Listening to a sine

By: Edward Terhaar, P.E.

The Sinusoidal (Sine) Rumble Strip Design Optimization Study presents results of sound-level monitoring of four types of centerline rumble strips installed along Trunk Highway (TH) 18 in Mille Lacs and Aitken counties in Minnesota.

This study follows an extensive study completed in 2015 that compared three alternative longitudinal edge-line rumble strips along two roadways in Polk County, Minn., sponsored by the Minnesota Local Road Research Board. In the Polk County study, the California design provided adequate driver feedback while generating less exterior noise than the standard Minnesota Department of Transportation (MnDOT) design. The Pennsylvania design did not provide enough driver feedback even though it generated less exterior noise than either the California or Minnesota designs.

These studies are in response to objections raised by some landowners about the unwanted noise caused by vehicles traveling over rumble strips when they drift over the edge or centerline of the roadway. By changing and modifying the design, the ultimate goal is to provide the maximum safety by capturing the driver’s attention through in-vehicle-generated sound levels while minimizing the associated external noise generated by the rumble strips.

To reduce this unwanted exterior noise, alternate rumble strip designs have been created and tested. One such design is the sinusoidal rumble strip. As the name implies, this design uses a sinusoidal wave-shaped rumble strip to create the noise and vibration necessary to alert the driver. With a less abrupt design than the standard MnDOT rumble strip, the exterior noise level is reduced.

Making it rumble

In the Sinusoidal Rumble Strip Design Optimization Study completed by Wenck Associates, David Braslau Associates and MnDOT, sound levels were monitored for four types of centerline sinusoidal rumble strips installed along TH 18 in Mille Lacs and Aitken counties in Minnesota. All four of the centerline rumble strips evaluated were a 14-in. wavelength sinusoidal design but with different geometric configurations. A single strip 14 in. wide and a double strip of two 8-in.-wide strips spaced 4 in. apart were tested, each with two different depths—3⁄8 in. and 1⁄2 in.

An evaluation of motorcycles and bicycles was carried out at the MnROAD facility near Albertville, Minn., to determine how various rumble strip configurations affected rider performance. An overall summary of the survey data indicates a preference for rumble strip designs which were 14 in. wide with a maximum depth of 3⁄8 in. Designs with two strips spaced 4 in. apart were the least desirable, according to the motorcyclist evaluations due to the raised section located between the two rumble strips.

Tests on TH 18 were performed with three different vehicles—passenger car, pickup truck and a Class-35 tandem dump truck. A single speed of 60 mph was used, as this was shown to provide the most meaningful data from previous studies. For each of the designs, an initial test was performed with vehicles traveling on normal pavement, followed by three passes on the rumble strip.

One-third octave band sound levels were taken 50 ft and 75 ft from the edge of the roadway, as well as inside the vehicle adjacent to the driver. Video recordings were taken 50 ft from the edge of the roadway. Digital audio recordings were captured for each of the sound-level readings. The maximum observed pass-by level was used for the comparative analysis.