Rumble Strips and Rumble Stripes

Decision Support Guide for the Installation of Shoulder and Center Line Rumble Strips on Non-Freeways

Overview of Current and Successful Installation Practices

Nearly all State agencies have a systematic policy providing standard drawings for CLRS and SRS on rural, two-lane, undivided highways or multilane, divided highways. Few agencies use different rumble strip designs for the outside shoulder for the two facility types. Most agencies that distinguish between the two facility types provide drawings specifying rumble strips on the inside and outside shoulders on multilane divided highways. Nearly all agencies supplement the systematic approach with a high crash corridor approach for roadways that do not meet systematic policy. This flexibility is important because roadways with the highest risk for ROR crashes (i.e., roadways with narrow lane and shoulder widths) often do not meet the criteria for systematic policies. Flexibility is also important for modifying standard designs to accommodate both their installation and the needs of those affected by their presence. This section provides an overview of current agency systematic policies and successful high crash corridor installation practices.

Current Systematic Policies

Agencies use systematic policies to develop a set of criteria that, if met, automatically qualify corridors for rumble strip installation. Systematic policies most often apply to new construction, reconstruction, or resurfacing. This work is performed while the contractor is already on-site performing other activities and ensures the pavement is in good condition and will not be resurfaced again in the near term. Alternatively, agencies install rumble strips as a retrofit if the pavement quality is sufficient and there is no scheduled paving activity in the near term (the length of time varies by agency).

Systematic policies provide criteria for installation and a standard specification for rumble strip dimension and layout. Criteria for installation are generally not crash or risk-based; they are based on roadway geometry, roadway users, and traffic operations. The written policy specifies minimum (or maximum) values for which rumble strips may be considered. The SRS and CLRS sections provide examples of criteria agencies use for systematic policies. Standard drawings provide details on basic rumble strip dimensions, locations, and breaks. Rumble strips are typically broken for intersections and bridges; standard drawings may provide details on where the breaks occur and may address non-standard applications at locations such as tapers, auxiliary lanes, and driveways (if necessary). Additionally, the standard drawing may provide information on bicycle gaps.

Agencies have found increased buy-in for rumble strips when stakeholders are included in developing the language for systematic policies. For example, the Montana Department of Transportation engaged the bicycle community in developing their policy and received feedback on language such as "The ideal clear space between the shoulder rumble strip and the edge of the paved shoulder is 4 ft."⁽¹⁸⁾ The Montana Department of Transportation has modified the design to allow for bicycles and has used quality control to try to ensure that the 4-foot space is maintained. Engaging stakeholders also increases buy-in and leads to better solutions for high crash corridor solutions when the corridor does not meet the criteria for systematic installation. If stakeholders feel that their voice is heard, they will be more open to understanding the effectiveness of rumble strips and will be more willing to work toward a solution that includes more system-wide installation, even on roadways with bicycle activity.

The next two sections provide an overview of systematic installation policies and standards for SRS and CLRS, respectively. These sections highlight the variability in current practices and the practices of the agencies that have been more successful in obtaining buy-in for rumble strip installation for non-freeway applications. The discussion focuses on rural, two-lane highways and applies to multilane highways.

Shoulder and Edge Line Rumble Strips

Table 1 provides an overview of SRS and ELRS design dimensions and systematic installation criteria. Note that offset dimensions show the maximum under the agency’s policy. The offset is zero for ELRS. The following installation criteria are commonly used by agencies (note blank cells indicate no policy specifically dealing with that particular criterion):

• Minimum posted speed limit. The minimum posted speed limit typically serves as a surrogate for level of urbanization. Rumble strips are used in rural areas and are typically avoided in urban areas, unless justified by crash history, due to noise complaints.
• Shoulder width. The minimum width is most commonly specified to allow for adequate room for bicycle activity; however, several agencies provide rumble strips on narrower shoulders due to increased risk for severe crashes, while other agencies do not specify a minimum shoulder width.
• Minimum distance to nearby residences. Few agencies specify a minimum distance to nearby residences, but for those that do, the distance is most commonly near 650 ft.
• Asphalt condition. There are several criteria used for asphalt condition:
• Structural condition. Several agencies look for the asphalt condition to be "good."
• Maximum pavement age. In addition to new pavements, several agencies utilize pavement age as a criterion in lieu of structural condition.
• Minimum pavement depth. Several agencies note a minimum depth for the surface coat that allows for rumble strip installation within the surface.
• ELRS option. This column shows agencies that have policies specifying the use of ELRS. In the case of those marked N (no), their policy did not directly address the possibility of combining the SRS and edge line marking, or their policy only indicated a SRS with a specified offset distance from the edge line marking or pavement joint.

Other installation criteria not shown below include the following:

• Minimum AADT. Few State agencies specific a minimum AADT threshold for installation. Minimum AADT thresholds range from 400 to 3,000 vehicles per day.
• Minimum pavement width. Several states specify SRS or ELRS in lieu of CLRS if the pavement width is below a certain value.
• Minimum distance to residences. A few states have a minimum distance to nearby residences. Specified distances range from 100 ft to 2,000 ft. The distance is most commonly about 650 ft.

Figure 7 provides an overview of the dimensions used in Table 1. The dimensions are consistent with those provided in the Key Definitions section.

Figure 7 . Illustration. SRS rumble strip dimensions.⁽⁶⁾

Table 1. Agency systemic SRS installation criteria.

Table 1 also provides the standard dimensions and bicycle gaps agency use in practice. Agency practices vary most in the length dimension used for rumble strips. Two surveyed agencies have a standard 4-inch rumble strip application while many agencies have a minimum standard of 12 inches or more. Two agencies had policies that were not specific to non-freeway applications and one agency had no standard drawings or referenced dimensions. There is also wide variety in the run length for bicycle accommodation, although most agencies apply a 10- to 12-foot gap for bicycle crossing.

Many of the agencies listed have one design pattern for all installations and have no flexibility built into the policy. Other agencies have built-in flexibility. The flexibility is most commonly specifying rumble strip length and offset based on shoulder width. For example, Tennessee standards include rumble strip length from 4 to 16 inches and offset ranging from edge line to 12 inches. The dimension and location of the rumble strip varies based on the available shoulder width. For available paved shoulder widths 8 feet or greater, rumble strips are 16 inches with a 12-inch offset. For available paved shoulder widths of 2 to 8 feet, rumble strips are 8 inches and ELRS are specified. For shoulders less than 2 feet, a 4-inch ELRS is specified. Many agencies allow for variable rumble strip length and offset to maximize the clearance (allow for 4-foot shoulder).

Flexibility should be built into the policy. Flexibility is important in policy, especially if the policy specifies parties that are involved in the decision-making process. For example, the Minnesota Department of Transportation (MnDOT) policy provides districts discretion for lateral placement to abate noise concerns and accommodate bicyclists.⁽¹⁹⁾ Additionally, the policy provides flexibility in rumble strip length based on pavement width, flexibility in offset to accommodate bicyclists with input from the State Bicycle Coordinator, and flexibility to gap rumble strips on the inside of horizontal curves with nearby residences if a Safety Edge® or wider shoulder is installed.

Center Line Rumble Strips

Table 2 provides an overview of CLRS design dimensions and systematic installation criteria. Note that the spacing is listed as 12/24 inches for several agencies. This indicates that the spacing is 12 inches between rumbles followed by a 24-inch gap. The following installation criteria are commonly used by agencies:

• Minimum posted speed limit. The minimum posted speed limit typically serves as a surrogate for level of urbanization. Rumble strips are used in rural areas and are avoided in urban areas, unless justified by crash history, due to noise complaints.
• Minimum pavement, lane, or shoulder width.
• Minimum distance to nearby residences. Few agencies specify a minimum distance to nearby residences, but the distance is most commonly near 650 ft.
• Asphalt condition. There are several criteria used for asphalt condition:
  • Structural condition. Several agencies look for the asphalt condition to be "good."
  • Maximum pavement age. In addition to new pavements, several agencies utilize pavement age as a criterion in lieu of structural condition.
  • Minimum pavement depth. Several agencies note a minimum depth for the surface coat that allows for rumble strip installation within the surface.
• Passing zone option. Fifty percent of the agencies surveyed had no specific policy regarding CLRS and passing zones. Of those agencies with policies, 22 out of 25 indicate that rumble strips should continue through passing zones. Note that one of the three agencies specifying CLRS be discontinued in passing zones is reviewing its policy to allow CLRS installation in passing zones. A recent study in Minnesota reviewed all two-lane fatal head-on crashes and found that passing is rarely a contributing factor.⁽⁵⁾ Based on this finding, breaking at passing zones would be counter-productive.

Other installation criteria not shown below include:

• Minimum AADT. Few State agencies specify a minimum AADT threshold for installation. Minimum AADT thresholds range from 1,500 to 3,000 vehicles per day.
• Shoulder width. Few States have a minimum shoulder width requirement; however, some States use the minimum shoulder width to specify whether edge line rumble strips may be used in place of CLRS.
• Minimum distance to residences. A few states have a minimum distance to nearby residences. Specified distances range from 100 feet to 2,000 feet. The distance is most commonly about 650 feet.

Figure 8 provides an overview of the dimensions used in Table 2. The dimensions are consistent with those provided in the Key Definitions section.

Figure 8 . Illustration. CLRS rumble strip dimensions.⁽⁶⁾

As with SRS, the length dimension varies most for CLRS among agency practices. The most common applications are 12 and 16 inches, but range from 8 to 20 total inches. Several agencies use two rumble strips, straddling the center line joint; however, most agencies mill one rumble strip centered on the joint. Agencies also differ in spacing for CLRS. SRS are nearly universally spaced 12 inches center-to-center, whereas CLRS are sometimes spaced 12 inches, or 12 inches and then 24 inches. Agencies do this to differentiate the noise and vibration between CLRS and SRS. The intention is to alert drivers to which direction they have drifted in order to maneuver in the appropriate direction.

There is less flexibility in rumble strip length for CLRS than for SRS; however, it is important that systematic policies address flexibility for noise mitigation and design standards address rumble strip placement with respect to RPMs.

Table 2. Agency systematic center line rumble strip installation criteria.

Successful High Crash Corridor Installation Practices

Many high-risk locations may not qualify for systematic installation but may benefit from rumble strip installation based on crash history. Highway corridors, with narrow shoulders for example, may not provide adequate clear space for bicyclists with rumble strip implementation but may have a history of high ROR crash counts. Practitioners can use the methods provided in the Rumble Strips and Safety Management section to identify the need for and potential benefits of rumble strips in these corridors. Most agencies reviewed do not provide specific guidelines for how and when to install rumble strips in these cases. Additionally, these corridors will have the greatest potential for installation issues due to special considerations. Consideration of the potential benefits and trade-offs is paramount, and the agencies with the most success installing rumble strips have written processes or requirements, including who is involved in the final decision-making. Successful policies include relevant stakeholders in the decision-making process once identifying the need.

Several agencies identify key personnel involved with decision-making or identify personnel who are typically included in a rumble strip decision-making committee. Examples of personnel who may be involved in the decision-making process include the following (note that agencies differ in the titles of individuals and names of key offices):

• Designers.
• Traffic engineers.
• Safety analysts.
• Bicycle or non-motorized coordinators.
• Environmental engineers.
• Planners.
• Maintenance personnel.

Each of these personnel may be considered at the local/county, regional/district, or State/central office level of the organization and concurrence among personnel is paramount. Additionally, stakeholders may be included in the process or notified as early as possible to allow time for feedback. Potential stakeholders include municipalities, local bicycle groups, and adjacent roadway property owners and residents. Their feedback is critical, and should be considered in combination with potential safety benefits. This also provides the agency an opportunity to provide stakeholders with information on the safety benefits, including specific performance measures calculated in the safety analyses. This may help the agency to promote rumble strips to stakeholders.

A few agencies also note the importance of project decision documentation. Due to the potential safety impact of decisions, it is important to document the need and the decision whether or not to install rumble strips, and why. Documentation is also important for explaining the benefits to stakeholders and to those who may perceive a disbenefit to their installation. The Texas Department of Transportation (TxDOT) noted that if they demonstrate the historical crash reduction factors for rumble strip installations to the general public they have fewer complaints after installation.

The Montana Department of Transportation has a specific process for new construction, reconstruction, rehabilitation, and overlay corridors where the shoulder width is greater than 1 foot but less than 4 feet. The procedure includes the following steps:⁽¹⁸⁾

1. Complete an economic analysis targeting roadway departure crashes to determine if rumble strips are justified.
2. The Planning Division determines if and how bicyclists use the highway corridor using bicycle route maps, bicycle use heat maps, or other methods.
3. If rumble strips are justified and the roadway is determined to be a high priority bicycle route, then a Rumble Strip Committee meeting is convened by the Project Design Manager. The committee evaluates the route and recommends and documents the appropriate action in the appropriate report (such as the scope of work report). The members of the committee include members from the Traffic and Safety Bureau, Planning Division, Highways Bureau, and the District. Other divisions are included on an as-needed basis.

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