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HFG RURAL ENVIRONMENTS Version 1.0 RUMBLE STRIPS Introduction Shoulder rumble strips (SRS) are raised or grooved patterns on the shoulder of a travel lane intended to provide a tactile/haptic and auditory alert to drivers who stray onto the shoulder. When a vehicle's wheels traverse an SRS, they generate both an increase in sound and haptic (physical) vibrations that drivers feel through their seat, foot pedals, floor, and steering wheel. SRSs are best suited for warning inattentive or drowsy drivers who are leaving the travelled way. SRSs can potentially wake drivers who fall asleep; however, this result typically requires a greater level of sound and vibration. In general, SRSs must produce sound and vibration levels that are easily detectable, yet not so loud and jarring that they startle drivers. The design challenge is balancing the need to provide alerts in a variety of situations (e.g., in heavy trucks or to sleeping drivers) with the need to avoid potentially undesirable startling effects and difficulties that SRSs can cause bicyclists. Previous safety evaluations of SRSs confirm their overall effectiveness. For example, Griffith (1) indicates there is a medium- high level of predictive certainty that SRSs reduce all single-vehicle run-off-road (SVROR) crashes by 21% on rural freeways and by 18% on all freeways (i.e., both rural and urban). FHWA (2) also indicates that continuous SRSs reduce injury SVROR crashes by 7% on rural freeways and by 13% on all freeways. Rumble strips have been shown to significantly reduce the run-off- road crash rate on some rural highways by up to 80% (3). Design Guidelines COMMON ROADWAY SOUNDS AND ASSOCIATED DB LEVELS SRS should produce an audible sound dB Sound dB Sound between 6 and 15 dB louder than 60 Freeway driving from 85 Heavy traffic background noise levels. inside car 70 Freeway traffic 90 Truck 75-80 Inside heavy truck cab 95-100 Motorcycle 85 City traffic inside car 110 Car horn EFFECTS OF DIFFERENT SRS DIMENSIONS ON AUDITORY / TACTILE ALERTS Characteristic Suitable Values Direct Effect on Driver Implications for Effectiveness Lateral 6+ in. from lane Drivers encounter the alert sooner, the The sooner the warning occurs, the placement / edge (but depends closer it is to the lane edge. more space drivers have to recover offset on other factors) before reaching the road edge. Groove Width 16 in. (12 may be Wider SRS will produce Sounds presented for longer durations acceptable if the sounds/vibrations for a longer duration are generally easier to detect. shoulder is narrow) as the vehicle laterally traverses it. Groove Depth 7/16 in. Deeper grooves increase sound and Louder sounds and vibrations are easier vibration alert levels. to detect relative to background noise levels. Groove 11-12 in. Narrower groove separation slightly Drivers generally perceive higher tones Separation increases the frequency. as sounding more urgent. Longitudinal None if shoulder Gaps of 12 ft or less can reduce the Effectiveness will be lower than without Gaps not shared with chance that a vehicle will miss the SRS gaps because alert duration will be bikes completely. shorter over gap sections. 12 ft if shoulder shared with bikes Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 16-6
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HFG RURAL ENVIRONMENTS Version 1.0 Discussion Torbic et al. (4) found that there is no conclusive evidence indicating a clear minimum level of stimulus that a shoulder or centerline rumble strip must generate in order to alert an inattentive, distracted, drowsy, or fatigued driver. However, the applicable research literature generally indicates that rumble strips that generate a 3 to 15 dBA increase above the ambient in- vehicle sound level can be detected by awake drivers. Also some evidence suggests that a sudden change in sound level above 15 dBA could startle a driver. However, a rumble strip generating more than a 15 dBA increase above the ambient sound level should not be automatically assumed to cause negative impacts (e.g., an increase in crashes), but rather to increase the potential for startling drivers who encounter the rumble strip. Related guidance for in-vehicle warning tones typically recommends sound intensity levels for auditory-only warnings (unaccompanied by vibrations) of between 10 and 30 dB above background noise levels, while not exceeding 90 dB overall. The SRS guidelines differ significantly from the guidance for in-vehicle warning tones because of the presence of haptic vibrations with the SRS. In particular, at least for passenger vehicles, background vibration levels are low in small vehicles and even a small change in vibration can be clearly detected. Laboratory driving simulator studies show that usually drivers easily detect steering wheel or brake pedal vibrations of 1.2-1.5 Nm torque presented over half a second. In contrast to passenger vehicles, cab vibrations in heavy trucks are significant and the size and weight of heavy trucks reduce the vibrations generated by SRS; therefore, the vibration component of SRS is viewed to have minimal benefit for alerting heavy truck drivers. It is also worth noting that the effectiveness of SRS for waking sleeping drivers has not been closely examined and that it is likely that SRS are much less effective in this application. The primary reasons for this lesser effectiveness are that greater stimulus levels are required to wake a sleeping driver rather than to merely alert a distracted or drowsy driver and that the increased arousal caused by traversing rumble strips is brief and insufficient (5). The rationale for the "suitable values" in the guidelines table is discussed in further detail in FHWA (6), Spring (7), and Torbic et al. (4). For the most part, the values also accommodate bicycle traffic on the shoulder. Design Issues An important consideration when installing SRS on non-controlled-access roadways is the impact on bicyclists (and possibly motorcyclists) because several aspects that improve the alerting aspects of rumble strips (e.g., depth) also make SRS more challenging to traverse. A key factor in the suitability of a shoulder for accommodating both SRS and bicycle traffic is the shoulder width (see table below). The MUTCD (2) also recommends a 12-ft gap in 60-ft cycle, which will result in 80% coverage of the shoulder with rumble strips and exactly 1½ times cycle length for lane line stripping. Other options are to use a 40-ft cycle, consisting of a 28-ft long rumble strip with a 12-ft gap. This generally coincides with the MUTCD-recommended cycle for rural lane line markings. Both patterns should provide gaps at sufficient frequency to allow bicyclists to cross the rumble strips in advance of hazards or intersections, though the 40-ft cycle will provide gaps more frequently for a given speed. Shoulder Width (ft) Is there a Problem? Reasoning 0-1.9 No Shoulder is too narrow for SRS or bicyclists. 2-3.9 Yes Shoulder may be wide enough for SRS or bicyclists. 4-5.9 Yes Shoulder may be wide enough for both SRS and bicyclists. 6+ No Shoulder is wide enough for SRS and bicyclists. Cross References Countermeasures for Pavement/Shoulder Drop-offs, 16-4 Key References 1. Griffith, M.S. (1999). Safety evaluation of rolled-in continuous shoulder rumble strips installed on freeways. Transportation Research Record, 1665, 28-34. 2. FHWA (2007). Manual on Uniform Traffic Control Devices for Streets and Highways. Washington, DC. 3. Harwood, D.W. (1993). NCHRP Synthesis of Highway Practice 191: Use of Rumble Strips to Enhance Safety. Washington, DC: Transportation Research Board. 4. Torbic, D.J., Hutton, J.M., Bokenkroger, C.D., Bauer, K.M., Harwood, D.W., Gilmore, D.K., et al. (2009). NCHRP Report 641: Guidance for the Design and Application of Shoulder and Centerline Rumble Strips. Washington, DC: Transportation Research Board. 5. O'Hanlon, J.F., and Kelley, G.R. (1974). A Psychophysiological Evaluation of Devices for Preventing Lane Drift and Run-Off-Road Accidents. Goleta, CA: Human Factors Research, Inc. 6. FHWA (n.d.). Synthesis of Shoulder Rumble Strip Practices and Policies. Washington DC. Retrieved from http://safety.fhwa.dot.gov/roadway_dept/research/exec_summary.htm. 7. Spring, G.S. (2003). Shoulder Rumble Strips in Missouri. (RDT 03-007, Final Report). Rolla: University of Missouri. 16-7