Rumble Strips and Rumble Stripes

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

INTRODUCTION

BACKGROUND

Center line and shoulder rumble strips are proven safety countermeasures for reducing roadway departure crashes, including head-on crashes and run-off-road (ROR) crashes. According to a recent study, ROR left or right crashes account for 64.4 percent of all single-vehicle crashes and, of those, 95.1 percent of the time the critical reasons were driver related.⁽¹⁾ Further analysis indicated the dominant critical reasons for passenger cars were internal driver distraction, steering overcompensation, poor directional control, too fast for curve, and sleeping. The dominant critical reason for large trucks was sleeping. According to the Fatality Analysis Reporting System (FARS) data, approximately 55 percent of fatal crashes are those targeted by rumble strips.⁽²⁾ Moreover, for rural two-lane highways, that number increases to approximately 66 percent. According to a 2009 National Highway Traffic Safety Administration study using events from the 100-Car Naturalistic Driving Study, approximately 66 percent of ROR events were departures to the right and 31 percent were to the left.⁽³⁾ Furthermore, Leuer found, using 2009 to 2013 data, that 13.4 percent of fatal crashes on Minnesota rural, two-lane highways were ROR left and 16.7 percent were ROR right, indicating that center line rumble strips are as important at preventing ROR crashes as shoulder rumble strips.⁽⁴⁾ In a separate study, Leuer found that while head-on crashes account for only 5 percent of crashes on Minnesota rural, two-lane highways, they account for more than 18 percent of the fatal crashes.⁽⁵⁾ Further analysis indicated that approximately 65 percent of fatal head-on crashes were the result of the vehicle drifting over the centerline and 31 percent were the result of control loss.

Rumble strips are a relatively low-cost countermeasure and economic analyses have indicated benefit-cost (B/C) ratios that exceed 100 to 1 (i.e., 100 dollars saved for every 1 dollar spent). For this reason, shoulder rumble strips have been installed nearly system wide for the Interstate system and most freeways and expressways. However, agencies have had varying degrees of success installing center line and shoulder rumble strips on multilane and two-lane roadways. The three primary concerns for rumble strips for these highway types include the following:

• Inconvenience for bicyclists. Standard rumble strip dimensions used by most agencies are difficult to traverse if bicycle gaps are not provided, as they make the rider uncomfortable and may lead to loss of control.
• External noise pollution. Rumble strips alert motorists through noise and vibration. The noise generated by rumble strips is different than background traffic noise and is intermittent. This can create a disturbance for nearby residents and for special environmental conditions (e.g., noise-sensitive wildlife habitats) if there are frequent incidental contacts.
• Pavement durability. There is concern that milling rumble strips into longitudinal joints or into the wearing course will allow water infiltration causing premature pavement deterioration.

Purpose of this guide

The purpose of this guide is to inform agencies on center line and shoulder rumble strip installation. It describes methods for identifying appropriate locations for installation, assessing the potential crash reductions and B/C ratio, and developing performance metrics for safety. Additionally, this guide discusses special considerations for rumble strip installations, identifies variability in current practices, and provides a decision-support framework for installing rumble strips.

The framework covers policy development for systematic rumble strip installation and provides a flowchart for decision-making for sites that can benefit from installation but do not meet criteria for systematic installation. Rumble strips fulfill a systemic need; however, the framework is also applied to sites that are identified based on crash history, such as for Highway Safety Improvement Program (HSIP) selection. Within this framework, this guide describes who may be involved in the decision-making process, at what points, and provides an overview of safety performance measures that can be presented to policy makers and stakeholders. Performance metrics described in this guide can be used to inform stakeholders of rumble strip benefits.

Intended Audience

This guide is intended for practitioners in transportation planning, highway design, traffic operations, highway maintenance, and traffic safety concerned with reducing target crashes through the installation of center line and shoulder rumble strips. This includes practitioners in Federal, State, local, and Tribal agencies tasked with improving highway safety using proven, low-cost, safety strategies. This also includes consultants working for these agencies.

Guide Organization

The guide is organized as follows:

• Introduction. This section provides background on the safety countermeasure, the purpose of the guide, and key definitions related to rumble strips.
• Rumble strips and safety management. This section discusses the methodologies for implementing rumble strips as a safety countermeasure and methodologies for estimating the safety effectiveness and economic impact.
• Special considerations. This section discusses key factors related to the impacts of rumble strips on bicyclists and motorcyclists, noise impacts on nearby residents, and perceived impacts to pavements.
• Overview of current and successful practices. This section identifies current practices agencies use for installation of rumble strips and identifies successful practices and methods for installing rumble strips based on high crash corridor analyses.
• Decision-support framework for rumble strip installation. This section provides a framework for agencies to follow based on current successful practices of agencies with widespread rumble strip installation.
• Case studies. This section includes case study examples where agencies weighed the decision to install rumble strips when there was potential concern for roadway users other than vehicles, nearby residents, and pavement condition.
• Other resources. This section provides links to related resources.

Scope of this Guide

The information presented in this guide focuses on rural, non-freeway applications. Additional information is presented for urban areas as appropriate. The guide does not focus on freeways, as there are fewer trade-off concerns and agencies have generally been successful at installing rumble strips on these facilities. The guide supports the installations of both center line and shoulder rumble strips.

How to Use this Guide

It is recommended that the entire guidance document be reviewed before application. This guide provides an overview of installation approaches and provides methodology for selecting high crash corridors for treatment, estimating countermeasure effectiveness, and conducting B/C analysis. These methods can be used to inform decision-making for the following:

• Systemic safety issues.
• HSIP selections.
• Recommendations stemming from road safety audits.
• Trade-off analysis when considering multiple options for treatment or non-treatment.

Additionally, this guide provides an overview of current agency practices and a model decision-making framework for installing rumble strips. This information can be used to inform rumble strip policy development and for identifying alternative designs for rumble strip installation where the standard practice cannot be applied.

Key Definitions

This section introduces important concepts and characteristics of rumble strips. Rumble strips are characterized by their location, type, and dimensions. Each of these characteristics are described in greater detail in the subsections that follow.

Placement

Rumble strip placement is defined as center line (CLRS), shoulder (SRS), or their combination (CLRS+SRS). SRS can be further defined by their offset from the edge line pavement marking. If the SRS is applied in conjunction with the pavement marking, then it is characterized as an edge line rumble strip or stripe (ELRS). If the SRS is located outside the pavement marking, then it is simply referred to as SRS. Throughout the document SRS and ELRS are collectively referred to as SRS, unless specifically talking about ELRS. Figure 1 shows and installation of combined CLRS and SRS.

Transverse rumble strips are placed within the lane to warn drivers of upcoming unexpected changes, such as traffic signals, changes in alignment, or the need to change lanes. Transverse rumble strips are not a focus application for this guide and will not be discussed further.

Figure 1 . Photograph. Combination milled CLRS and SRS.

Type

Currently, there are two main types of rumble strips used on rural, non-freeway facilities, milled and raised. Milled rumble strips, which are most prevalent, are milled into the roadway surface using a rotary milling machine. They function by allowing the tire to drop into the groove, which creates both sound and vibration. The sound level has been studied and has been shown to be a function of the dimensions of the milled rumble strip, which are explained in the next section. Figure 1 provides an example of milled rumble strips. Recently, some agencies have also begun studying and specifying dimensions for milled sinusoidal rumble strips, which are intended to reduce the external noise produced while providing sufficient noise and vibration to alert the driver of roadway departure.

Although research suggests that milled rumble strips are the most effective application type, raised rumble strips have been applied in States with warmer climates in cases where milled rumble strips cannot be installed. Raised rumble strips include side-by-side raised pavement markers, rumble bars, or plastic inserts within thermoplastic pavement markings. Profiled thermoplastic pavement markings have been developed to help with nighttime, wet visibility and, may have some very limited rumble characteristics. Figure 2 provides an example of profiled thermoplastic pavement markings in Washington. Locations without snowplowing activities may use profiled thermoplastic pavement markings or other raised rumble strips; however, milled rumble strips are preferred. Raised rumble strips may be considered in areas where milled rumble strips are not practical, such as bridge decks or on thin surface courses (e.g., chip seals).

Figure 2 . Photograph. Example profiled thermoplastic pavement marking.

Dimensions

Figure 3 provides a graphical representation of rumble strip dimensions, which is explained as follows:⁽⁶⁾

1. Offset: This is the distance from the pavement marking (delineating the edge of the traveled way) to the inside edge of the rumble strip.
2. Length: Dimension of the strip that is perpendicular to the travel directions of the roadway. This is often referred to as the transverse width of the rumble strip.
3. Width: Dimension of the strip that is parallel to the travel direction of the roadway.
4. Depth: The maximum distance from the surface of the roadway to the bottom of the rumble strip.
5. Spacing: The distance between adjacent rumble strips. It is most often measured from the center of the strip to the center of the adjacent strip.
6. Gap: The distance from edge of rumble strip to edge of rumble strip when there is a break in the pattern. Gaps are commonly used to allow bicycles to cross the rumble strip pattern, to allow passing vehicles to cross CLRS, and to allow for turning movements at intersections and driveways.

Figure 3. Illustration. Rumble strip dimensions.⁽⁶⁾