The Minnesota Department of Transportation (MnDOT) has just started a major project to install low-noise ‘mumble’ strips and striping to increase road safety and reduce disturbance at more than 80 intersections across 20 counties in the state.
Technically referred to as sinusoidal rumble strips, the lower noise ‘mumble’ strips are similar to traditional rumble strips, which warn drivers when they are approaching an intersection or departing their lane. Mumble strips have a wave pattern ground into the pavement longitudinally that lessens the external noise produced when vehicles travel across them. Traditional latitudinal rumble strips do not have the wave pattern. The new mumble strips will be installed on two-lane state highways that have intersecting minor roads, with the work beginning this week and lasting through the summer. The mumble strips will be installed on both sides of the lane and pavement markings will go over both sets of strips, with white on the right and yellow on the left.
More than 40% of all fatal and serious crashes happen at intersections, and according to a Federal Highway Administration (FHWA) study, a similar safety improvement design has helped reduce injury crashes by more than 30%. There were more than 1,850 serious or fatal crashes between 2008 and 2012 across Minnesota, so MnDOT implemented a rumble strip policy requiring centerline rumbles to be placed on all rural, undivided roadways with a posted speed limit of 55mph (88km/h) or higher where paved surface is constructed, reconstructed or overlaid after April 1, 2012. Centerline rumbles are shown to reduce crashes on rural two-lane roads by 9% in total.
One of the intentions of the rumble strip is to get the driver’s attention through noise, and this can be disturbing to residences near state highways. For rumbles to be effective, there must be a vehicle interior noise increase of 6-15dB, with MnDOT’s existing rumble strip design resulting in a 15dB increase. In the past, MnDOT used edgeline rumbles, which was found to create a fair amount of nuisance noise as some drivers ‘drive the fog-line’. In order to reduce the amount of nuisance noise, the agency’s current rumble policy encourages the use of centerline and shoulder rumbles instead. Due to their design, mumble strips rely more on interior vibration, and create less exterior noise.
“This will be our first large-scale installation of these strips. The goal with mumble strips is to encourage motorists to reduce their speeds, which can reduce the severity of crashes, should they occur,” explained Derek Leuer, MnDOT traffic safety engineer. “This is a low-cost strategy that we can implement to make rural intersections safer. The mumble strips are usually installed within a day and should have minimal impact on motorists, nearby residents and businesses. We will evaluate the intersections with the mumble strips for driver speed, crash frequency and crash severity. If the evaluations show that drivers are reducing speeds and there are fewer crashes, we could consider installing more of these intersection configurations in the future.”