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.

The sinusoidal rumble strip uses a wave-shaped rumble strip to create the noise and vibration necessary to alert the driver.

Observed interior and exterior sound levels were generally similar to previous sinusoidal rumble strips tested, but there were measurable variations between the four different designs. The shallower strips increased the interior sound level, not greatly different from the deeper strips, but generated slightly lower sound levels measured 50 and 75 ft from the highway centerline.

Rumble strip designs 1 and 4 as described in the report created lower exterior sound-level increases but created interior levels similar to designs 2 and 3. The external results correspond to the depth of the rumble strip design, with designs 1 and 4 having a maximum depth of 1⁄8 in. less than designs 2 and 3. The interior sound-level increases are similar for all four designs, but vary by vehicle type. All the designs created increases greater than 10 dBA for the passenger car, which is a desirable level for gaining the attention of the driver. For the pickup truck, the interior sound-level increases ranged from 4.5 to 6.8 dBA, while the increases for the dump truck ranged from 0.8 to 2.7 dBA.

As in the earlier study, estimates of sound level with distance from the roadway were made using a typical outdoor sound-propagation model, using one-third octave band source levels taken from the maximum pass-by levels at 50 ft. These were then compared with the background sound-level spectrum measured in Polk County, which is representative of rural areas near two-lane roadways. Using the concept of sound detectability developed originally for the Army Tank Automotive Command in the early 1970s, the detectability of the rumble strips was calculated. “Detectability” level is normally lower than “audibility” level since it is associated with actively listening for a sound compared with passively hearing a sound. For example, in a restaurant, one can hear people at the next table but not pay much attention to what is being said. This can be called “passive” hearing. On the other hand, when one tries to understand carefully what is being said at the next table, this can be called “active” listening.

For the passenger car, all four of the sinusoidal designs created less exterior sound levels than the standard Minnesota rumble strip design. Theoretical detectability of the no-strip and Strip 4 sound level with a car extends out to about 2,000 ft. With the pickup, Strip 4 is detectable up to 2,500 ft. Sound from the truck with no rumble strip can be detectable at more than 3,000 ft, with sound from the rumble strips heard at slightly farther distances. As described in the previous study in Polk County the detectability distance for the standard Minnesota rumble strip design was well over 3,000 ft.

The 14-in-wide, 1⁄16-1⁄2 in. depth design was recommended for further implementation by MnDOT. While all the sinusoidal designs provided adequate driver feedback and minimal exterior noise for passenger cars, this design also gave good results for pickup trucks. This is important because pickup trucks make up a significant portion of the vehicle fleet. The single 14-in.-wide strip also was more desirable for motorcycle riders compared to the double 8-in. strips.

The full study report and background information can be found at http://dotapp7.dot.state.mn.us/projectPages/pages/projectDetails.jsf?id=13607&type=CONTRACT.

About The Author: Terhaar is a principal, Traffic Engineering, with Wenck Associates., Maple Plain, Minn.