Minnesota: Are You Ready to Mumble?

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.

A California-style sinusoidal rumble strip, installed along a Polk County Highway.
A California-style sinusoidal rumble strip, installed along a Polk County Highway.

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.

The Minnesota rumble strip, left, and California rumble strip, right.
The Minnesota rumble strip, at left and also pictured in top photo, and California rumble strip, right.

Related Resources

Rumble Strip Noise Evaluation study

MnDOT looks for solution to noisy highway rumble strips – Crossroads article

2 thoughts on “Minnesota: Are You Ready to Mumble?”

  1. Rumble strips are just a horrible thing for bike riders. This design looks even worse then the current ones. A continuous
    trench to cross that collects debris!

Leave a Comment

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s