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

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143 S E C T I O N 1 1 This section presents the primary conclusions from this research related to the design and application of shoulder and centerline rumble strips. This section also summarizes key unresolved issues related to the design and application of shoulder and centerline rumble strips. Conclusions The conclusions of this research are as follows: • Shoulder rumble strips are an effective low-cost crash mit- igation measure. The most reliable and comprehensive estimates to date of the safety effectiveness of shoulder rumble strips are for freeways and rural two-lane roads. Estimates of the safety effectiveness of shoulder rumble strips for rural multilane divided highways are also avail- able but are not considered as reliable as the estimates for freeways and rural two-lane roads. The lack of reliable esti- mates on the safety effectiveness of shoulder rumble strips for other roadway types does not necessarily mean that shoulder rumble strips are ineffective on these roadway types; rather, the safety effects of shoulder rumble strips on these other facility types are not known at this time. • The best available estimates of the safety effectiveness of shoulder rumble strips are as follows: – Rolled shoulder rumble strips on urban/rural freeways are expected to reduce SVROR crashes by 18 percent and SVROR FI crashes by 13 percent. – Shoulder rumble strips on rural freeways are expected to reduce SVROR crashes by 11 percent and SVROR FI crashes by 16 percent. – Shoulder rumble strips on rural two-lane roads are expected to reduce SVROR crashes by 15 percent and SVROR FI crashes by 29 percent. – Shoulder rumble strips on rural multilane divided high- ways are expected to reduce SVROR crashes by 22 per- cent and SVROR FI crashes by 51 percent. • Given their proven safety benefits for several roadway types, the likelihood that shoulder rumble strips are effective on other roadway types, the low cost of installation, and rela- tively few concerns (i.e., noise, bicyclists, pavement perfor- mance, and visibility), shoulder rumble strips are considered appropriate for installation along a range of roadway types including freeways, on- and off-ramps, multilane divided and undivided highways, and two-lane roads in both rural and urban areas. • On rural freeways, shoulder rumble strips should be placed as close to the edgeline as possible to maximize the safety benefits of the measure, taking into consideration other factors such as pavement joints. • Centerline rumble strips are also an effective low-cost crash mitigation measure for undivided roadways with two-way traffic. The most reliable and comprehensive estimates to date of the safety effectiveness of centerline rumble strips are for rural and urban two-lane roads. The lack of reliable estimates on the safety effectiveness of centerline rumble strips for other roadway types does not indicate that center- line rumble strips are ineffective on these roadway types; rather, the safety effects of centerline rumble strips on other facility types are not known at this time. • The best available estimates of the safety effectiveness of centerline rumble strips are as follows: – Centerline rumble strips on urban two-lane roads are expected to reduce TOT target crashes by 40 percent and FI target crashes by 64 percent. – Centerline rumble strips on rural two-lane roads are expected to reduce TOT crashes by 9 percent, FI crashes by 12 percent, TOT target crashes by 30 percent, and FI target crashes by 44 percent. • The safety benefits of centerline rumble strips for roadways on horizontal curves and on tangent sections are for prac- tical purposes the same. • Given their proven safety benefits for several roadway types, the likelihood that centerline rumble strips are effective on Conclusions and Recommendations for Future Research

other roadway types, the low cost of installation, and rela- tively few concerns, centerline rumble strips are considered appropriate for installation along a range of roadway types including multilane undivided highways and two-lane roads in both rural and urban areas. • For roadways where bicyclists are not expected (e.g., free- ways), shoulder rumble strip patterns should be designed to produce sound level differences in the range of 10 to 15 dBA in the passenger compartment; and on roadways where bicyclists can be expected or near residential or urban areas, shoulder rumble strip patterns should be designed to pro- duce sound level differences in the range of 6 to 12 dBA in the passenger compartment. • Centerline rumble strip patterns should be designed to produce sound level differences in the range of 10 to 15 dBA in the passenger compartment, except near residential or urban areas where consideration should be given to designing centerline rumble strips to produce sound level differences in the range of 6 to 12 dBA in the passenger compartment. • Statistical models developed in this research to predict the sound level difference in the passenger compartment when traversing rumble strips can be used to design rumble strip patterns that produce the desired alerting properties. Pre- dictive models are available that include, as independent variables, 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, shoulder rum- ble strips can be designed with relatively narrow lengths (e.g., 6 in. [152 mm]) and still generate the desired sound level differences in the passenger compartment. Recommendations for Future Research The key unresolved issues associated with shoulder rumble strips that should be addressed in future research are as follows: • Better quantify the safety effectiveness of rumble strip applications on different types of roads: The most reliable and comprehensive estimates on the safety effectiveness of shoulder rumble strips are available for freeways and rural two-lane roads. Estimates on the safety effectiveness of shoulder rumble strips along rural multilane divided high- way (nonfreeways) are also available but are not consid- ered as reliable as the estimates for freeways and rural two-lane roads. The safety effectiveness estimates for free- ways, rural two-lane roads, and rural multilane divided highways are considered appropriate only for the respec- tive roadway types. The safety benefits of shoulder rumble strips along urban freeways (by themselves), urban freeway on-ramps and off- ramps, urban multilane divided highways (nonfreeways), urban multilane undivided highways (nonfreeways), urban two-lane roads, rural freeway on-ramps and off-ramps, and rural multilane undivided highways (nonfreeways) have not been quantified at this time due to limited mileage of shoulder rumble strip treatments along these respective roadway types. In the future it is desirable to calculate reli- able safety estimates for these roadway types. Given the cur- rent state of applications, this issue should likely not be addressed for at least another 3 to 5 years to allow for more installations along the respective roadway types. • Determine the optimal placement of shoulder rumble strips on rural two-lane roads: Conclusive evidence shows that on rural freeways rumble strips placed closer to the edgeline are more effective in reducing SVROR FI crashes compared to rumble strips placed farther from the edgeline. However, for other roadway types (e.g., rural two-lane roads), there is no conclusive evidence based upon crash statistics to indicate that offset distance influences the safety effectiveness of shoulder rumble strips. Further investiga- tions, potentially through kinematic modeling, should be made to assess the optimal placement of shoulder rumble strips along roadway types, focusing primarily on rural two- lane roads. • Determine the optimal longitudinal gaps in rumble strips to provide accessibility for bicyclists while maintaining the effectiveness in reducing lane departures: It may be possible to provide accessibility for bicyclists, while still pre- serving the effectiveness of rumble strips for motor vehicles, by providing longitudinal gaps in rumble strips. Moeur (99) addressed this issue from a bicyclist’s perspective. However, this research did not account for vehicle speed and trajec- tory. In addition, the Moeur study did not vary the length of the rumble strip patterns, and the trajectories of bicyclists as they navigate from the outside of the rumble strip along the shoulder to the inside of the rumble strip near the travel lane are a function of bicycle speed, gap length, and rumble strip groove length. Further investigation into these issues is desirable. • Better quantify the safety effectiveness of shoulder rumble strips in varying conditions: – Along varying roadway geometry. Studies concerning the safety effectiveness of shoulder rumble strips have utilized crash data collected over long segments of high- way, such that the study segments included both tan- gents and horizontal curves. No distinction has been made in these studies or in the present research between 144

tangent and horizontal curve sections, and there are no studies that have analyzed the effectiveness of shoulder rumble strips at horizontal curves only. Therefore, there is no definitive information about potential differences in the safety effectiveness of shoulder rumble strips between tangents and horizontal curves. – Effectiveness of rumble strips installed on the inside (left) or outside (right) shoulder. Crash data suggest that the probability of leaving the travel lane on the out- side (right) of the road differs from the probability of leaving the travel lane on the inside (left) of the road. This implies that different safety benefits should be expected for shoulder rumble strips installed on the inside (left) shoulder as compared to those installed on the out- side (right) shoulder (100). However, this effect has not been quantified. The inability to distinguish between SVROR-left and SVROR-right crashes in the electronic databases assembled during this research prohibited the evaluation of this issue. The key unresolved issues associated with centerline rumble strips that should addressed in future research are as follows: • Better quantify the safety effectiveness of centerline rumble strips in varying conditions: – Along passing zones and no-passing zones: Safety eval- uations of centerline rumble strips have not distinguished between the safety benefits in passing zones and no- passing zones. Research by Miles et al. (38) suggests that centerline rumble strips do not significantly change driv- ing behaviors in passing zones, but no accident analyses have been conducted to support this conclusion. – Along different roadway types: The most reliable and comprehensive estimates on the safety effectiveness of centerline rumble strips are available for urban and rural two-lane roads. The safety benefits of centerline rumble strips along urban multilane undivided highways (non- freeways) and rural multilane undivided highways (non- freeways) likely differ from the safety estimates for urban and rural two-lane roads. The safety benefits have not been quantified at this time due to limited mileage of centerline rumble strip treatments along these respective roadway types, but in the future it is desirable to calcu- late reliable safety estimates for these roadway types. Given the current state of applications, this issue should likely not be addressed for at least another 3 to 5 years to allow for more installations along the respective roadway types. The key unresolved issues associated with both shoulder and centerline rumble strips that should be addressed in future research are as follows: • Determine the minimum level of stimuli (i.e., sound or vibration) necessary to alert a drowsy or inattentive driver: The minimum level of stimuli necessary to alert a drowsy or inattentive driver is a key human factors issue for which there is little reliable information. Recommended design thresholds are provided in this research, but the recommen- dations are based on common practice and engineering judgment. Further human factors research is needed to better assess noise thresholds for design purposes and to gain a better sense of the combined properties of noise and vibration in alerting inattentive, distracted, drowsy, and/or fatigued drivers. • Determine optimum dimensions of shoulder/centerline rumble strips necessary for effective vehicular warning with least potential for adverse effects: Regression models have been developed to predict the in-vehicle sound level differential generated by rumble strips based on a number of factors including rumble strip dimensions (length, width, depth, and spacing), vehicle speed, vehicle angle of depar- ture, rumble strip location (shoulder or centerline), pave- ment surface type (concrete or asphalt), rumble strip type (milled or rolled), and pavement surface condition (wet or dry). The noise prediction models developed during this research are the most comprehensive developed to date for determining optimum dimensions for rumble strips; how- ever, the goodness-of-fit of the models is relatively low. Fur- ther investigations should be undertaken to improve the goodness-of-fit of these models, expand on the types of vehicles for which the models are calibrated, and assess appropriate ranges of the variables for design purposes. • Determine the impact of rumble strips on pavement per- formance: Pavement deterioration due to the installation of rumble strips is a concern for many transportation agen- cies. Very little scientific research has been conducted to address these concerns, but through observational reports most of the pavement performance concerns appear to be unwarranted. Along the same lines, only a few transporta- tion agencies consider pavement performance issues in their current rumble strip policies. Research is needed to determine the effect of rumble strips on long-term pave- ment performance and to provide guidance on minimum pavement depths for rumble strip installation. • Determine the visibility/retroreflectivity of pavement markings installed on milled shoulder/centerline rumble strips: Some transportation agencies have reported con- cerns over the visibility and retroreflectivity of pavement markings when rumble strips are installed on the edgeline or centerline (i.e., edgeline or centerline rumble strips). These agencies note that potential problems may occur under nighttime conditions especially if snow, salt, sand, or debris collect in the grooves of the rumble strips. Con- flicting evidence as to whether this is an actual problem is 145

found in the literature. The majority of studies suggest that visibility/retroreflectivity of pavement markings placed over rumble strips is higher compared to pavement markings placed on flat pavement, particularly during wet-night conditions. It does not appear that definitive research has been performed to completely resolve this issue. • Assess the impact of noise produced by rumble strips on adjacent residents: This as a common problem cited by transportation agencies, although the problem is more related to rumble strips installed in the travel lane as com- pared to on the shoulder or centerline. This may become more of an issue as rumble strips are being installed (or are considered for installation) more frequently in urban areas. It does not appear that definitive research has been per- formed to completely resolve this issue. Future research should be undertaken to develop noise prediction models based on the rumble strip pattern and type, terrain, dis- tance to nearby residents, and the type of building con- struction used in nearby residential communities. A pro- cedure for selecting mitigation measures should then be developed to reduce noise propagation to residents that will experience noise levels above a predetermined mini- mum level. • Quantify the safety effectiveness of dual application treatments: An attempt was made during this research to quantify the safety effectiveness of dual applications of rumble strips (i.e., sites with both centerline and shoulder rumble strips installed along the same roadway segment); however, limited mileage of such sites prohibited formal evaluation of the safety effectiveness of this treatment. In the future, it is desirable to calculate reliable safety esti- mates for the dual treatment of both centerline and shoul- der rumble strips installed along the same roadway section. Given the current state of applications, this issue should likely not be addressed for at least another 3 to 5 years to allow for more installations of this dual treatment. 146