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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Public Roads - Jan/Feb 1999

A Silver Bullet: Shoulder Texture Treatments

by Ann Walls

I want to tell you a story. This is one of those stories you've probably heard once or twice before. It begins, as all good stories, with "once upon a time" and ends with "if only it were simpler, cost less, and could be implemented easily." The middle part is, of course, the story of implementing new and improved safety devices, appurtenances, and roadside features. It's the "silver bullet" story.

You see, many of us in the safety community have been waiting for a hero like the Lone Ranger to ride in, shoot a couple of silver bullets into their safety problems, and end them. It's a pretty good story, but most of the time, it's simplistic and far-fetched.

Most highway safety professionals know that safety usually doesn't come cheaply, and we all know that funds are limited. The safety appurtenances we see every day are still evolving and improving. While new and better devices are indeed being developed all the time, unfortunately motorists, pedestrians, and bicyclists continue to be killed and injured at alarming rates.

Waiting for a silver bullet or an "ultimate" solution to be developed isn't usually the best way to deal with today's highway safety problems. Nevertheless, every once in a while, we do develop a silver bullet that is simple, inexpensive, and effective and provides immediate results. One of our "silver bullet" safety enhancements is the proper selection and installation of shoulder texture treatments, also known as "rumble strips." Many states and local agencies have had good results with them.

Shoulder texture treatments were originally introduced as a safety countermeasure to help fatigued or drowsy drivers. During the 1970s, they were being rolled into new bituminous shoulders or cast into new concrete shoulders. They were found to be very effective on long, monotonous sections of rural highways and in other areas where drivers were likely to suffer from fatigue.

The development of small milling machines to cut shallow channel sections into existing shoulders soon followed, and while this approach was relatively expensive, several states began to implement and evaluate the results of rumble strips. During the last decade, better equipment and newer devices have significantly reduced the cost of implementing rumble strips.

Scope of the Problem

Almost 60 percent of rural fatal crashes occurred on tangent alignments and level roads, and 82 percent of these crashes are away from highway junctions. Numerous studies have shown that driver fatigue was a contributing factor in many of these accidents. Accident reports, however, tend to underestimate the effect of driver fatigue because drivers are not fond of admitting that they went to sleep or were dozing off before running off the road.

Are many drivers actually falling asleep at the wheel and driving off the road? They certainly are! Information from various sources supports the contention that a significant portion of highway crashes involve drowsy or sleeping drivers. The Pennsylvania Turnpike Commission estimated that before they started using rumble strips, 57 percent of all run-off-the-road crashes on the turnpike were caused by drivers falling asleep or being inattentive. Similarly, the New York Thruway Authority estimated that 40 percent of all crashes on the thruway were related to drivers falling asleep. These aren't unique situations; most organizations that have studied run-off-the-road crashes have reported that between 40 percent and 60 percent of these crashes are related to driver fatigue.

Cost of Rumble Strips

Interestingly, the cost of implementing shoulder surface treatments is surprisingly low where there is sufficient shoulder width available.

Early in the 1990s, the average cost of rolled-in rumble strips was being reported at between 7 to 10 cents per linear foot. At the same time, the cost of milled-in rumble strips was running usually between 35 to 50 cents per linear foot, but sometimes, it was more than a dollar per linear foot.

Currently, the milled-in treatments are becoming more common. The cost generally runs between 20 to 35 cents per linear foot, but some costs are as low as 12 cents per linear foot for longer sections of work. Of course, the cost of providing traffic control during construction adds to the overall expense, but the overall cost of work-zone traffic control has also gone down as more effective equipment has entered the marketplace.


The results, particularly on highways with long, monotonous tangent sections where drivers tend to drift off to sleep, have been remarkably good. Of course, most of these devices have been used only on major divided highways or in areas with a documented history of accidents related to fatigued drivers.

Reviews have shown that up to 80 percent fewer run-off-the-road accidents have been reported after the installation of "audible roadway delineators," as one state calls its rumble strips.

In one of the larger studies in California, a reduction of 49 percent in run-off-the-road accidents was reported on a 254-kilometer section of highway after the treatment was put in place.

Presenting the Facts

The objectives of the Federal Highway Administration (FHWA) are to reduce the number and the severity of crashes by accelerating the acceptance and implementation of shoulder surface treatments. Given the magnitude of the benefits reported in the use of these audible delineation and warning devices, it seems entirely appropriate to make the use of rumble strips a routine practice in conjunction with resurfacing operations and new construction on all rural sections of the National Highway System.

One of FHWA's first activities was to survey their field offices for best practices and policies. FHWA also developed two informational brochures - one targeted at highway managers and the other for the driving public - to make people aware of the potential benefits of shoulder treatments.

The brochure for the general public says, "When the road rumbles, it's time to take a break!" This brochure is meant to educate the public about the drowsy driver problem, show how rumble strips can act as an audible and tactile warning, and teach drivers how to steer the vehicle back onto the roadway without losing control.

Developing the Best Guidance and Technical Information

FHWA, through its Office of Highway Safety, is currently evaluating the information received from the field to determine the best state-of-the-art practices, programs, and policies. FHWA hopes to have this information available in the near future in the form of a technical advisory.

In addition, while working with FHWA field offices, several excellent sources of current information, including evaluations and cost data, were identified. In particular, the New York Division researched and published a report entitled The Effectiveness and Use of Continuous Shoulder Rumble Strips. This report is now being enhanced and made into an interactive, multimedia CD-ROM. The CD will present information on issues and effectiveness of rumble strips, installation information, and design details.

The FHWA Office of Highway Safety will be working with individual states to enhance their programs and to accelerate the application of audible delineation and warning devices on shoulders.

Noise and Bicycles

Some potential adverse effects will also be addressed. The two negatives that most often come up are the noise - particularly in built-up areas - and the potential adverse effects of rumble strips for bicyclists. Interestingly, as the novelty of shoulder surface treatments wears off in an area, the noise generated by truck drivers wanting to try them out tends to drop off significantly.

Bicycle safety is another matter. FHWA is aware that many highways need to safely accommodate an increasing number of bicyclists and pedestrians. The problem is that narrow shoulders or the placement of surface texture treatments can force the bicycle rider into the travel lane for motor vehicles. However, it is interesting to note that on highways with fairly wide, paved shoulders that are used by bicyclists and that have rumble strips, the audible delineation and warning serves to effectively separate the motor vehicle and non-motorized vehicle traffic.

Because rumble strips could also be an important solution to run-off-the-road problems in metropolitan areas, areas where bicycles are present, and areas where pedestrians walk along the roadside, it is also necessary to develop guidelines on the application of audible roadway delineation on all sections of the National Highway System.

This is exactly what FHWA wants to do. FHWA's tasks are to identify the best treatments and to strive for consistency in the design and application of shoulder texture treatments.

A key element in the selection and installation of rumble strip patterns is the need to provide a smooth, clear, and paved surface for bicycles. This can be done by locating the rumble strip close to the pavement edge line. Another rumble strip safety enhancement for bicyclists is the use of a skip pattern in the rumble strip that allows the bicyclist an opportunity to enter and exit the bike path with out having to cross over any of the surface treatments.


Some people contend that audible delineation and warning devices only temporarily awaken the driver and the driver may fall asleep again. Of course, this is possible; nonetheless, field experience indicates that significant reductions in run-off-the-road accidents can be anticipated when shoulder treatments are applied.

The reported benefit-to-cost ratios of shoulder treatments (where sufficient shoulder width already exists) range from 20-to-1 for rolled-in strips to 5 (or more)-to-1 for milled-in strips. The ratios for milled-in shoulder treatments have been improving over the last several years as equipment and application become more cost-effective and efficient.

FHWA is working with the American Traffic Safety Services Association and other industry groups to collect, develop, and present technical information. FHWA will be discussing issues and answers with industry at the 29th Annual American Traffic Safety Services Association Convention and Traffic Expo in San Antonio, Texas, in February 1999.

For additional information about this conference, call Ann Walls at (202) 366-6836. Information about the development of the technical advisory is available from Brian Gilleran at (202) 366-0915. For information about development and distribution of the CD-ROM - for which several states are sharing information and participating in the development of technical information, guidelines for application, and sample shoulder surface treatment details and specifications - call Jim Growney at (518) 431-4224, ext. 212, or Emmett McDevitt at (518) 431-4125, ext. 231.

Ann Walls has been a marketing specialist in the Office of Highway Safety for three years. She is a member of the Safety Outreach and Work-Zone Safety teams, and she is a member of the American Marketing Association. She received a bachelor's degree in business management (with an emphasis in marketing) from the University of Maryland.

Let's Rumble

A Rumble Strip Web site is scheduled to premier at the Annual Meeting of the Transportation Research Board in January 1999. The Web site will enable transportation professionals and the public to access up-to-the-minute information about shoulder rumble strips, their use in mitigating run-off-the-road crashes, and issues that affect their implementation. At the time of publication, the anticipated URL (web address) for the site is The Web site is logically organized, and it links sections of information that examine all aspects of rumble strips and their use. Specifically, the site:

  • Addresses problems, solutions, cost-benefit issues, and frequently asked questions about rumble strips.
  • Provides several state reports on the effectiveness of rumble strips and offers a multimedia demonstration of the noise and vibration of rumble strips.
  • Reviews technical specifications and installation data about the four types of rumble strips and about the policy issues involved.
  • Discusses the issues concerning rumble strip implementation, including the effect on the surrounding community, safety for bicyclists, and the concerns of road maintenance professionals.
  • Presents resources concerning rumble strips, including a library of implementation reports, outreach materials, multimedia archives, a Who's Who of rumble strip contacts, and links to other rumble strip resources.

In addition, users will be able to access the "Ask the Expert" feature to get answers to specific questions not addressed elsewhere on the site.

The Rumble Strip Web site is designed to be a central, easily accessible place for information about this important traffic safety topic.

For information about rumble strips, their application, or this Web site, contact either Jim Growney of the Albany, N.Y., office of FHWA's Eastern Resource Center by telephone at (518) 431-4224, ext. 212, or by e-mail at or Emmett McDevitt of FHWA's New York Division Office by telephone at (518) 431-4125, ext. 231, or by e-mail at