reduction or other maneuvering.
Why does WSDOT use rumble strips?
WSDOT’s Strategic Highway Safety Plan (SHSP) – also referred to as Target Zero – provides a comprehensive framework of specific goals, objectives, and strategies for reducing traffic fatalities and serious injuries. The overall goal of the SHSP is a safe and efficient surface transportation system, with no deaths or serious injuries on state highways by 2030.
Between 2006 and 2008, run-off-the-road crashes accounted for 30% of all serious injuries and 39% of all fatalities. During that same time period, 10% of all serious injuries and 19% of all fatalities were linked to opposite-direction (cross centerline) multivehicle collisions. Rumble strips are an effective and efficient countermeasure for reducing run-off-the-road and cross centerline crashes.
Shoulder Rumble Strips
Shoulder rumble strips are well documented regarding their effectiveness, primarily in reducing the frequency of vehicles departing the roadway to the right on undivided highways and to both the left and right on divided highways.
Milled 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 found a 40% reduction in run-off-the-road crashes.
The Interstate 82 experience led to systemwide application on Washington’s rural interstate system. After extensive installation, another before-and-after comparison was conducted for segments of Interstate 5, Interstate 90, and US 395. That comparison evaluated 56 miles and determined a 35% reduction in run-off-the-road crashes.
Centerline Rumble Strips
Centerline rumble strips are well documented as an effective countermeasure for crossover collisions, which are those where a vehicle strays out of the lane to the left and collides with an oncoming vehicle or object off the roadway. WSDOT began experimenting with CLRS in 1995 and, once effectiveness was verified, began a systematic program of installation throughout the state.
A March 2011 study conducted by WSDOT compares collision records from 2002 to 2009, before and after CLRS treatment, on nearly 500 miles of state highway. For crossover collisions, the study results show a 45% reduction in All Injury Severities and a 49% reduction in Fatal & Serious Injury Severity.
The following chart depicts findings from that study. It shows the crossover collision rate per year of the studied miles (orange columns) superimposed on the statewide yearly accumulated miles of CLRS installed (gray). The column trends indicate that, as the mileage of CLRS treatment has increased, the crossover rate has decreased.
Crossover Collision Crash Rate and Miles CLRS Installed
Roadway Rumble Strips
The effectiveness of roadway rumble strips is not as well established as shoulder rumble strips or centerline rumble strips. They are usually deployed to warn drivers of changing conditions in the roadway ahead. Their use is typically as a countermeasure for emerging crash trends associated with excessive speed through curves or on the approach to a stop condition, where signing and delineation have not achieved the desired results.
Are there other impacts associated with rumble strip placement?
- Rumble strip usage on the shoulders of undivided highways demands strategic application because bicycle usage is more prevalent along the shoulders of these roadways. 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.
- Some residents near highways where rumble strips are installed have reported that they find the noise levels objectionable.
- Milled-in rumble strips have been linked to pavement deterioration when placed on pavements with inadequate structure. For this reason, they should not be placed on pavements with inadequate structure, nor should they be placed too close to the pavement edge.
For these reasons, WSDOT has developed policies and standards regarding the application of rumble strips on state routes. Chapter 1600 of the WSDOT Design Manual provides guidance on the use of rumble strips. Rumble strip patterns are presented in Section M of WSDOT’s Standard Plans.
What is in the future for rumble strips in Washington State?
- WSDOT is currently experimenting with edge line rumble strips (ELRS), also known as rumble stripes. These narrower millings are installed beneath the right-hand lane line (fog stripe). They feature less encroachment on the shoulder, but also present less tire contact area. Demonstrated to be effective by studies in other states, they may also prove valuable in Washington and find some use where wider SRS are not recommended.
- WSDOT is conducting an ongoing assessment of noise levels associated with rumble strips.
- There are football-shaped rumbles, currently in use in other states, that are reported to be bicycle friendly. They are not universally regarded as being as effective as the conventional, rectangular standard, and are a proprietary design. WSDOT is not planning to install them at this time.
- Commercial truck and automobile manufacturers have been installing Lane Departure Warning (LDW) systems on some vehicle models in recent years. This technology employs a variety of sensors to alert the driver of an impending lane departure (“virtual rumbles”). This could make the physical placement of rumble strips (SRS and CLRS) on roadways unnecessary. However, even in the event that LDW systems become standard equipment on all new vehicles sold in the U.S., there will be many older models unequipped with this technology for many years thereafter. Consequently, milled rumble strips should be an effective safety countermeasure for many years to come.
What is the process for conducting an engineering analysis of shoulder rumble strips along undivided highways as referenced in Design Manual Chapter 1600?
Apply the following criteria in evaluating the appropriateness of rumble strips on the shoulders of undivided highways:
- Use on rural roads only.
- Use where posted speed is 45 mph or higher.
- Provide for at least 4 feet of usable shoulder between the rumble strip and the outside edge of shoulder. If guardrail or barrier is present, increase the dimension to 5 feet of usable shoulder. Field verify these dimensions.
- Ensure shoulder pavement is structurally adequate to support milled rumble strips.
- Do not place shoulder rumble strips on downhill grades exceeding 4% for more than 500 feet in length along routes where bicyclists are frequently present.
Collect crash history for the most current three-year (or longer) period 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.
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.
This calculation provides a general idea of whether a location is experiencing a high number of run-off-the-road crashes. However, this preliminary evaluation process does not yield conclusive evidence regarding the need for shoulder rumble strips. When the run-off-the-road crash experience for a proposed location approaches or exceeds the statewide average value, it is an indication that this location warrants additional evaluation for shoulder rumble strips.
Continue the analysis with the collision data from the preliminary evaluation.
- 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 whether these contributing circumstances exist.
- 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.
- Analyze the direction of travel and whether the crash occurred with the vehicle leaving the roadway to the left or to the right (WSDOT data analysis has shown that 60–65% of our run-off-the-road accidents are run-off-the-road to the right). Crashes with vehicles leaving the roadway to the left on an undivided highway that have crossed the opposing lane(s) of travel 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.
- Perform a Benefit/Cost (B/C) 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 1. The B/C analysis normally shows a very favorable investment because of the relatively low cost of installing shoulder rumble strips.
- If rumble strips are selected as a countermeasure 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 Program during this process.
- Remember to discontinue rumble strips at locations where horizontal offset is insufficient (such as bridges or locations with traffic barriers).
Coordinate with WSDOT's Bicycle & Pedestrian Coordinator
Determine whether your project is along a rural bicycle touring route or accommodates regularly scheduled bicycle touring events. Also, ask if the route is developing into a bicycle route.
The Headquarters Bicycle and Pedestrian Coordinator is a good resource for evaluating the use of shoulder rumble strips. Request information that quantifies the number of riders using the segment and what their associated peak volume and duration is.
This information will help balance the bicycle usage with the run-off-the-road vehicular crash experience.
If a rumble strip treatment is selected, work together with the Regional Bicycle and Pedestrian Coordinators to determine which pattern is appropriate. Standard Plan M-60.20 provides guidance for rumble strip patterns appropriate for use on undivided highways. Consider all of the following in the selection of a pattern:
- Routes with minimal bicycle usage afford opportunities to use a 16-inch-wide rumble strip where plenty of shoulder width is available. As remaining shoulder width becomes marginal, give more consideration to the narrower patterns.
- For routes with moderate to heavy bicycle usage, restrict the selection to a 12–inch-wide pattern (Standard, Type 2).
- On routes with higher run-off-the-road crash experience, consider the wider patterns with longer spaces between gaps (Standard, Type 3), if this application is consistent with guidance on bicycle usage.
- Where moderate to heavy bicycle usage occurs in locations where shoulders are known to collect considerable amounts of debris, consider the pattern with more frequently spaced gaps (Standard, Type 4).
- For 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 rumble strips, they may present an appropriate treatment for a specific site.
Coordinate with WSDOT's Maintenance Department
If a rumble strip treatment is chosen, consider the conditions on the shoulder that bicyclists may encounter. In some locations (such as forested areas), more frequent shoulder maintenance may be needed. Identify the associated maintenance personnel with which to discuss regular or more frequent sweeping, etc. This discussion should focus on optimizing the state’s maintenance funds. The discussion should take into account such factors as who will benefit (e.g., how many users), when will maintenance activities be needed, and what specific locations have the greatest need. Budget limitations require that any such activity be strategic with respect to both location and timing.
Consult your Regional/Headquarters Bicycle and Pedestrian Coordinator for all conditions where shoulder rumble strips are being considered at 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 Coordinators.
The Washington State Highway Log provides a reference for determining the following features of highway sections:
- Rural roads – Identifies the Functional Classification of sections
- Posted speed – Lists the designated Legal Speed of sections
- Shoulder width – Gives the shoulder width (as well as surface type: asphalt, gravel, etc.) of sections; this must be verified in the field. Consider discontinuing SRS at locations with barrier along the shoulder if the horizontal offset is reduced.
The Statewide Travel and Collision Data Office provides a resource for determining the collision history of roadway sections.
Links to other information sources on rumble strips
Share your thoughts on rumble strips with WSDOT
We are presenting this information to aid in the knowledge and discussion of the use of rumble strips in Washington State.
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Roadside Safety Engineer