Guidance for the Design and Application of Shoulder and Centerline Rumble Strips (2009)

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S U M M A R Y The primary objective of this research was to develop guidance for the design and applica- tion of shoulder and centerline rumble strips as an effective motor vehicle crash reduction measure, while minimizing adverse effects for motorcyclists, bicyclists, and nearby residents. The focus of the research was on (1) summarizing previous research and existing policies on the design and application of shoulder and centerline rumble strips, (2) quantifying the safety effectiveness of shoulder rumble strips on different roadway types, (3) providing guidance on the safety effectiveness of shoulder rumble strips placed in varying locations with respect to the edgeline, (4) quantifying the safety effectiveness of centerline rumble strips on different road- way types, (5) quantifying the safety effectiveness of centerline rumble strips along varying roadway geometry, and (6) developing statistical models for predicting noise levels within the passenger compartment of a vehicle for use in designing rumble strip patterns. There are sev- eral shoulder and centerline rumble strip design and application guidelines that can be devel- oped based on converging findings from the existing literature and results from this research. Shoulder rumble strips may be considered for implementation on a range of roadway types, including urban and rural freeways, on- and off-ramps, multilane divided highways, multilane undivided highways, and two-lane roads. Criteria that may be considered for determining whether implementation is appropriate include shoulder width, lateral clear- ance, traffic volume, bicycles, pavement type, pavement depth, area type, speed limit, and crash experience. The most reliable and comprehensive estimates to date of the safety effec- tiveness of shoulder rumble strips are for freeways and rural two-lane roads. The safety effec- tiveness estimates for shoulder rumble strips and the standard errors (SE) for the estimates are the following: Urban/Rural Freeways • Rolled shoulder rumble strips [based on results from Griffith (1)]: – 18 percent reduction in single-vehicle run-off-road (SVROR) crashes (SE = 7) and – 13 percent reduction in SVROR fatal and injury (FI) crashes (SE = 12). Rural Freeways • Shoulder rumble strips [based on combined results from this research and Griffith (1)]: – 11 percent reduction in SVROR crashes (SE = 6) and – 16 percent reduction in SVROR FI crashes (SE = 8). Rural Two-Lane Roads • Shoulder rumble strips [based on results from this research and Patel et al. (2)]: – 15 percent reduction in SVROR crashes (SE = 7) and – 29 percent reduction in SVROR FI crashes (SE = 9) Guidance for the Design and Application of Shoulder and Centerline Rumble Strips 1

2Estimates on the safety effectiveness of shoulder rumble strips along rural multilane divided highways are also available but are not considered as reliable as the estimates for free- ways and rural two-lane roads. The safety estimates for rural multilane divided highway are as follows: Rural Multilane Divided Highways • Shoulder rumble strips [based on results from Carrasco et al. (3)]: – 22 percent reduction in SVROR crashes and – 51 percent reduction in SVROR FI crashes. The lack of reliable estimates on the safety effectiveness of shoulder rumble strips along other roadway types does not indicate that shoulder rumble strips are ineffective on these roadway types. Rather, their safety effects are not known at this time. Transportation agencies specify different offset distances from the edgeline to install shoul- der rumble strips. The safety evaluation performed during this research found statistically sig- nificant evidence that on rural freeways, rumble strips placed closer to the edgeline are more effective in reducing SVROR FI crashes compared to rumble strips placed further from the edgeline. Therefore, for rural freeways, it is recommended that shoulder rumble strips be placed as close to the edgeline as possible, taking into consideration other factors such as pavement joints. For other roadway types, such as rural two-lane roads, there is no sta- tistically significant evidence to indicate that offset distance influences the safety effectiveness of shoulder rumble strips. Therefore, based strictly on safety, there is no current basis for rec- ommending that transportation agencies change their current policies concerning the place- ment of shoulder rumble strips with respect to the edgeline on these other roadway types. Centerline rumble strips may be considered for implementation on a range of roadway types, including urban and rural multilane undivided highways and rural two-lane roads. Cri- teria that may be considered for determining whether implementation is appropriate include lane width, traffic volume, pavement depth, area type, speed limit, and crash experience. The most reliable and comprehensive estimates to date on the safety effectiveness of centerline rumble strips are for those installed on urban and rural two-lane roads. The safety effective- ness estimates for milled centerline rumble strips and the standard errors for the estimates are as follows: Urban Two-Lane Roads • Centerline rumble strips (based on results from this research): – 40 percent reduction in total (TOT) target crashes (SE = 17) and – 64 percent reduction in FI target crashes (SE = 27). Rural Two-Lane Roads • Centerline rumble strips [based on combined results from this research and Persaud et al. (4)]: – 9 percent reduction in TOT crashes (SE = 2), – 12 percent reduction in FI crashes (SE = 3), – 30 percent reduction in TOT target crashes (SE = 5), and – 44 percent reduction in FI target crashes (SE = 6) (based on results from this research). Target crashes are defined as head-on and opposite-direction sideswipe crashes. Similar to shoulder rumble strips, the lack of reliable estimates on the safety effectiveness of centerline rumble strips along other roadway types does not indicate that centerline rumble strips are ineffective on these roadway types. Rather, their safety effects are not known at this time.

Prior to this research, it was not known whether the same safety benefits of centerline rum- ble strips should be expected along different roadway alignments. Results of this research show that the expected reductions in crashes due to the installation of centerline rumble strips on horizontal curves and tangents are very similar. Thus, it is concluded that the safety effec- tiveness of centerline rumble strips is for practical purposes the same for both curved and tan- gent alignments. It is difficult to specify optimal dimensions of shoulder and centerline rumble strips because fundamental research has not been conducted on the stimuli levels necessary to alert inatten- tive, distracted, drowsy, or fatigued drivers. Based upon a review of previous research and exist- ing practices, it is recommended that for roadways where bicyclists are not expected (e.g., freeways) that rumble strip patterns should be designed to produce sound level differences in the range of 10 to 15 dBA in the passenger compartment; on roadways where bicyclists can be expected or near residential or urban areas, rumble strip patterns should be designed to pro- duce sound level differences in the range of 6 to 12 dBA in the passenger compartment. Sev- eral statistical models were developed as part of this research that predict the sound level difference in the passenger compartment when traversing rumble strips. Transportation agen- cies can utilize these models to develop rumble strip patterns for use on a range of roadway types and operating conditions. The independent variables of the predictive models include the four primary rumble strip dimensions (i.e., length, width, depth, and spacing), vehicle speed, angle of departure, pavement type (asphalt or concrete), pavement condition (wet or dry), rumble strip type (milled or rolled), and location (shoulder or centerline). In situations where it is desirable to provide more lateral clearance for bicyclists or for installing shoulder rumble strips on roads with very narrow shoulders, the predictive models indicate that rum- ble strips can be designed with relatively narrow lengths (e.g., 6 in. [152 mm]) and still gener- ate the desired sound level differences of 6 to 12 dBA in the passenger compartment. Further guidance is provided in the main text of the report concerning the design and application of shoulder and centerline rumble strips. 3