SUMMARY

Effective Low-Noise Rumble Strips

The purpose of this research was to propose a rumble strip design that would minimize exterior noise on the outside of the vehicle yet generate sufficient auditory and vibratory response on the inside of the vehicle to alert the operator of a roadway lane departure. A review of the previous research indicated that this could best be accomplished with a rumble strip of a sinusoidal profile. However, systematic investigation of the parameters of such designs were limited and generally not comparable due to the different measurement methods used. As a result, the first portion of this research was to develop suggested test procedures for exterior noise exposed to residents along the roadway and for the operators inside the vehicles. Standard procedures existed for measuring the pass-by noise for vehicles for the purpose of evaluating pavements regarding tire-pavement noise. These were slightly modified to apply to rumble strips, as well as pavement in the travel lane. For interior noise and vibration, procedures were not standardized, particularly for the application evaluating rumble strips.

To develop proposed standard interior procedures, nine vehicles from four design categories were tested on two types of rumble strips in California: sinusoidal strips with a 14-inch wavelength and a nominal peak-to-peak depth of inch; and more traditional cylindrical ground depressions 5 inches in length and a ⅜-inch peak depth spaced every 12 inches in the direction of the roadway. The results of these measurements (see Chapter 2) were used to finalize the test procedures and to determine four groupings of light vehicle types to be represented in future rumble strip evaluations.

Using these measurement procedures, testing of existing sinusoidal rumble strips in Indiana and Michigan was conducted (see Chapter 3). These included 12-, 14-, 18-, and 24-inch sinusoidal wavelengths, peak-to-peak depths of ⅜ to ½ inches, and recesses of 0 or ⅛ inch. The results generally confirmed that sinusoidal strips could produce sufficient interior noise and vibration to alert vehicle operators of a lane departure; however, because of site pavement differences and the small sample of wavelengths, additional evaluations were required. For this purpose, 20 different sinusoidal strip designs were installed along a two-lane highway in the state of Washington. These were created to evaluate the design parameters of wavelength, peak-to-peak amplitudes, and recesses more consistently. These design parameters spanned the range included in the Midwest strips; however, the wavelengths were concentrated in the range of 14 to 16 inches. In addition to these, the standard non-sinusoidal strips used by Washington State Department of Transportation (WSDOT) were installed and tested (see Chapter 4). These were identical to those tested in California except with cylindrical ground depressions 6 inches in length. All the experimental sinusoidal strips produced lower pass-by levels than the standard design by 3.4 to 13.9 decibels (dB) when properly summed (band-passed) in frequency range from 31.5 to 200 hertz (Hz) to account for the high overall off-strip levels due to the chip seal of the highway surface.

Eight of the sinusoidal designs (13-, 14-, 15-, and 16-inch) produced higher interior noise levels than the WSDOT standard design. Three (13-, 14-, and 15-inch) sinusoidal design bands also produced larger on/off increments than the standard strips. The absolute interior vibration levels for the sinusoidal designs were less than the WSDOT design; although, three were within 2.6 dB. The increments for these three were less by 1 to 2 dB than the WSDOT design; however, all exceeded the 10 dB goal by about 4 dB.

Based on this research, a design proposal for sinusoidal strips was developed. The wavelength of the sinusoidal shape is 14 to 15 inches. A non-recessed design is suggested to minimize the overall depth of the cut; however, recessed up to ⅛ inches could be used with little degradation in strip performance. The peak-to-peak dimension of sinusoid is suggested to be to ½ inches.