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

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

135 S E C T I O N 1 0 This section summarizes the implications from the key research findings for design and application of shoulder and centerline rumble strips. In formulating policies regarding the design and application of shoulder and centerline rumble strips, transportation agencies should address the following six key issues: 1. On what roadways is it appropriate to install shoulder/ centerline rumble strips? 2. What type of rumble strips will be used? 3. What will the dimensions be? 4. Where will the rumble strips be installed, relative to either the edgeline or to the centerline? 5. Should the rumble strip be installed in a continuous pat- tern or with intermittent gaps? 6. What features or areas might necessitate an interruption in the rumble strip pattern? After rumble strips are installed, transportation agencies should also address maintenance issues. In particular, trans- portation agencies should consider adopting a policy on the preparation of rumble strips prior to pavement surface overlays. Guidance is provided below on each of these issues. First, guidance is provided on these issues as they specifically relate to shoulder rumble strip policies. Second, guidance is provided on these issues as they specifically relate to centerline rumble strip policies. Implications on Shoulder Rumble Strip Policies Roadway Types Where it is Appropriate to Install Shoulder Rumble Strips Shoulder rumble strips may be considered for implementa- tion on a wide range of roadway types, including urban free- ways, urban freeway on-ramps and off-ramps, urban multi- lane divided highways (nonfreeways), urban multilane undivided highways (nonfreeways), urban two-lane roads, rural freeways, rural freeway on-ramps and off-ramps, rural multilane divided highways (nonfreeways), rural multilane undivided highways (nonfreeways), and rural two-lane roads. When developing a policy on which roadway type (or types) it is appropriate to install shoulder rumble strips, and for help in prioritizing actual sites for the installation of shoulder rumble strips, the following criteria have been con- sidered by one or more transportation agencies. Guidance is provided on common values and ranges of values used by transportation agencies. The values provided here are based upon common practices by agencies rather than being sub- stantiated by research. Also, some criteria may be consid- ered for certain roadway types, but not others. • Shoulder Width: Minimum shoulder widths for rumble strip application range from 2 to 10 ft (0.6 to 3.0 m), with 4 ft (1.2 m) being the most common value. Minimum shoulder widths may differ by roadway type. • Lateral Clearance: Minimum lateral clearances range from 2 to 7 ft (0.6 to 2.1 m), with 4 ft (1.2 m) and 6 ft (1.8 m) being the most common values. Some agencies may prefer to define the lateral clearance to be the dis- tance from the outside (i.e., right) edge of the rumble strip to the outside edge of the shoulder, while others may measure the clearance to the nearest roadside object rather than the outside edge of the shoulder. • ADT: Minimum ADTs for rumble strip application range from 400 to 3,000 ADT, but in most cases fall between 1,500 and 3,000 ADT. • Bicycles: Agencies address bicycle considerations in sev- eral ways, including: (a) not installing rumble strips on roads with significant bicycle traffic or if the roadway is a designated bicycle route, (b) adjusting the dimensions of the rumble strips, (c) adjusting the placement of the Rumble Strip Application and Design Criteria

rumble strips, (d) adjusting the minimum shoulder width and/or lateral clearance requirements, and/or (e) provid- ing gaps in periodic cycles. Guidance provided in the AASHTO Guide for the Development of Bicycle Facilities (98) should also be considered. • Pavement Type: Some agencies only install shoulder rumble strips on asphalt surfaces. Pavement type also influences whether rolled rumble strips can be used. • Pavement Depth: Minimum pavement depths range from 1 to 6 in. (25 to 152 mm). • Area Type: Some agencies only install shoulder rumble strips in rural areas, primarily due to potential noise dis- turbance. • Speed Limit: Minimum speed limits used by agencies ranged from 45 to 50 mph (72 to 80 km/h). Some agen- cies also adjust the rumble strip dimensions depending upon the speed limit. • Crash Frequencies/Rates: Some agencies establish a threshold value, such as the statewide average for the given roadway type. Reliable estimates for the safety effectiveness shoulder rum- ble strips provide useful information for highway agencies. The most reliable and comprehensive estimates to date of the safety effectiveness of shoulder rumble strips are for free- ways and rural two-lane roads. For consistency with previ- ous sources, the results from Griffith (1) are indicated as applying to rolled rumble strips. The combined results of this research and the Griffith study include both milled and rolled rumble strips and are therefore indicated as applying to should rumble strips in general. There is no indication of any substantive differences in safety between milled and rolled rumble strips. The safety effectiveness estimates with their associated standard errors are as follows: Urban/Rural Freeways – Rolled shoulder rumble strips [based on results from Griffith (1)]:  18 percent reduction in SVROR crashes (SE = 7) and  13 percent reduction in SVROR 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). Estimates on the safety effectiveness of shoulder rumble strips along rural multilane divided highway (nonfree- ways) are also available, but they are not considered as reli- able as the estimates for freeways and rural two-lane roads. The safety estimates for rural multilane divided highway (nonfreeways) are as follows: Rural Multilane Divided Highways (nonfreeways) – Shoulder rumble strips [based on results from Car- rasco et al., (3)]:  22 percent reduction in SVROR crashes and  51 percent reduction in SVROR FI crashes. The estimates above are considered appropriate only for the roadway types for which they are shown. In all likelihood, the safety benefits of shoulder rumble strips vary by road- way type because the different types of roadways have varying geometric design standards (i.e., lane widths, shoulder widths, roadside, etc.), accommodate varying traffic volumes and distributions, serve different driver pop- ulations, and accommodate a range of operating speeds. It should be clearly stated that the lack of reliable estimates of the safety effectiveness of shoulder rumble strips along the other roadway types does not indicate that shoulder rum- ble strips are ineffective on these other roadway types. Rather, it should be understood that the safety effects of rumble strips on these roadway types are simply unknown at this time. The safety effects have not been quantified due to limited mileage of shoulder rumble strip installations along these respective roadway types. As a final note regarding the safety effectiveness of shoul- der rumble strips, shoulder rumble strips are expected to reduce SVROR crashes involving heavy vehicles on rural freeways by approximately 40 percent, but no evidence exists to suggest that shoulder rumble strips reduce SVROR involving heavy vehicles on rural two-lane roads. There- fore, if a problem of SVROR crashes involving heavy vehi- cles is identified along a rural freeway, then installation of shoulder rumble strips can be expected to mitigate these types of crashes. However, if a similar problem is identified along a rural two-lane road, then it is unknown how effec- tive shoulder rumble strips will be at mitigating such a prob- lem. Also, evidence suggests that shoulder rumble strips reduce SVROR crashes that occur during low-lighting con- ditions on rural two-lane roads. This may be due to the positive guidance that rumble strips provide when the delineation of the roadway is limited. Therefore, in situa- tions where SVROR crashes during low lighting conditions are noted, shoulder rumble strips may be considered as a potential safety improvement. Type of Rumble Strips to Use A variety of shoulder rumble strip types are used in North America. These include milled, rolled, raised, or formed. 136

Based on noise and vibration research, milled rumble strips generally provide higher in-vehicle noise and vibration lev- els than rolled rumble strips. Also, because one of the pri- mary advantages of milled rumble strips over other types is that they can be installed at any time on new or existing pavements, milled rumble strips are the preferred rumble strip type among most agencies. Assuming the alerting properties are sufficient, nothing necessarily prohibits the use of the other types of rumble strips. The primary con- cern associated with rolled rumble strips, besides the alert- ing properties, is that rolled rumble strips are sensitive to the pavement temperature at the time of installation. When the temperature is too low, the indentations may not reach the specified depth. When the temperature is too high, the asphalt may not be stable enough to attain the specified pat- tern. Also, the use of raised rumble strips is usually restricted to warmer climates due to maintenance difficulties resulting from snow removal in northern climates. Dimensions of Shoulder Rumble Strips For transportation agencies to begin determining the dimensions of shoulder rumble strips, agencies first must start by determining the desired alerting properties of the rumble strip pattern for the given roadway type. There is no conclusive evidence from practice or research to serve as a basis in answering this question; however, the literature pro- vides a range of recommended values for this design crite- rion. To alert an inattentive, distracted, drowsy, or fatigued driver, the literature indicates that rumble strips should gen- erate a 3 to 15 dBA increase above the ambient in-vehicle sound level. However, there is also some evidence suggest- ing that sudden changes in sound level above 15 dBA could startle a driver. Considering the research methodologies of the studies that have investigated this issue and based upon the recom- mended values presented in the literature that may or may not be supported through research, it is recommended that rumble strips should, at minimum, be designed to generate sound levels 3 dBA above the ambient in-vehicle sound and should not generate sound levels greater than 15 dBA above the ambient in-vehicle sound. These limits of 3 and 15 dBA increases above the ambient in-vehicle sound level should be viewed as minimum and maximum design values. Rumble strips that generate less than a 3 dBA increase in the ambi- ent sound level will likely not have the alerting properties to prove effective in reducing target crashes, and rumble strips that generate more than 15 dBA above the ambient sound level have the potential to startle drivers, which would be an undesirable result. To take a slightly more conservative approach when designing rumble strips, it is recommended that an increase in 3 dBA above the ambient in-vehicle sound level be viewed as an absolute minimum design value and that an increase of 6 dBA above the ambient in-vehicle sound level should be viewed as the desired minimum design value. This recommendation provides for some tolerance in the desired response. At a 3 dBA increase in ambient sound, a driver will likely be able to notice the change in sound level. However, the literature does not provide sufficient detail to qualify the state of the driver, nor the driver’s response. It is likely that an inattentive or distracted driver will notice a 3 dBA change in sound level, but it is not known whether drowsy or fatigued drivers will (a) notice this change in ambient sound, nor (b) respond correctly to the stimulus. It is more likely that a 6 dBA increase in the ambient sound level will alert the full range of target drivers (i.e., inatten- tive, distracted, drowsy, or fatigued drivers) sufficiently to enable drivers to correct their steering in an appropriate manner. Regarding the upper end of the scale, there is conflicting information. Some literature suggests that an increase of 15 dBA above the ambient sound level is necessary to alert drivers, while other information suggests that sound changes above 15 dBA could produce a startle reaction. Therefore, an increase of 15 dBA above the ambient sound level appears to be a reasonable maximum design value, recognizing that if a rumble strip pattern generates more than a 15 dBA increase above the ambient sound level, it should not be automatically assumed that the rumble strip will cause neg- ative impacts (e.g., an increase in crashes), but rather could increase the potential for startling drivers who encounter the rumble strips. Having recommended three design values (i.e., mini- mum design value [3 dBA], desirable minimum design value [6 dBA], and maximum design value [15 dBA]) for consideration in developing a rumble strip policy, further guidance is provided that considers bicyclists needs. On roadways where bicyclists are not expected, such as on freeways, it is recommended that rumble strip patterns be designed to generate approximately 10 to 15 dBA above the ambient in-vehicle sound level, but for roadways where bicyclists may be expected, it is recommended that rumble strip patterns be designed to generate between 6 to 12 dBA above the ambient in-vehicle sound level. Another way to consider this is that for roadways where bicyclists are not expected, relatively aggressive rumble strip patterns may be used, while for roadways where bi- cyclists are expected, more bicycle-tolerable rumble strip patterns should be used. This also implies that different rumble strip patterns should be used for different road- way types. While the difference in the safety effectiveness of rumble strips designed using the lower and higher decibel ranges is unknown, there is no indication from safety studies and predictive modeling that this difference would be substantial. 137

Having specified recommended design thresholds or limits for sound level differences to be generated by rumble strips, transportation agencies have the option to conduct their own field research to identify rumble strip patterns that generate the desired thresholds, or agencies could look at previous research results to identify potential patterns. Another option that transportation agencies now have for determining the dimensions of rumble strips is to utilize the noise prediction models developed through this research. The models in Tables 82 and 83 provide the greatest flexi- bility and explain the greatest variation in the sound level difference generated by the rumble strips. It is recom- mended that one or both of these predictive models be applied to establish rumble strip patterns for application under certain operating conditions. Because of the type of information confounded within the state indicator vari- ables in the model from Table 83, unless an agency is from one of the four states represented in the model, it is rec- ommended that an agency assume the base conditions when using this respective prediction model. In addition, in situations where the pavement surfaces for the through travel lanes and shoulders differ, it is recommended that the coefficients for the “Concrete” indicator variable be halved (i.e., divided by 2) and the “Concrete” indicator be a value of 1. As a starting point or for comparison purposes, two rum- ble strip patterns are worth noting here. First, the most com- mon dimensions of milled shoulder rumble strips used throughout United States are: – Length: 16 in. (406 mm); – Width: 7 in. (178 mm); – Depth: 0.5 to 0.625 in. (13 to 16 mm); and – Spacing: 12 in. (305 mm). Based upon the noise prediction models, this pattern gener- ates a sufficient amount of noise in the upper range of the recommended design thresholds. These dimensions can be considered a relatively “aggressive” pattern and are consid- ered appropriate for use on roadways where bicyclists are not expected (e.g., freeways). Second, there is consensus that a milled rumble strip pattern with the following dimensions provides a reasonable compromise between the needs of bicyclists and motorists (i.e., bicycle-tolerable rumble strip pattern): – Width: 5 in. (127 mm), – Depth: 0.375 in. (10 mm), and – Spacing: 11 to 12 in. (280 to 305 mm). Notice above that in specifying the dimensions of the bicycle-tolerable rumble strip patterns, the length dimen- sion is not provided. One of the key dimensions concern- ing shoulder rumble strips is their length. For milled rum- ble strips, typical lengths of patterns are 12 and 16 in. (305 and 406 mm), but at least one state transportation agency has adopted a policy that allows lengths as short as 6 in. (152 mm) for milled rumble strips. The desire to install milled rumble strips with groove lengths less than the typical 12 to 16 in. (305 to 406 mm) is (a) to keep this dimension as narrow as possible to provide additional lateral clearance for bicyclists, (b) due to pavement performance issues when rumble strips are installed on roadways with narrow shoul- ders, and/or (c) simply to install rumble strips on roadways with narrow or nonexistent shoulders where rumble strips might not otherwise be installed. Based upon the unit increase above the ambient sound level per unit increase of the length dimension of the noise prediction model, there is approximately a 2.5 to 3.6 dBA difference in sound level above the ambient when comparing a rumble strip with a 6 in. (152 mm) length to a rumble strip with a typical length of 16 in. (406 mm), holding all other dimensions con- stant. Considering that the range of recommended design thresholds for roadways where bicyclists may be expected is 6 dBA (i.e., an increase between 6 to 12 dBA above the ambient sound level is desirable), a rumble strip with a 16 in. (406 mm) length that is expected to generate an increase above ambient sound between 9 and 12 dBA can be reduced in length to 6 in. (152 mm) and still be expected to generate a sound level difference above the desirable range of 6 dBA. Thus, it can be concluded that rumble strips designed with narrower lengths (e.g., 6 in. [152 mm]) can generate the desired sound level differences to alert inattentive, distracted, drowsy, or fatigued drivers. Example No. 2 in Section 9 clearly illustrates this. It is not necessarily recommended that all rumble strip patterns have a narrow length of 6 in. (152 mm), but in those situations where it is desirable to design a rumble strip with a narrower length for a particular reason (e.g., to provide more lateral clearance for bicyclists or for very narrow shoulders), it is likely that a rumble strip pattern can be designed as such and still generate an increase in the sound level difference sufficient to alert inattentive, distracted, drowsy, or fatigued drivers. One question often raised when discussing the design dimensions of rumble strips is whether shoulder rumble strips should be designed specifically to consider heavy vehicles. Based upon the results of the safety evaluation of shoulder rumble strips on rural freeways, the results imply that the current dimensions of shoulder rumble strips installed along rural freeways provide sufficient levels of stimuli to alert inattentive and drowsy drivers of heavy vehicles and that it is not necessary to design rumble strip patterns that are “more aggressive” based strictly on the needs of drivers of heavy vehicles. 138

Finally, in an effort to minimize the adverse effects of rumble strips on nearby residents, it is recommended that rumble strip patterns designed to generate between 6 to 12 dBA above the ambient in-vehicle sound level should be applied on roadways in close proximity to residential areas, rather than rumble strips designed to generate between 10 to 15 dBA above the ambient in-vehicle sound level. Although these design criteria were not necessarily estab- lished with the intent to address issues related to noise gen- erated outside of the vehicle, use of less aggressive patterns (i.e., patterns designed to generate between 6 to 12 dBA above the ambient in-vehicle sound level) in close proxim- ity to residential areas seems logical in an effort to minimize the adverse effects of the rumble strips on nearby residents. Placement of Shoulder Rumble Strips Relative to the Edgeline Typical offset distances range from 0 to 30 in. (0 to 762 mm). Some transportation agencies prefer to install shoulder rum- ble strips close to the travel way on the inside portion of the shoulder or in some cases on the edgeline, while other states install rumble strips more toward the middle of the shoul- der. Reasons for the varying policies include providing suf- ficient lateral clearance for bicyclists to ride along the shoulder without encountering the rumble strips, minimiz- ing the number of motor vehicle encounters with the rum- ble strips so as not to cause excessive noise for nearby residents, and providing the capability to install rumble strips along roads with narrow or nonexistent shoulders. Based upon the analyses designed to determine the impact that rumble strip placement (i.e., offset distance from the edgeline) has on the safety effectiveness of shoul- der rumble strips, there is conclusive evidence to show 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 edge- line. Therefore, for rural freeways, it is recommended that shoulder rumble strips be placed as close to the edgeline as possible or even on the edgeline, taking into consideration other factors such as pavement joints to provide the max- imum safety benefit from this treatment. For other roadway types, such as rural two-lane roads, there is no conclusive evidence to indicate that offset dis- tance impacts the safety effectiveness of shoulder rumble strips. Therefore, based strictly upon the safety benefits, there is no current basis for recommending that trans- portation agencies change their current policies concern- ing the placement of shoulder rumble strips with respect to the edgeline on these other roadway types. On divided highways, consideration may be given to spec- ifying different offsets for rumble strips installed on the right (outside) shoulder compared to rumble strips installed on the left (median) shoulder. For transportation agencies that adopt this policy, typically the offset for the left (median) shoulder is less than the offset for the right (outside) shoulder. Regarding the placement issue, it is also important to note that the AASHTO Guide for the Development of Bicycle Facil- ities (98) states that rumble strips are not recommended where shoulders are used by bicyclists unless there is (a) a minimum 1 ft (0.3 m) offset between the travel lane and the rumble strip, (b) a 4 ft (1.2 m) lateral clearance from the rumble strip to the outside edge of the paved shoulder, or (c) 5 ft (1.5 m) to adjacent guardrail, curb, or other obstacle. If existing conditions preclude achieving the minimum desirable clearance, then AASHTO policy indicates the length of the rumble strip may be decreased or other appro- priate alternative solutions should be considered. Finally, some transportation agencies have reported concerns over the visibility and retroreflectivity of pave- ment markings when rumble strips are installed on the edgeline (i.e., edgeline rumble strips). These agencies note potential problems may occur under nighttime conditions especially if snow, salt, sand, or debris collect in the grooves of the rumble strips. Conflicting evidence as to whether this is an actual problem is found in the literature. However, the majority of studies suggest that visibility/ retroreflectivity of pavement markings placed over rum- ble strips is higher compared to standard edgeline pave- ment markings, particularly during wet-night conditions. Thus, concerns over the visibility and retroreflectivity of pavement markings should not prohibit the use of edgeline rumble strips. Use of a Continuous or Intermittent Pattern Primarily to better accommodate the needs of bicyclists, consideration may be given to providing intermittent gaps in the rumble strip patterns, compared to a continuous pat- tern. Based upon research and current practice, it is com- mon to provide periodic gaps in the rumble strips of 10 or 12 ft (3.0 or 3.6 m), in 40 or 60 ft (12 or 18 m) cycles. Pro- vision of intermittent gaps enables bicyclists to maneuver from one side of the rumble strips to the other without hav- ing to encounter the indentations/grooves. Features or Areas That Might Necessitate an Interruption in the Shoulder Rumble Strip Pattern Within a shoulder rumble strip policy, consideration should be given to specific features or areas where the rumble strip pattern should be discontinued or interrupted to avoid adverse consequences (e.g., pavement deterioration, noise, etc.). Specific features or areas along the shoulder or roadway 139

where it is common to discontinue or interrupt shoulder rumble strips include the following: – Intersections, driveways, and turn lanes; – Entrance and exit ramps; – Structures (i.e., bridges); – Areas where the lateral clearance drops below a spec- ified value and/or areas where the lateral clearance is limited due to adjacent guardrail, curb, or other obstacles; – Residential areas; – Catch basins and drainage grates; – Pavement joints; and – Median crossings. Concerning further guidance on ways to minimize the impact of shoulder rumble strips on nearby residents, con- sideration should be given to terminating the rumble strips 656 ft (200 m) prior to residential/urban areas. This thresh- old value is based upon studies that showed when rumble strips were terminated 656 ft (200 m) prior to residential or urban areas, the noise impacts proved tolerable to nearby residents. Preparation of Shoulder Rumble Strips Prior to Overlayment of the Pavement Surface Once shoulder rumble strips are in place, transportation agencies need to consider a policy on how they plan to prepare rumble strips prior to overlaying the shoulder surface so that rideability and/or pavement integrity are not compromised. Based upon one observational study, it is recommended to prepare areas with rumble strips prior to overlayment either by (1) milling, inlaying, and overlay- ing or (2) by simply milling and overlaying. Other prepara- tion approaches such as shim and overlay or simply overlay will likely result in some degree of reflection in the area of the former rumble strips. Implications on Centerline Rumble Strip Policies Roadway Types Where it is Appropriate to Install Centerline Rumble Strips Centerline rumble strips may be considered for implemen- tation on a wide range of roadway types including urban multilane undivided highways (nonfreeways), urban two- lane roads, rural multilane undivided highways (nonfree- ways), and rural two-lane roads. When developing a policy on which roadway type or types it is appropriate to install centerline rumble strips, and for help in prioritizing actual sites for the installation of centerline rumble strips, the fol- lowing criteria have been considered by one or more trans- portation agencies: – Lane width, – ADT, – Pavement depth, – Area type, – Speed limit, and – Crash frequencies/rates. Valuable information that may help agencies decide on which roadway types they should install centerline rum- ble strips are reliable estimates on the safety effectiveness of this treatment. The most reliable and comprehensive estimates to date of the safety effectiveness of centerline rumble strips are for those installed on urban and rural two-lane roads. The safety effectiveness estimates for centerline rumble strips with their associated standard errors are as follows: Urban Two-Lane Roads – Centerline rumble strips (based on results from this research):  40 percent reduction in 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 (based on results from this research) (SE = 6). The expected safety benefits of centerline rumble strips are for practical purposes the same whether installed along horizontal curves or tangents. Target crashes are defined to be head-on and opposite-direction sideswipe crashes. The estimates above are considered appropriate only for the roadway types for which they are shown. In all like- lihood, the safety benefits of centerline rumble strips vary by roadway type because the different types of roadways have varying geometric design standards (i.e., lane widths, shoulder widths, roadside, etc.), accommodate varying traffic volumes and distributions, serve different driver populations, and accommodate a range of operating speeds. The lack of reliable estimates of the safety effectiveness of centerline rumble strips along the other roadway types does not indicate that centerline rumble strips are ineffec- 140

tive on these other roadway types. Rather, it should be understood that the safety effects are simply unknown at this time. The safety effects have not be quantified at this time due to limited mileage of centerline rumble strip installations along these respective roadway types. Also, limited mileage of dual applications of rumble strips (i.e., centerline and shoulder rumble strips installed on the same road section) along rural two-lane roads prohibited formal evaluation of the safety effectiveness of this treat- ment along this respective roadway type; however, because the safety effect of this treatment is unknown and not quantified does not imply that the treatment is ineffective. Finally, concerns have been expressed about the poten- tial of motorcyclists losing control of their motorcycles when they encounter centerline rumble strips. Based upon a recent study, conclusive evidence exists to show that centerline rumble strips add no measurable risk to motor- cyclists. Therefore, there is no need to consider potential adverse effects for motorcyclists when developing a cen- terline rumble strip policy. Similarly, there is no need to prohibit the use of centerline rumble strips on roadways with significant motorcycle traffic. Type of Rumble Strips to Use Nearly all transportation agencies in North America that install centerline rumble strips use milled rumble strips. As indicated above for shoulder rumble strips, the primary advantages of milled rumble strips over other types is that they can be installed at any time on new or existing pavements. Dimensions of Centerline Rumble Strips The general principles of the related discussion above for shoulder rumble strips hold true for determining the dimensions of centerline rumble strips. Regarding the recommended design threshold values for centerline rumble strips, it is recommended that centerline rumble strip patterns be designed to generate approximately 10 to 15 dBA above the ambient in-vehicle sound level. Due to the placement of the rumble strips in the center of the roadway, bicyclists should very rarely encounter the rum- ble strips themselves, so bicyclists rarely need to be con- sidered in design dimensions of centerline rumble strips. On the other extreme, crash data presented in Section 2, Crashes and Heavy Vehicles, suggest that heavy vehicles should potentially be considered in the design of center- line rumble strips. Designing centerline rumble strips to generate approximately 10 to 15 dBA above the ambient in-vehicle sound level should be more than sufficient to alert drivers of heavy vehicles, based upon the results of the safety evaluation of shoulder rumble strips. The noise prediction models in Tables 82 and 83 are applicable for designing centerline rumble strips. The fol- lowing are the most common dimensions of milled center- line rumble strips used throughout North America: – Length: 12 or 16 in. (305 to 406 mm); – Width: 7 in. (178 mm); – Depth: 0.5 in. (13 mm); and – Spacing: 12 in. (305 mm). Based upon the noise prediction models, this pattern gen- erates a sufficient amount of noise in the upper range of the recommended design thresholds. Near residential or urban areas, consideration should be given to designing centerline rumble strip patterns that generate between 6 to 12 dBA above the ambient in- vehicle sound level to minimize the impacts on nearby residents. Placement of Centerline Rumble Strips Relative to the Centerline Pavement Markings The placement of centerline rumble strips can be within the pavement markings, extend into the travel lane, or on either side of the centerline pavement markings. The most common type of application is to install centerline rumble strips that protrude into the travel lane, followed by center- line rumble strips that are within the limits of the painted centerline pavement marking. Only a few transportation agencies currently install centerline rumble strips on either side of the centerline pavement marking. It should be noted that the safety estimates provided above for center- line rumble strips do not directly consider the placement of the rumble strips relative to the centerline pavement markings. The discussion above for shoulder rumble strips related to concerns over the visibility and retroreflectivity of pave- ment markings when rumble strips are installed on the edgeline (i.e., edgeline rumble strips) also applies to center- line rumble strips. In summary, concerns over the visibility and retroreflectivity of pavement markings should not pro- hibit the use of centerline rumble strips. Features or Areas That Might Necessitate an Interruption in the Centerline Rumble Strip Pattern Within a centerline rumble strip policy, consideration should be given to specific features or areas where the rum- ble strip pattern should be discontinued or interrupted to 141

avoid adverse consequences. The following are specific features or areas along the roadway where it is common to discontinue or interrupt centerline rumble strips: – Intersections and driveways; – Passing zones; – Structures (i.e., bridges); and – Residential areas. Regarding the discontinuation of centerline rumble strips at passing zones, it is currently not known what percent- age of transportation agencies that install centerline rum- ble strips permit this treatment within passing zones versus the percentage of agencies that prohibit it. The safety estimates provided above do not directly account for passing zones. Treatment sites with and without pass- ing zones were included in the safety evaluation. Also, pre- vious research indicates that centerline rumble strips have little or no influence on driver behavior in passing zones. Therefore, there is no conclusive evidence to recommend that centerline rumble strips should be discontinued within passing zones. The discussion above for shoulder rumble strips related to minimizing the impacts of rumble strips on nearby resi- dents also applies to centerline rumble strips. In summary, consideration should be given to terminating centerline rumble strips 656 ft (200 m) prior to residential/urban areas. Preparation of Centerline Rumble Strips Prior to Overlayment of the Pavement Surface The general principles of the related discussion above for shoulder rumble strips hold true for centerline rumble strips. In summary, it is recommended to prepare areas with rumble strips prior to overlayment either by (1) milling, inlaying, and overlaying or (2) by simply milling and over- laying. Other preparation approaches such as shim and overlay or simply overlay will likely result in some degree of reflection in the area of the former rumble strips. 142