In the search for a quieter rumble strip, Minnesota may have found a winner in California.
California’s standard rumble strip design outperformed Minnesota’s and Pennsylvania’s in a comparison study along a rural highway near Crookston, Minnesota. (Read the recently published report.)
“California’s rumble strip still gave significant feedback to drivers, but it was significantly less noticeable outside the vehicle,” said engineering consultant Ed Terhaar, who performed a noise analysis with acoustical engineer David Braslau on behalf of the Minnesota Local Road Research Board.
Although they serve as an effective warning to drivers, rumble strips can cause unwanted noise when a vehicle drifts over a centerline or edgeline.
Both the LRRB and the Minnesota Department of Transportation, which is sponsoring a companion study, are interested in finding a new design that still captures the driver’s attention, but minimizes the sound heard by neighboring residents.
Polk County tests
Terhaar and Braslau’s research showed that Minnesota and California’s designs produce a similar level of interior noise. Although external decibel levels are not that different from each other either, Minnesota’s rumble strip has a considerably stronger tone that can be heard further away.
“California’s sound is less sharp, less intrusive and less noticeable,” Braslau said. “Minnesota’s has a really sharp peak. So while the absolute sound level of California’s isn’t all that much lower, its perception is less.”
Testing was performed using three different vehicles – a passenger car, pickup truck and semi-trailer truck – at three different speeds – 30, 45 and 60 miles per hour.
In general, Pennsylvania’s rumble strip had both a quieter interior and exterior sound than California’s and Minnesota’s.
Like Pennsylvania, California’s rumble strip has what is called a sinusoidal design – a continuous wave pattern that’s ground into the pavement (it’s the style commonly used in Europe and has been called a “mumble strip” because it’s quieter). The main difference between the two is that California’s wave length is 14 inches, while Pennsylvania’s is 24 inches.
Minnesota’s design is much different than the sinusoidal pattern used by the other two states.
“It’s not a continuous wave – it’s basically chunks of pavement taken out at certain intervals with flat pavement in between. It’s more of an abrupt design, whereas California and Pennsylvania’s are more continuous and smooth,” Terhaar explained.
The next step for researchers is to test variations of the California rumble strip design at MnDOT’s Road Research Facility (MnROAD).
The 8-inch rumble strip tested in Crookston is the typical edgeline design used by Polk County, but it was found to be too narrow for semi tires, so MnDOT will look at wider designs in its follow-up study. Researchers will also look at the impacts to motorcyclists and bicyclists, as well as the California rumble strip’s centerline striping capability.
MnDOT looks for solution to noisy highway rumble strips – Crossroads article