State of The Practice for Shoulder and Center Line Rumble Strip Implementation on Non-Freeway Facilities

EXECUTIVE SUMMARY

Center line rumble strips (CLRSs), shoulder rumble strips (SRSs), and edge line rumble strips (ELRSs) are proven safety countermeasures intended to alert drivers when they leave the roadway across the center line or edge line through the generation of noise and vibration. Previous research has quantified the noise level generated by rumble strips and safety benefits on non-freeway facilities. Additionally, researchers have identified trade-offs for rumble strips, including inconvenience for bicyclists and motorcyclists, excessive noise for nearby residents, and potential for pavement life or pavement marking degradation.⁽¹⁾ These trade-offs have made it difficult for some State transportation departments to identify appropriate situations for implementing rumble strips on non-freeway facilities as well as identify an appropriate rumble strip design given a set of constraints.

This project had two main objectives. The first objective was to identify the state of knowledge and practice among State transportation departments for the use and design of CLRSs and SRSs, identify any information gaps, and develop a research plan to address those gaps. This document fulfills that objective. The second objective was to develop a decision support guide to inform agencies on CLRS and SRS installation. This objective was fulfilled with the preparation of Decision Support Guide for the Installation of Shoulder and Center Line Rumble Strips on Non-Freeways, which can be found on the Federal Highway Administration (FHWA) Rumble Strips Web site.⁽¹⁾

The project team developed the guide and final report based on a literature review, current practices review, and follow-up interviews with six departments. The literature review focused on the following areas:

• Rumble strip design.
• Noise and vibration testing methods and findings (inside and outside the vehicle).
• Impacts on bicyclists and motorcyclists.
• Pavement condition impacts.
• Pavement marking visibility.
• Operational effectiveness.
• Safety effectiveness.
• Expert system development.

The literature review showed that there has been ample research quantifying the safety effectiveness of CLRSs and SRSs on crash frequency and severity, but they do not account for reduced dimensions that are becoming more common to address bicycle accommodations and noise issues. Rumble strip effects on bicyclists and noise issues have been reviewed independently of each other and independently of safety effects; currently, there is no research identifying a relationship between sound level and safety. Additionally, few research studies made recommendations for one practice of rumble strip design over another.

The current practices review focused on departmental policies and standard drawings, and the project team conducted follow-up interviews with the Connecticut Department of Transportation, Louisiana Department of Transportation and Development, Minnesota Department of Transportation, Montana Department of Transportation, and Texas Department of Transportation, as well as FHWA’s Eastern Federal Lands. The results of the follow-up interviews are provided in appendix B and were used to supplement information found in the policies and standard drawings in the current practices review. The purpose of the current practices review was to identify currently used design dimensions for CLRSs and SRSs as well as rumble strip implementation strategies, selection criteria, special considerations, and modifications.

The current practices review identified that rumble strip implementation strategies include systemic, high-crash corridor, systematic approaches, and combinations of each. Using the systemic safety approach, departments implement rumble strips on corridors based on risk features that are correlated with higher severity focus crash types (e.g., fatal and incapacitating injuries on the fatal, incapacitating injury, nonincapacitating injury, possible injury, and no injury scale). In this approach, corridor crash history is not considered for identifying rumble strip treatment locations. Rather, crash data analyses are used to identify risk factors associated with fatal and severe injury run-off-road (ROR) crashes, fatal and severe injury head-on crashes, or other focus crashes. While the systemic approach to safety focuses on identifying locations for rumble strip installation based on risk, the systematic approach focuses on installing rumble strips system-wide (often while completing other construction activities) with exceptions for installation that are based on policy. Alternatively, departments have traditionally used crash frequency (e.g., locations with a higher number or higher than expected number of crashes) or crash rate to justify additional corridors for installing rumble strips on an as-needed basis. This approach may also be referred to as a “case-by-case” approach because installation must be considered for each corridor based on multiple factors, and the decision to install or not is made independently in each instance based on these factors. Most department policies do not address systemic installation and instead focus on systematic installation strategies and provide exclusion criteria for rumble strip implementation. Systematic installation strategies often consider average daily traffic, pavement condition, posted speed limit, lane width, shoulder width, total pavement width, and pavement maintenance. The results of the current practices review guided the development of the decision support guide (i.e., the overview of current practices and model decisionmaking framework).⁽¹⁾

Additionally, through the current practices review, the authors identified that while departments have their own standard designs for rumble strips, they do allow some flexibility, especially with sites that have a history of ROR crashes and bicyclist concerns. Most departments specify that high-crash locations can be identified through crash data and will choose to install rumble strips at these locations even if the systematic criteria are not met. However, it is important to show the effectiveness of the rumble strip design because the installation may impact the usability of the roadway for bicyclists or may burden nearby residents with increased noise or perceived noise activity. Many departments struggle with these considerations, and these considerations led to the recommendation for developing a rumble strip decision-support guide.⁽¹⁾

The purpose of the rumble strip decision support guide is to inform departments on CLRS and SRS installation on non-freeway facilities.⁽¹⁾ The draft guide was reviewed by a panel of State transportation departments and FHWA representatives. The guide describes methods for identifying appropriate locations for installation, assessing the potential crash reductions and benefit/cost ratio, and developing performance metrics for safety. As part of this research, the project team used processes from agencies that have had success with installing rumble strips to develop a model decisionmaking framework, which is a four-step process that leads decisionmakers to determine whether to install a standard rumble strip, modified rumble strip, alternative treatment, or no treatment. The framework considers whether the segment meets the department’s criteria for systematic installation or is justified by crash history if pavement condition, bicyclists, or noise are concerns.

A gap analysis was conducted based on the literature review and current practices review. The gap analysis identified that departments struggle with the optimal design and location of rumble strips given the geometry and context of the roadway. Additionally, departments struggle with identifying when noise issues will be a concern and what the optimal sound level should be. To date, no research studies have explored the impacts of rumble strips on pedestrian or bicyclist safety (i.e., no crash modification factors have been developed). Most assessments of pavement condition are anecdotal, and there is little quantitative research identifying the impacts of rumble strips on pavements or longitudinal joints. Finally, few safety studies have reported the dimensions of rumble strips included in the research, making it difficult to identify the safety effectiveness of different designs, particularly narrower and shallower rumble strips.

The project team developed an action plan based on the results of the gap analysis, which serves as a list of objectives for future research grouped under overarching goals. The goals, in no particular order, include the following:

• Establish safety effects of rumble strips.
• Identify performance measures for noise trade-off analysis.
• Identify performance measures for bicyclists for trade-off analysis.
• Establish effects of pavement condition and depth on deterioration.
• Develop an optimization tool based on the previous four goals and existing rumble strip implementation policies.
• Identify impacts of current and future in-vehicle technologies on rumble strip needs.
• Identify the impacts of rumble strips on driver behavior.