CHAPTER 6

Conclusions and Recommended Further Research

Suggested Testing Procedure

The first objective of this research was to develop a measurement program to assess rumble strip performance effectively and consistently at both the exterior pass-by and vehicle interior. From previous research, it is known that the magnitude of pass-by noise levels and the inputs to the operator differ across various vehicle types on any given rumble strip design. Based on this and testing completed in Northern California in October 2018, it was determined that vehicles representing four vehicle groupings should be used for testing rumble strips. These groupings include compact cars, sedans, mid-size SUVs, and large, full-frame SUVs or pickup trucks. Each of the vehicles within these groupings typically has different features that affect their response to rumble strips, including vehicle weights, interior dimensions, tire sizes, and road isolation. The results from these four groupings averaged together can help avoid biases.

For pass-by noise, the existing AASHTO procedure, modified to reflect both on- and off-strip driving conditions, is suggested. That is, a microphone was positioned at the standard 25-foot location along the wayside of the travel lane for off-strips pass-by measurements. To maintain 25 feet from the center line of the test vehicle to the microphone location, a microphone was positioned 3 feet farther from the standard microphone location to measure pass-by levels when the vehicle’s passenger-side tires are fully on the shoulder rumble strips. To measure pass-by levels on the centerline strips, it is suggested that data be collected when the vehicle is traveling in the far lane of a two-lane roadway in the opposite direction. In this case, the microphone was positioned 6 feet closer to the roadway to maintain 25 feet from the center line of the vehicle to the microphone when the driver-side tires are located on the centerline strips. For the safety of the pass-by field operator, further displacement of the microphone is not recommended for off-strips measurements in the far lane. Instead, it was estimated that subtracting 1 dB from the centerline microphone data would provide the approximate off-strips levels to compare to the on-strips levels for the centerline rumble strips.

Multiple interior noise and vibration measurement locations were considered. After comparing the noise and vibration results to each other in all test vehicle categories and to subjective tactile responses to rumble strips experienced by the driver, primary and secondary measurement locations were established for both noise and vibration. For interior noise, both microphones are located along the center line of the headrest of the front passenger seat. The primary microphone location, identified as the CC microphone, was hung from the ceiling of the cabin 7 inches forward of the headrest and 29 inches above the seat when the seat is positioned halfway between the extreme forward and rear positions. The secondary microphone, identified as the FC microphone, was also 7 inches forward of the headrest and 29 inches above the seat when the seat is positioned at the most forward position. For interior vibration, the primary tri-axle accelerometer was secured to the ST of the passenger seat, and the secondary tri-axle accelerometer was secured to the SC. At each frequency band in the spectra, amplitudes from all three directions of each accelerometer were energy summed for a single spectrum at each vibration location.

One-third octave band data was collected at all four interior sensors in 0.1-second increments for a total of 8 to 10 seconds. Two consecutive seconds of consistent data were averaged for each usable test run. Since noisy pavement could influence the measurements, OBSI testing of the off-strips pavement is also suggested to identify and quantify such pavements. This testing would follow the standard AASHTO T 360 test procedure. For increment calculations at all test sites, it is recommended that the on and off levels be calculated by using the energy summation of the lower ⅓ octave bands as opposed to overall A-weighted levels. For interior noise and vibration, this “band-pass” would be for the bands from 31.5 to 315 Hz. For pass-by data, this would be the bands from 31.5 to 200 Hz. This is particularly important for sinusoidal rumble strips and those with noisy pavements, such as chip seals and aged pavements. This can be done for both interior noise and vibration measurements and exterior pass-by measurements. The effect of noisy pavements reducing the difference in on/off increments was apparent in one of the two test sites in California, three of the four sites in the Midwest, and all of those in Washington. If OBSI measurements cannot be performed to quantify tire-pavement noise, the use of band-passing is particularly important.

Suggested Rumble Strip Design

The second objective of this research was to suggest a rumble strip design capable of effectively alerting the driver of lane departure while reducing exterior noise levels projected along the wayside. This objective was intended to minimize disruption to neighbors living along the roadways having rumble strips without reducing the safety effectiveness of rumble strips to alerting drivers of lane departure.

Previous research showed that minimum on/off increments of 10 dB on the vehicle interiors would be required to effectively alert the driver of lane departure. While a criterion was not previously established to reflect minimal pass-by noise levels, on/off increments of 5 dB or less were used in this research to demonstrate minimal noise along the wayside. To achieve both requirements, the following sinusoidal rumble strip design parameters are suggested:

• A sinusoidal wavelength of 15 inches is suggested with tolerance of ±½ inches.
• Recess of 0 inches is suggested. However, a recess of up to ⅛ inches can be tolerated if necessary for other reasons.
• A peak-to-peak amplitude of to ½ inches is suggested.

The width of the rumble strip perpendicular to the direction of travel was not systematically evaluated; however, a minimum width of 12 inches is suggested to ensure that width of the tire contact patch is fully engaged with the strip design.

Recommended Further Research

During the study, vehicle operators and passengers in the test vehicles noted that the noise and vibration was sufficient to easily determine when light vehicles were on any of the sinusoidal strips. In evaluating the objective interior noise and vibration increments, the minimum on/off increment of 10 dB is supported in the literature to be sufficient to effectively alert the driver of lane departure. For pass-by noise, the maximum on/off increments of 5 dB or lower was also applied. The suggested design from this research achieved these goals. It is suggested that consideration should be given to subjectively verifying these criteria using untrained subjects under controlled testing conditions.

In the Caltrans research on sinusoidal strips, limited testing of a 4-yard dump truck was included. These measurements produced interior noise and vibration increments of less than

10 dB for both the sinusoidal and the conventional cylindrical ground strips (see Figure 5) (Donavan 2018). The cause of this disparity was not determined. Possible causes could be due to the larger tire diameter used on the truck compared to the light vehicles, the greater mass of truck, and/or a greater amount of road isolation designed into the heavier vehicles. The largest interior increment for the 4-yard dump truck on the sinusoidal strips was 7.2 dB for the steering column acceleration. Because of the disparity between light vehicles and heavier trucks, research on the response of medium and heavy trucks to both conventional and sinusoidal rumble strips is recommended. Ideally, additional tests would be performed using the strips along Washington SR 105.

The phenomenon noted in this research, as documented in Appendix D, where the maximum in the time history of the ⅓ octave band containing the sinusoidal rumble strip repetition rate lags the A-weighted pass-by level should be investigated further. Because of this phenomenon, the peak sound pressure level in the ⅓ octave band containing the sinusoidal frequency may be overlooked by future researchers and users in applications of the recommended test procedure.

For advancing the standardization of the sinusoidal strip rumble test procedure, issues regarding repeatability and reproducibility should be examined more thoroughly, as has been done for some of the recent AASHTO procedures related to vehicle noise. This would include the uncertainty due to the effect of different test vehicles used in the procedure. This was done in the current research to a limited degree by using eight vehicles, two from each vehicle category. However, for standardization, it would be appropriate to examine this more thoroughly. Other issues regarding standardization would be examining the reproducibility with multiple vehicle operators and the effect of different pavement types.

Beyond the initial noise and vibration performance of the sinusoidal rumble strip designs, longer-term research should be considered. These could include assessing how well the sinusoidal profile is maintained over time, potential rehabilitation methods, and their cost. The possible decline in the ability of the strips to produce adequate noise and vibration over time should also be investigated. It is recommended to also evaluate the positive and negative effects of striping over the strips.