This Sinusoidal 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 that compared three alternative longitudinal edge line rumble strips along the edge of two roadways in Polk County, Minnesota, sponsored by the Minnesota Local Road Research Board. 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. All four of the centerline rumble strips evaluated in this report were a 14 inch wavelength sinusoidal design but with different geometric configurations. A single strip 14 inches wide and a double strip of two 8 inch wide strips spaced 4 inches apart were tested, each with two different depths — 3/8 inch and 1/2 inch. An evaluation of motorcycles and bicycles was carried out at the MnROAD facility near Albertville, Minnesota, 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 inches wide with a maximum depth of 3/8 inch. Designs with two strips spaced 4 inches 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 in the previous study. 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 feet and 75 feet from the edge of the roadway, as well as inside the vehicle adjacent to the driver. Video recordings were taken 50 feet from the edge of the roadway. Digital audio recordings were captured for each of the sound level readings. The maximum observed pass-by level is used here for the comparative analysis. Observed interior and exterior sound levels were generally similar to the California strip tested in the previous study at 60 mph, 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 feet from the highway centerline. Rumble strip designs 1 and 4 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 inch less than designs 2 and 3. The interior sound level increases are similar for all four designs but vary by vehicle type. All of the designs created increases greater than 10 dBA for the passenger car, which is a desirable level for gaining 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 feet. 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 feet. With the pickup Strip 4 is detectable up to 2,500 feet. Sound from the truck with no rumble strip can be detectable at more than 3,000 feet, with sound from the rumble strips heard at slightly farther distances. As described in the previous study in Polk County, Minnesota, the detectability distance for the standard Minnesota rumble strip design was well over 3000 feet. In summary, it is the authors’ opinion that Rumble Strip Design 3 (14 inches wide, 1/16 — 1/2 inch depth) be considered for further implementation by MnDOT. While all of the sinusoidal designs provided adequate driver feedback and minimal exterior noise for passenger cars, Design 3 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 inch wide strip of Design 3 was also more desirable for motorcycle riders compared to the double 8 inch strips of Designs 2 and 4. Additional feedback from motorcycle riders should be obtained for this design since the rumble strips installed at MnROAD were less than ½ inch in depth. In areas where there is extreme sensitivity to external noise, Rumble Strip Design 1 (14 inches wide, 1/16 — 3/8 inch depth) would be an acceptable alternate design. (Author/publisher)
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