Recommended Guidelines for Curb and Curb-Barrier Installations (2005)

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25 CHAPTER 3 SUMMARY OF STATE SURVEYS ON CURBS AND CURB–BARRIER COMBINATIONS INTRODUCTION The objective of this research project was to develop design guidelines for using curbs and curb–barrier combinations on roadways with operating speeds greater than 60 km/h. Seven tasks were identified to accomplish this objective. The second of these tasks was to conduct a survey of transportation agen- cies to determine current practice, guidelines, and standards pertaining to the use of curb and curb–barrier combinations on higher-speed roadways. The survey was also intended to identify problems experienced by transportation agencies and solutions developed to counter those problems. The research team composed a three-page survey (pro- vided in Appendix A) and distributed it to all 50 states. The survey included 12 questions covering the types of curbs used, the guidelines for using them, the typical functional pur- poses of curbs, alternatives to using curbs, safety problems encountered, using curbs in combination with barriers, curb research, and voids for establishing guidelines. Twenty-seven states completed and returned the survey. Their responses are organized by topic. In lieu of the state’s name, when refer- ence is made to individual states, a numeric identifier is used. TYPES OF CURBS USED BY THE STATES According to AASHTO, curbs are used extensively on all types of urban highways but should be used cautiously on rural highways. There are two general classifications of curbs: vertical curbs and sloped curbs. Some raised aspect or verti- cal element is required to be considered a curb. Vertical curbs are relatively high and steep-faced. They used to be called barrier curbs, but this terminology is no longer used because these curbs are not redirective devices or traffic barriers. AASHTO Type A curb, shown in Figure 17, is a vertical curb that ranges in height from 150 to 225 mm. It is designed to inhibit or discourage vehicles from leaving the roadway. Vertical curbs should not be used on freeways and are con- sidered undesirable on high-speed arterials. AASHTO rec- ommends that vertical curbs not be used where design speeds exceed 65 km/h, except in predominantly urban or rapidly developing urban areas in the intermediate speed range. Sloped curbs are designed to be low with flat sloping faces so that vehicles can cross them readily. AASHTO Type B, C, D, E, F, and G curbs, shown in Figure 18, are all typical sloped curbs. Types B, C, and D are considered to be mount- able under emergencies. The vertical portion on the lower face of Types C, D, and F is constructed as an allowance for future resurfacing. All the sloped curbs shown in Figure 18 can be used as shoulder curbs to control drainage, improve delineation, and reduce erosion. The survey respondents indicated the type or types of curbs they used for facilities with a design speed of 65 km/h or greater: AASHTO Type A vertical curb or Type B, C, D, E, F, or G sloped curbs. A summary of the results is presented in Table 5. Five states indicated that they used Type A vertical curb although two of those states indicated that it was not used for speeds much greater than 65 km/h. Three other states employed a curb similar to Type A with a few minor modifi- cations in the dimensions. Thirteen states employed a Type B sloped or similar curb. Type B was used by more of the responding states than any of the other curbs. Type C and Type D had a similar response, with seven states using each or a comparable version with slightly modified dimensions. Types E and G also had a similar response. Type F had the lowest response rate: only one state used a sloped curb that was similar to Type F. Additionally, seven states identified curbs used in their jurisdictions that could not be categorized with the AASHTO curbs. Seven distinct curbs were identi- fied, shown in Figure 19. Most of the states had guidelines or policies in place for when vertical or sloped curbs should be used. Only six states indicated that they did not have policies, two of which indi- cated that they followed AASHTO guidelines. Eight states indicated that they limit the use of curb by facility type. State 8 did not use curbs on roadways with design speeds greater than 70 km/h with the exception of asphalt concrete dikes. State 13 also restricted their use to roadways with design speeds less than 70 km/h, but noted that exceptions exist, particularly in urban areas. State 25 limited their use to facilities with design speeds under 80 km/h. State 3 limited their use to non–access-controlled highways. State 7 and State 10 limited their use to urban streets. State 18 only

used 76-mm asphalt or 100-mm lip curbs for design speeds greater than or equal to 80 km/h. State 19 responded that it did not restrict their use, but noted that the policy was that curbs are undesirable for use on roadways with design speeds greater than 80 km/h. State 24 also responded that it did not restrict their use, but its roadside design guide prohibits the use of nonsloped curb on new construction projects on high- ways with operating speeds greater than or equal to 80 km/h and along the mainline of Interstates, freeways, or high-speed parkways. TYPICAL FUNCTION OF CURBS AASHTO lists drainage control, pavement edge delin- eation, right-of-way reduction, aesthetics, delineation of pedes- trian walkways, reduction of maintenance operations and assistance in orderly roadside development as purposes of 26 curbs. States were asked to separately rank the functional purposes of vertical and sloped curbs used in their state. Most states identified drainage control as a primary or secondary purpose for vertical curb. Walkway support and pavement delineation were also highly rated as typical functional pur- poses of vertical curb. Only one state said the primary use of vertical curb was to protect vehicles from steep slopes. Respondents also identified other functional purposes of ver- tical curb not listed on the survey, including minimizing right-of-way impacts, access control, accommodating pedes- trians, aesthetics, erosion control, delineating edge parking, and traffic channelization. The primary functional purpose of sloped curb was most often listed as drainage. Twenty-four states listed it as the pri- mary or secondary purpose. Pavement delineation was listed by 14 states as a primary or secondary functional purpose. One state listed walkway support as a primary functional purpose, and several others listed it as a secondary purpose. Protecting vehicles from slopes was listed as a secondary pur- pose by three states. Respondents also wrote in other func- tional purposes of sloped curb including erosion control, minimizing right-of-way impacts, access control, pedestrian needs, channelization, and delineation. ALTERNATIVES TO USING CURBS Eight survey respondents had found an alternative to using curbs for one or more of the functional purposes mentioned in the previous section. State 5 used sloping freeway curbs with catch basins to prevent embankment erosion in the gutter on Figure 17. AASHTO vertical curb Type A (1″ = 25.4 mm). Figure 18. AASHTO sloped curbs (1″ = 25.4 mm).

27 AASHTO curb Number of states using this curb Number of states using a curb similar to this curb Total Type A Vertical 5 3 8 Type B Sloped 6 7 13 Type C Sloped 5 2 7 Type D Sloped 6 1 7 Type E Sloped 3 1 4 Type F Sloped 0 1 1 Type G Sloped 2 3 5 TABLE 5 Vertical and sloped curb use among the states surveyed A1 A2 A3 A4 A5 A6 A7 Figure 19. Curbs used by various states that could not be classified as AASHTO curbs.

depressed freeway sections. State 4 used portable New Jer- sey shape concrete barrier along the edge of highways for drainage channelization. State 12 used pavement striping or flexible tubes in lieu of curbs for channelization. State 16 used open ditches for drainage but noted that it is not appro- priate at most locations where curbs are specified. State 19 used drainage ditches or swales in place of curbs for drainage and berm guards or other safety barriers to protect vehicles from steep slopes. State 9 used concrete traffic separators in the median. State 25 used a unique curb and gutter adjacent to the travel lane on facilities of greater than 80 km/h. State 24 used a traversable curb that is more like a gutter or berm than a curb. The curb has no reveal and is 100 mm high and 305 mm wide with a 1:3 slope across the top. The curbs used by States 25 and 24 are included in Figure 19 as A5 and A6, respectively. PREVIOUS CURB SAFETY PROBLEMS IDENTIFIED BY THE SURVEY Only five states indicated that they had experienced safety problems when using curbs alone on higher-speed roadways. However, two states indicated that they did not use vertical curbs on facilities with posted speeds in excess of 70 km/h and one state indicated it only had ten miles of curbed high-speed highway in its system. Of the five states that experienced safety problems, only four states provided further information. State 25 had experienced cross-median fatalities with curbs along higher-speed roadways; median guardrail was installed to resolve this. State 9 indicated that it had had problems with vertical curb installed on a 90 km/h urban Interstate in the 1960s and was replacing the vertical curb with sloped curb or concrete barrier. State 19 had experienced problems with vaulting and rollover of vehicles in the tests performed for the Midwest State’s Regional Pooled Research Program. State 27 also experi- enced problems with vaulting. This state reported that curbs are not typically used alone, but rather in combination with a guardrail and for protection from runoff or erosion of a steep slope. CURB–BARRIER COMBINATIONS The states were also asked about curb and barriers used in combination. The survey included the illustration in Figure 20. The survey respondents provided the type of curb used, the type of barrier used, the offset distance from the edge of the travel lane to the face of the guardrail or barrier (distance A in the illustration), and the offset from the face of the curb to the face of the guardrail or barrier (distance B in the illustra- tion). Many states indicated that they tried not to use the two in combination. Three states said they did not use them in combination on higher-speed roads. Seven states responded that guardrail was used but did not specify the type of guard- 28 rail. The authors assume they are referring to a form of non- yielding, metal beam guardrail (i.e., not cable). The type of curb used differed greatly. Regarding offset distances, four states recommended the curb be placed offset from the bar- rier, though at distances that varied greatly; one state placed the curb under the barrier; and most states placed the curb face flush with the barrier face. The responses are summa- rized in Table 6. The states were also asked if they had experienced any safety problems with curb–barrier combinations on higher- speed roads. Three states had experienced safety problems. State 6 had experienced vaulting with the 150-mm curb that was resolved by only using sloped curbs on higher-speed facilities. State 19 had also experienced vehicles vaulting. The respondent did not elaborate on how this problem was solved but did state that even with the curb face flush to the barrier, vehicle wheels can get caught between the curb and the guardrail. State 27 had experienced W-beam rail failure at the splice and switched to ten-gauge rail on all Interstates and ramps for roadside applications. PREVIOUS CURB-RELATED RESEARCH CONDUCTED BY THE STATES Survey respondents were asked if their states had con- ducted any research related to curbs or curb–barrier combi- nations. Seven states indicated that they had conducted or were currently participating in research on curbs or curb– barrier combinations. Three states had participated in the Pooled Fund Study by the Midwest Roadside Safety Facil- ity entitled “Guardrail and Guardrail Terminals Installed over Curbs.” One state had conducted crash tests of 100-mm and 150-mm curb beneath guardrail and determined that the 100-mm-high curb met the criteria of NCHRP Report 350, Recommended Procedures for the Safety Performance Eval- uation of Highway Features. The 150-mm-high curb did not meet the criteria. VOIDS FOR ESTABLISHING GUIDELINES The states were asked to identify the most critical void for establishing guidelines for using curbs and curb–barrier com- Figure 20. Schematic drawing used in the State Survey to identify curb and barrier placement along roadways.

binations that they would like to see addressed in this research project. Most indicated that they needed more guidance on the use of curbs and curb–barrier combinations at various speeds and functional classes, especially for high-speed facil- ities. They wanted to know the appropriate guardrail and curb to use for all speed and functional scenarios. Two states indi- cated they would also like guidance on the appropriate curb to be used with sidewalks. One state asked for guidelines for transitional sections (suburban to urban) of highway projects in developing areas. One state wanted to address the need for additional lane widths for each configuration. Two states iden- tified the need for consideration of the practicalities of con- struction and maintenance in curb–barrier combinations. One of those two states had experienced problems with the com- binations when milling, paving, or removing snow. The other state had experienced problems placing guardrail in pave- ment when installing guardrail in front of the curb. Additionally, responding states also identified the follow- ing voids: • Consideration of bumper height and vehicle center of gravity, • Drainage alternatives, 29 • Usage of impact attenuators on medians, • Curb–barrier combinations at bridge approaches with bridge rails, • Safety impacts without a shoulder, • Influence of asphalt concrete dikes and W-beam, • Amount of allowable curb reveal to permit resurfacing without resetting the curb, • Placement of sand barrels behind the curb, and • Curb trajectory information. ADDITIONAL INFORMATION The state that originated the problem statement has a unique traversable curb (Type A6 in Figure 19) that it wanted to be included in the study. The curb has a nominal width of 300 mm with a 13 slope across the top and no vertical reveal. The curb is intended to permit pavement delineation and pro- vide drainage control while minimizing the potential desta- bilization hazard to errant vehicles. To their knowledge, the curb had not been crash-tested. One state was interested in design, maintenance, and con- struction issues concerning curb–barrier combinations. They ID Curb type Barrier type Distance from edge of lane to barrier Distance from face of curb to barrier face 1 A Guardrail 0.6 m 0.15 m 3 A (similar) Strong-post (steel) 2.4 m 0 4 Sloped W-beam 0.6 m 0 6 C W-beam 0.6 to 3.7 m 0 8 Asphalt dike W-beam 0.6 to 3.0 m 0 9 Asphalt (100-mm max) W-beam 1.8 to 3.0 m 0 10 A Guardrail 2.4 m 0 11 Vertical and sloped W-beam or T-beam 3.0 m 2.4-3.0 m 12 B or G W-beam Varies 0 or 0.23 m 13 B W-beam Varies 0-0.23 m 14 A W-beam 0.3 m 0 15 G (similar) W-beam Varies 0.05 m or 0.25 ma 16 B and G Strong-post, steel-plate Varies 0 17 B Guardrail 2.4 or 3.0 m 0 18 100-mm limit W-beam 1.2 or 3.0 m 0 or 0.6 m behind post 18 100-mm curb Raised median 1.2 or 3.0 m 0 or 0.6 m behind post 19 A or G Steel-plate beam, concrete Shoulder (1.8-3.0 m) 0 or behind guardrail 20 Sloped or vertical (100- mm max) W-beam Varies 0 or behind guardrail 21 A F-shape concreteb Shoulder (2.4 m) Behind barrier or 3 m 22 C Guardrail 0.3 m 0 23 Sloped W-beam Shoulder 0 or >0.46 m 24 Auxiliary type VI Variesc Varies <0.3 m or >3.0 m 25 Shoulder berm gutter Guardrail Shoulder Under guardrail 26 Sloped (50 mm) Guardrail Varies 0-.3 m 27 C Strong-post (steel) blocked-out or 3-cable Varies 0 a If curb/gutter is outside paved shoulder, 50 mm; when it contacts the lane, 250 mm. b The transition is rolled down from barrier to curb. c Varies, but not cable, concrete barriers, or attenuating devices. TABLE 6 Summary of curb–barrier installation practices among the states

had recently sent a questionnaire to all DOTs asking if the surveyed state installed curb in conjunction with strong-post guiderail. If so, the questionnaire inquired about installation, milling and overlay procedures, and whether the surveyed state had experienced any complications with the guiderail and curb combination while milling, installing, snow plow- ing, or overlaying. Regarding curb–barrier combinations, one state recom- mended placing the face of the barrier at the back edge of the curb. They felt this would lessen the number of nuisance hits of the guardrail and lower damage to vehicles that may rub the curb slightly. The state that identified bumper height and vehicle center of gravity as needing to be addressed indicated that the auto- mobile industry should be included in any research since that 30 industry has a large impact on the success or failure of road- side safety features. SUMMARY The survey of the states indicated that most states discour- aged the use of curbs on roadways with design speeds over about 70 km/h. The most common type of sloped curb used by the states was the AASHTO Type B curb. Most states located guardrails such that the face of the curb and the face of the barrier were flush consistent with current AASHTO recommendations. There was a perceived need for better guidelines on the use of curbs on higher-speed roadways and on the use of curbs in conjunction with roadside barriers.