Sinusoidal Rumble Strips aka “Mumble Strips”
Rumble strips, or “Milled-In Audible Roadside Delineators” (MIARD), are useful tools in the traffic engineer’s toolbox to alert inattentive motorists that they have wandered out of the travel lane, and either off of the road, or into the opposing lane of traffic. Unfortunately, the vibration and grating noise used to prevent collisions also produces unwanted exterior noise which is annoying to nearby land users. Traditional rumble strip designs also attract the ire from various bicycling and motorcycling groups, as they can disrupt rider control of two-wheeled vehicles.
Consequently, traffic engineers need to delicately balance the need for traditional rumble strip installations with adjacent land uses and other road users. Locations too close to populated areas will undoubtedly generate complaints, as will locations with high concentrations of two-wheeled vehicles.
European highway authorities have developed rumble strips with a sinusoidal wave pattern, which accomplish the desired objective, without generating as much road noise. The Danish Road Institute published a pilot study titled Traffic noise at rumble strips on roads: a pilot study on sinusoidal rumble strips in 2008. Conclusions from their study indicated that traditional rumble strips spaced on 1 foot intervals generated exterior road noise 3-7 dB greater than sinusoidal designs.
The Wirtgen Group W 50 DC milling machine can be outfitted with a rumble strip kit which produces both American “rumble” and European “mumble” patterns. Not only can the European sinusoidal mumble pattern be seen at this location on California Route 20, from Keys Boulevard to Orchard Shores Drive near Clearlake Oaks, the Google Maps car actually photographed the work being performed!
CalTrans District 1 also installed mumble strips in July 2012 on a seven-mile segment of US Highway 101 in Humboldt County. Mumble strips were installed at this location due to sensitive sound receptors, including nesting Marbled Murrelets, Spotted Owls, Osprey, and Bald Eagle habitat, as well as nearby homes and campgrounds.
Surface Preparation Technologies, LLC of Mechanicsburg, PA, was kind enough to conduct a demonstration of their SINNUS sinusoidal rumble strip design to our agency on a 1/2 mile segment of Marne Highway in Mount Laurel, NJ. The SINNUS rumble strip differs from the European sinusoidal; it follows a wave pattern which repeats on 2 foot intervals in a cut varying between 1/8″ to 1/2″ below the pavement surface. SINNUS can optionally be cut with tapered edges. These design features are friendlier to two-wheeled vehicles and produce less exterior road noise. Pavement markings are expected to last much longer when installed in a SINNUS rumble strip, as not only are they entirely recessed, they also give an important vertical component for nighttime and wet reflective visibility.
Minnesota DOT’s (MnDOT) Office of Traffic, Safety and Technology is testing various sinusoidal rumble strip patterns, including the SINNUS rumble strip. They have published an excellent YouTube video of their demonstration site near Thief River Falls which clearly demonstrates the exterior noise differences between traditional American and the SINNUS rumble strip designs.
Sinusoidal rumble strips will be the new future in the traffic engineer’s toolbox to reduce head-on and roadway departure collisions.