Mumble strips show promise for fewer roadway departures with reduced road noise near residential and environmentally sensitive areas.

In 2014, more than half of traffic fatalities in the United States resulted from roadway departure crashes. The Federal Highway Administration defines roadway departure crashes as crashes that occur after a vehicle crosses an edge line, a center line, or otherwise leaves the traveled way. Sometimes the cause of these crashes is fatigued or distracted drivers drifting over the center line or the edge line of their lane. In such cases, one of the most effective safety countermeasures to reduce run-off-the-road and head-on crashes is the rumble strip.

A series of milled pavement corrugations or raised media affixed to the road surface (such as raised pavement markers) near the edge lines or center lines, rumble strips cause a combination of vibration and a staccato sound within the vehicle when they are struck. Together, the vibration and sound alert a driver that the vehicle is drifting out of the travel lane.

Since the 1990s, many studies of rumble strips’ safety performance have validated the safety benefits of this countermeasure when the design is the traditional milled cylinder shape and spacing--7 x 12 x 0.5 inches (18 x 30 x 1 centimeters) spaced 12 inches (30 centimeters) center-to-center--typically used in the United States. For example, a study under the National Cooperative Highway Research Program (NCHRP) found that, for head-on and opposite direction sideswipe collisions, milled center line rumble strips provide reductions in injury crashes of 45 percent on rural two-lane roads and 64 percent on urban two-lane roads.

Although the safety benefits are impressive, installations of rumble strips sometimes create complications. For example, rumble strips located on the shoulder and edge line can be difficult for bicyclists to traverse without slowing their speed significantly. In addition, the most common concern currently--and the one that sometimes results in the costly removal of rumble strips--is related to external noise when vehicles strike the rumble strips. Although this type of rumble strip is not in the driving lane, drivers may incidentally hit the rumble strip when making a passing maneuver or taking a curve too widely. Unfortunately, the unexpected and loud noise generated when a vehicle hits a rumble strip can be disruptive to those in the surrounding area. Transportation agencies often choose not to install rumble strips in the vicinity of residences or other noise-sensitive receptors because of these concerns, restricting the potential safety benefit of the countermeasure.

Many agencies have tried reducing dimensions of the milled cylinder shape to address some of the issues, but an entirely new design of rumble strips that uses an oscillating sine wave pattern has shown some potential to significantly reduce noise outside of the vehicle. Can researchers find an appropriate depth, width, and spacing of the sinusoidal pattern--dubbed the “mumble strip”--that will provide enough noise and vibration inside the vehicle to alert drivers that they are drifting from their lane? Read on to find out.

Research Into Quieter Pavements

California experienced an average of 1,370 fatalities from roadway departures annually from 2010 through 2014. To combat these types of fatalities, the California Department of Transportation (Caltrans) installs milled center line and shoulder or edge line rumble strips, with a recommendation that the shoulder should be a minimum of 5 feet (1.5meters) wide to facilitate use by bicyclists. The agency’s policy is to consider rumble strips in all resurfacing projects, and it encourages designers to use engineering judgment and consider the risks of run-off-road and head-on crashes when determining whether rumble strips are advisable.

As a result of its extensive experience dealing with the acoustic impacts of highway noise, Caltrans has a long-standing interest in traffic noise abatement. In the mid-2000s, for example, Caltrans conducted a study to compare noise measurements on various European pavements to similar data obtained for pavements in California and Arizona. The relationships that Caltrans researchers developed with the European researchers would become a key factor a few years later as they began to address noise issues related to rumble strips.

In 2005, the United Kingdom’s Department for Transport published an advisory based on research for a “quieter alternative to conventional rumble strips.” This research sought to refine the design of transverse rumble strips that are placed across the travel lane for a short distance to warn all motorists of an upcoming condition rather than narrow, continuous rumble strips at the edges of the travel lane to warn drivers who are moving outside the travel lane. However, the recommended sinusoidal design and profile would prove to be a starting point for later research on center and edge line rumble strips.

In 2007, Danish researchers published a seminal study that tested rumble strips designed specifically to generate low noise levels. The study tested five types of milled rumble strips, two depths of the sinusoidal design and three depths of cylinder shape, all more shallow than the depth that has proven to yield crash reductions in the United States. The Danish researchers calculated that, at distances exceeding approximately 82 feet (25 meters) from the road, passenger cars riding on sinusoidal indentations led to an increase of only 0.5–1 decibel in the maximum pass-by noise level compared to the noise of the same vehicles passing by on the pavement with no rumble strip. The cylindrical indentations gave a maximum increase of 2–7 decibels.

Note that a difference of 5 decibels is considered readily noticeable. NCHRP Report 641: Guidance for the Design and Application of Shoulder and Centerline Rumble Strips recommends that rumble strips create an increase in interior noise level of 6–15 decibels to be effective. The Danish study did not measure noise inside the vehicle, but multiple later studies have done so and found that the increase in interior noise varies based on vehicle type, vehicle speed, and rumble strip design.

The British and Danish studies came to the attention of staff at Caltrans’ Division of Environmental Analysis as the division was receiving increasing inquiries from its districts about how to deal with noise complaints related to rumble strip strikes. A number of citizens and Caltrans staff also expressed concerns about noise pollution resulting from rumble strip strikes in environmentally sensitive areas, especially in parks, nature reserves, and tribal lands. It was clear that district engineers needed a noise-reducing design alternative that would maintain safety benefits.

The Mechanics of Sound

“The public is always concerned about the noise levels generated by transportation infrastructure,” says Bruce Rymer, a senior engineer and acoustician with the Caltrans Division of Environmental Analysis. “With a little more thoughtful design, the noise levels can be turned down on elements like pavement, bridge decks, bridge joints, and rumble strips.”

In response, Caltrans contracted a motor vehicle noise and vibration engineer to begin studying whether existing rumble strip patterns could be improved to reduce noise. Caltrans has many miles of sound walls, but the most efficient noise control strategy is to turn down the volume at the noise source rather than disrupt the sound transmission path with a barrier or install some form of noise insulation at the receptor. Could researchers find a design that would lower noise outside the vehicle while maintaining or increasing it inside the vehicle?

The Mumble Strip

In 2009, researchers from the Danish Road Directorate (who had participated in the 2007 study) were on sabbatical in California and observed the research. It was a member of the Danish team who first coined the term “mumble strip” as a joke, but the U.S. researchers, amused by it, quickly picked up the term. Caltrans’ initial investigations and testing concluded that sinusoidal mumble strips could achieve the design goals of lowering exterior noise levels in the human hearing range and still provide sufficient driver warning. As expected, interior noise and vibration levels did vary depending on the test vehicle used.

Earlier quiet pavement research at Caltrans had found that some pavement surface textures are noisier than others and that raised (positive) texture is louder than recessed (negative) texture. Caltrans’ 2009 work resulted in a recommendation for an internal design concept that built on previous research for both quieter pavements and the Danish experiments with the sinusoidal rumble strip shape. Caltrans’ design goal was a sinusoidal pattern that minimizes the “harsh” impulse noise from existing rumble strip designs but still increases interior noise levels by 6 dB or more. The optimal wavelength or period for the sinusoid shape Caltrans was looking for would take into account U.S. standards for vehicle speed and tire width and diameter. The internal memo that resulted from the 2009 Caltrans research recommended an optimal period and depth regardless of speed.

Embracing the recommendation, Caltrans District 1 engineers began working to identify a potential test site for installation and to refine the approach to applying the concept in a test scenario. At the time, they were interested primarily in using the sinusoidal shape on edge lines and shoulders rather than on center lines. In July 2012, the district installed the mumble strips on a 7-mile (11-kilometer) road segment to demonstrate the design, followed by a study in September 2012.

After analyzing the data from the study, the engineers determined that the conceptual sinusoidal mumble strip pattern does lower roadside noise levels while maintaining interior cabin noise and vibration levels adequate to alert the driver.

Caltrans is pursuing a patent regarding its optimal sinusoidal rumble strip. Once the patent has been granted, the agency will look at how it can adjust its policy and approach toward applying rumble strips, including developing a set of recommendations on when and where its mumble strip design should or should not be applied.

“It’s far more cost effective to lower the noise generated at the source than to attempt to block it or provide noise insulation at the receiver,” says Rymer. “We need to think of our roadway design elements like washing machines and dishwashers--design them to be quiet.”

Minnesota Builds on Caltrans’ Research

As is the case for many States, fatalities and serious injuries as a result of roadway departure crashes are overrepresented in Minnesota. For example, of the 1,922 highway fatalities that the State experienced in 2010–2014, 52 percent were attributed to vehicles either running off the road or crossing the center line into oncoming traffic.

To combat this safety issue aggressively, in 2011 the Minnesota Department of Transportation (MnDOT) revised its policy on rumble strips. The revisions mandate that on rural trunk highways where the posted speed limit is 55 miles per hour (88 kilometers per hour) or greater, and the paved shoulder width is 4 feet (1.2 meters) or greater, shoulder rumble strips are to be placed on all rural highway projects that involve constructing, reconstructing, or overlaying shoulders. Center line rumble strips are subject to the same policy when constructing, reconstructing, or overlaying pavement, and the policy applies to both multilane undivided and two-lane undivided highways.

However, the resulting widespread application of rumble strips garnered its share of noise complaints from residents in certain areas. These complaints fueled MnDOT’s interest in California’s study of the sinusoidal shape. In 2015, MnDOT initiated the Sinusoidal Rumble Strip Design Optimization Study, which concluded in mid-2016. The final report is available at www.dot.state.mn.us/research/TS/2016/201623.pdf. This study built upon earlier work sponsored by the Minnesota Local Road Research Board, which compared three sinusoidal designs on the shoulders of county roads in Polk County, MN.

The goal of the 2015 MnDOT study was to determine an optimal sinusoidal design or designs that would lower nuisance noise levels, provide adequate warning to drivers who inadvertently are leaving a lane, and be safe for bicyclists and motorcyclists to traverse.

“In Minnesota, we’ve had a one-size-fits-all rumble strip,” says Will Stein, safety and design engineer with FHWA’s Minnesota Division. “So the idea was to give designers more tools. Much like design should be tailored to the highway’s context, safety measures like rumble strips can be better fitted to their surroundings.”

Phase 1: Broad Evaluation of Designs

MnDOT conducted the first phase of the study at its MnROAD test track near Albertville, MN. The purpose of phase 1 was twofold: (1)to subjectively evaluate a broad array of sinusoidal rumble strip designs and narrow them down to the most promising for more detailed field testing and noise measurement; and (2) to obtain feedback from motorcyclists and bicyclists on various designs within a safe, closed-track environment.

MnDOT researchers milled various sinusoidal configurations into the test track. They also milled MnDOT’s standard shape, which is the cylindrical design and dimensions proven in the NCHRP Report 641 to reduce crashes significantly. All of the sinusoidal rumble strips were milled to the same depth of 0.0625 inch (0.16 centimeter) at the high point and 0.375 inch (0.95 centimeter) at the low point. The differences among the designs included:

• Wavelength variations at 12, 14, and 16 inches (30, 36, and 41 centimeters).
• Widths of 14 inches (36 centimeters) for single-row rumble strips, and widths of 8 inches (20 centimeters) for double-row rumble strips. Double-row rumble strips were separated by 4 inches (10 centimeters) of pavement.
• Rumble strips with tapered edges versus straight vertical edges--to evaluate any difference for bicyclists and motorcyclists.

The MnROAD research team tested the array of rumble strip designs, as did the project’s technical advisory panel. The evaluation was subjective, but the broad consensus was that the 14-inch (36-centimeter) wavelength provided the best in-vehicle noise and vibration level. All wavelengths produced considerably less external noise compared to standard rumble strips.

Motorcyclists and bicyclists also evaluated the designs for traversability. Riders crossed over the rumbles to simulate passing maneuvers and rode along the rumbles to simulate tangential hits. Motorcyclists expressed clear preference for the single-row design, noting that it was more comfortable to traverse and provided more stability than the double-row configuration.

In addition, bicyclists noted that they preferred the sinusoidal shape because it is less jarring to ride over than the standard design. Neither group had a strong preference for the tapered versus vertical edges.

From this subjective feedback, which identified the 14-inch (36-centimeter) wavelength as the most promising sinusoidal shape, researchers selected four configurations for more detailed study on Minnesota State Highway 18 (MN 18).

Phase 2: Detailed Field Testing

For the MN 18 field testing, researchers conducted an additional subjective evaluation in combination with measurement of noise levels internally and externally using a noise meter. To measure noise levels within vehicles, they used a passenger car, a pickup truck, and a MnDOT tandem dump truck. They also measured noise levels externally at distances of 50 and 75 feet (15 and 23 meters) from the highway.

The study concluded that external noise was much lower for all four sinusoidal designs compared to the standard rumble strip. Internal noise for the passenger vehicle was strong for all four sinusoidal designs. The range for internal noise for the pickup did not vary greatly, but the 0.5-inch (1.3-centimeter) depth did provide a more audible warning. Noise levels within the dump truck, however, were difficult to detect above the engine and other noise from the truck itself. Consequently, areas that experience a high number of roadway departures for trucks may not be ideal candidates for installing sinusoidal rumble strips.

The study determined that the optimal sinusoidal design for asphalt pavements is the single-row 14-inch (36-centimeter) rumble strip that is 0.5 inch (1.3 centimeters) at its deepest point. For concrete pavements, Minnesota is considering a double row of 6-inch (15-centimeter) rumble strips to avoid milling through the joints. These will also be 0.5 inch (1.3 centimeters) deep. Both designs will use a 14-inch (36-centimeter) wavelength.

One of the additional benefits of MnDOT’s sinusoidal modified design is the increased durability of pavement marking. Theoretically, placing the pavement marking within the sinusoidal rumble will provide a greater lifespan for the pavement marking because the entire marking will be below the surface of the pavement. MnDOT found that a surface-applied latex will last 1 year, while recessed latex will last 3–4 years. The estimated longevity for an epoxy marking is 3–4 years; for recessed epoxy, the agency expects a 6–7 year lifespan.

Moving forward, MnDOT will revise its rumble strip policy to add a sinusoidal design option for noise-sensitive locations. However, Minnesota will not stop using traditional rumble strips. The current rumble design will likely remain the default, with the sinusoidal mumble design as an option when needed. District traffic engineers will make the decisions regarding applying the most aggressive safety design that is appropriate for the conditions.

Future Directions

Ongoing research into reengineering traditional rumble strips is motivated by the understanding that road designers need new tools that fit within the context of different areas and roadways so that they can take locational needs into consideration. To date, engineers from both Caltrans and MnDOT have described the research as “very promising.”

Although these studies into alternative designs have established interesting and informative results, researchers have examined only a few shapes and variations in depth in the United States. Experience with the standard rumble strip design indicates statistically significant crash reductions, but it will take approximately 5 years of study after installation of several hundred miles of the mumble strips (or any other shapes) before agencies have enough crash data to determine the actual safety performance of the alternatives.

Sinusoidal (Mumble) Designs Field Tested on MN 18
Strip 1: Single row on center line. Width: 14 inches (36 centimeters) Depth: 0.0625 inch (0.16 centimeter) (high point) to 0.375 inch (0.95 centimeter) (low point) Depicts an optimal sinusoidal rumble strip for Minnesota.	Strip 2: Two rows straddling center line. Width: 8 inches (20 centimeters) Depth: 0.0625 inch (0.16 centimeter) (high point) to 0.5 inch (1.27 centimeters) (low point)
Strip 3: Single row on center line. Width: 14 inches (36 centimeters) Depth: 0.0625 inch (0.15 centimeter) (high point) to 0.5 inch (1.27 centimeters) (low point)	Strip 4: Two rows straddling center line. Width: 8 inches (20 centimeters) Depth: 0.0625 inch (0.16 centimeter) (high point) to 0.375 inch (0.95 centimeter) (low point)

In addition, cost of installation is higher--and may be a factor in how often States use the sinusoidal rumble strip--because the shape requires continuous milling, which is a slower process and wears out the cutting heads more quickly. “There could be a 30 percent increase for small projects to account for the additional labor and cleanup costs,” says John Holbert, director of sales and marketing with Surface Preparation Technologies LLC. “Additional traffic control expense should also be anticipated due to the slower milling process.”

These extra costs may mean a finite number of safety dollars would not add as many miles of sinusoidal rumble strips as the traditional cylinder shape. However, the reduced noise concerns may make it possible for installations of sinusoidal rumble strips where the traditional shape would not be publicly acceptable, thereby extending the reach of this life-saving safety measure.

Lisa Kinner Bedsole is a senior technical writer with Leidos. She has supported FHWA’s Office of Safety outreach and communications efforts since 2005. She holds a B.A. in classics from the University of Mary Washington.

Ken E. Johnson is the State work zone, pavement marking, and traffic devices engineer in MnDOT’s Office of Traffic, Safety, and Technology. During his 25-year career, he has worked in surveys, land management, design, project management, and traffic engineering. He has a bachelor’s degree in civil engineering and a master’s degree in infrastructure systems engineering, both from the University of Minnesota. He is a registered engineer in Minnesota.

Cathy Satterfield is a safety engineer at FHWA’s Office of Safety focusing on reducing roadway departures and improving visibility. She holds a B.S. in civil engineering from the University of Minnesota and a professional engineer’s license in Idaho.

For more information, contact Cathy Satterfield at 708–283–3552 or cathy.satterfield@dot.gov.

The authors would like to thank Bruce Rymer for his contributions to the article.