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7 program is used to assess new roadway designs by using a (2) Test 3-10, which involves an 820C (e.g., Geo Metro) driver performance model to simulate the vehicle/driver impacting the guardrail at a speed of 100 km/h and an response when traversing the proposed roadway configuration. impact angle of 20 degrees. The Driver Performance Model in IHSDM estimates drivers' speeds and path choices along a roadway, and this informa- A guardrail system that meets the evaluation criteria for Tests tion is provided as input to VDANL, which estimates vehicle 3-10 and 3-11 in NCHRP Report 350 is generally considered kinematics such as lateral acceleration, friction demand, and acceptable for use on all TL-3 roadways within the United rolling moment. The information from VDANL is used to States. identify conditions that could result in loss of vehicle control (i.e., skidding or rollover). EFFECT OF CURBS ON VEHICLE STABILITY Full-Scale Crash Testing Dunlap, 1973 (11, 21) Although advancements in computer simulation programs The objective of Dunlap's research was to determine how have made it possible to accurately reproduce and predict far in front of the barrier the curb should be placed to achieve complex impact events, full-scale testing is still essential in the best redirection performance from the curbtraffic barrier evaluating the safety performance of curbs and other road- system. Dunlap examined all the test data available in the side appurtenances. To evaluate the performance of roadside early 1970s and found that the results were difficult to gen- safety barriers, impact conditions must meet the standard eralize. While there were cases of vehicles vaulting over a testing procedures accepted by the FHWA. The first proce- guardrail or bridge railing when a curb was used in front of dures document was published by the Highway Research the guardrail, in many cases the guardrail itself had structural Board in 1962 (18). The later revisions of the procedures problems so it was difficult to assess the contribution of the were made by the National Cooperative Highway Research curb to the failure. Dunlap performed computer simulations of a variety of curb and barrier combinations using HVOSM Program. The latest revisions of the testing procedures were to determine the risk of overriding the barrier. Dunlap's analy- published in NCHRP Report 350 in 1993 (19). sis indicated that for the six curb and barrier combinations From 1981 to 1992 crash tests were conducted according studied, vaulting was not expected to be a problem. This analy- to the test requirements specified in NCHRP Report 230 (20). sis, however, has several serious limitations not least of which The test conditions required for evaluation of guardrail in is the validity for barrier impact analysis of the HVOSM com- NCHRP Report 230 involved a 2000-kg sedan impacting the puter program that was being used at the time. Dunlap's work guardrail at a speed of 100 km/h and an angle of 25 degrees. does, however, illustrate two important points: (1) computer The most important change in NCHRP Report 350 was simulation is one possible method for assessing a variety of that the large passenger sedan had virtually disappeared from curbbarrier geometries and (2) the conventional wisdom the vehicle population, and new vehicle types, such as mini- that curbs should not be used in front of semirigid barriers vans, SUVs, and pickup trucks, had emerged in their place. warrants more careful investigation. Since the first testing procedures specified in Highway Research Circular 482 up until NCHRP Report 350, the large car sedan (i.e., a 2040-kg car) had served as the crash test Olsen et al., 1974 (22) vehicle representing the fleet of large passenger vehicles. NCHRP Report 350 replaced the large car with a 2000-kg Olsen and other researchers at Texas Transportation Insti- pickup truck. The challenges that the pickup truck introduced tute (TTI) conducted a study to investigate how various types to the crash testing procedures were due to its high, more for- of curbs affect vehicle response, such as redirection, trajectory, ward center of gravity making it much more unstable during path, roll, pitch, and accelerations. Their study involved full- impacts than its predecessor, the large sedan. scale tests and simulations of vehicles traversing various types The performance of a curb/guardrail combination are eval- of curbs. Eighteen full-scale tests were conducted on types B uated using test conditions specified in NCHRP Report 350 for and D curbs (see Table 1); nine full-scale tests were conducted evaluating the crashworthiness of the length of need (LON) on each curb type at speeds of 48, 72, and 97 km/h and at section of a guardrail. There are currently two tests that are 5-, 12.5-, and 20-degree encroachment angles. The HVOSM required in Report 350 to evaluate guardrail systems for use computer program was used to simulate vehicle impact with along high-speed roadways: three different curb types: AASHTO curb types B, D, and G. Although in the study, the curbs were referred to as C, E, and (1) Test 3-11, which involves a 2000P pickup truck (e.g., H curbs (which is consistent with the nomenclature of the Chevrolet 2500) impacting the guardrail at a speed of AASHTO Blue Book), the AASHTO Green Book now refers 100 km/h and an impact angle of 25 degrees, and to these curbs as B, D, and G, respectively. Nomenclature

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8 TABLE 1 Summary of full-scale test results from Olsen et al. (22) Test number Approach speed Encroachment Maximum (mph) angle bumper height (degrees) during vehicle trajectory (inches) Curb Type D N-2a 30.4 5.1 24.1 a N-3 45.6 5.0 24.3 N-4 59.3 4.6 23.9 N-5 32 11.6 20.8 N-6 45.3 11.1 23.7 N-7 63.6 12.6 23.5 N-8 32.7 18.5 23.5 N-9 41.8 18.7 21.9 N-10 63.0 17.6 23.3 Curb Type B N-11a 34.2 4.9 26.2 N-12 44.7 5.1 24.8 N-13 34.2 11.2 23.8 N-14 43.5 12.8 23.1 N-15 32.1 17.4 22.1 N-16 43.0 18.4 23.5 N-17 66.5 5.1 24.3 N-18 62.2 12.3 21.4 N-19 61.5 18.6 23.0 a Vehicle redirected throughout this document will use the Green Book designa- tors, including barrier configuration, lateral stiffness proper- tions. Twelve curb impacts were simulated on each curb type ties of the barrier, and impact conditions, as well as vehicle at impact speeds of 48, 72, and 97 km/h and at 5-, 12.5-, and characteristics, such as bumper shape and vehicle kinematic 20-degree encroachment angles. Impacts at 121 km/h were properties. The trajectory of the vehicle after mounting a also simulated at 5-, 10-, and 15-degree encroachment angles. curb must allow the vehicle to contact the guardrail, or other The test vehicle used in the study was a 1963 Ford four-door roadside device, at the appropriate height. sedan with heavy-duty suspension. The vehicle's mass was 1,905 kg, and the center of gravity of the vehicle was 610 mm above ground. The test vehicle is shown in Figure 4. Olsen et al. found that AASHTO types B, D, and G curbs, which are sloping curbs 150 mm or less in height, provide no redirection for a large passenger vehicle, such as a 1900-kg sedan, traveling at speeds greater than 72 km/h at encroach- ment angles greater than 5 degrees. They also found that type B and D curbs can produce, under certain speed and encroach- ment angles, vehicle ramping high enough to allow the bumper height to equal or exceed the height of a typical guardrail, as illustrated in Figure 5. Such vehicle trajectories may result in a vehicle vaulting over the top of the rail or snagging on the tops of the posts and flipping over. Whether the vehicle penetrates behind the barrier or is redirected is, of course, influenced by other fac- Figure 4. Vehicle used in Olsen et al.'s study (22).

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9 337 mm Curb Lateral Offset from Curb Figure 5. Possible trajectory of vehicle bumper relative to guardrail height. Olsen et al. found that for 150-mm-high AASHTO B and the bumper is close to the guardrail before the wheel D curbs an increase in either speed or impact angle resulted impacts the curb, then the dipping event would cause the in greater lateral distances to the maximum rise point and bumper to impact the guardrail just below the W-beam rail. higher vertical position of the vehicle at the maximum rise Note that the lower edge of the guardrail is 533 mm above the point. The encroachment angle had a more notable effect on pavement surface due to the 150 mm elevation of the curb; the maximum rise point and position than did vehicle speed, whereas, the lower edge of the rail is only 381 mm above when vehicle speed was greater than 100 km/h. ground level in normal configuration. An initial dipping The maximum rise height of the bumper, predicted from the motion of the bumper was not evident during impact with the simulations, was approximately 737 to 787 mm and occurred type G curb, and the bumper contacted the guardrail on the in the range of 2.44 to 3.0 m behind 150-mm-high curbs. The face of the W-beam in all impact cases. height of a typical W-beam guardrail is 686 mm, as shown The simulation study by Olsen et al. also demonstrated in the sketch in Figure 5. The maximum rise height during that the stiffness of the vehicle's suspension had little effect impact with the type G curb was only slightly affected by vehi- on vehicle trajectory. A summary of full-scale test results cle speed and encroachment angle. The maximum vertical rise performed in Olsen et al.'s study is given in Table 1 and a of the vehicle impacting the type G curb was less than 50 mm. summary of their HVOSM simulation results is given in Furthermore, the maximum rise height did not increase an Table 2. The HVOSM model had a disk wheel that was not appreciable amount for speeds greater than 48 km/h, indi- detailed enough to accurately simulate wheel contact with a cating that the maximum rise height during impact with the curb. The simulation results in NCHRP Report 150 predicted type G curb is relatively independent of vehicle speed and that full-size cars would be redirected by a 13-in.-high Type impact angle. X curb in 60-mph impacts up to 12.5 degrees. However, in It was concluded that the maximum rise point was depen- 60-mph crash tests, the test vehicles crossed the curb. The dent on the combination of vehicle roll and pitch caused by disparity between the test results and the HVOSM predic- striking the curb. When the wheel impacts the curb, the loads tions was more apparent in the high-speed tests, particularly are distributed to the other three wheels, particularly the other between the predicted roll and bumper rise and those values front wheel. If the impacting wheel rises too quickly, then the measured from the test data. vertical tire force will be sufficient to bottom out the suspen- sion, introducing shock loads. In addition, excessive pitch and roll angles are produced when the fully compressed sus- Ross and Post, 1975 (23 ) pension unloads. The effect that curb geometry has on damp- ing the roll angle during wheel impact obviously differs with Researchers at TTI conducted a study to evaluate auto- the height and the steepness of the curb face. The pitch and mobile behavior when traversing selected curb configura- roll angles produced by simulated collisions with type B and tions and sloped medians and, also, to evaluate the potential D curbs were as much as twice those produced by collisions for a vehicle to vault over roadside barriers placed in combi- with the type G curb. nation with curbs or sloped medians. HVOSM was used to Curbs that are 150-mm high and set in front of a 685-mm simulate vehicle impacts with 150-mm-high and 200-mm-high W-beam guardrail at a 0.61-m lateral offset may result in the curbs, modified curbs, and slopes. The researchers also com- vehicle impacting the guardrail at a point below the lower pared the effects of standard curb shapes to various retrofit edge of the rail, possibly causing snagging, as shown in alternatives, such as installing wedge-shaped asphalt plugs in Figure 5. During impacts with the 150-mm-high curbs, the front of the curbs and replacement of the curbs with slopes. bumper would dip down slightly and then began to rise as the It was concluded from the simulation results that traffic bar- vehicle crossed the curb. If the angle of impact is such that riers should not be placed near curbs due to the probability of

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10 TABLE 2 Summary of HVOSM simulation results from Olsen et al. (22) Curb Vehicle Impact Max Max Max bumper Lateral Bumper height speed angle roll pitch height above distance to above curb at (mph) (deg) angle angle curb max rise point 2-ft offset (deg) (deg) (inches) (ft) (inches) Type B (6-in.) 30 20 +8.8 2.9 22 5 12 45 20 -8.9 3.0 26 8 11 60 12.5 -13 2.0 27 7 13 60 20 -8 2.0 29 10 10 75 10 -15.5 2.0 30 6 13 75 15 -10.2 1.8 30 10 12 Type D (6-in.) 30 12.5 -9.5 2 21 4 13 30 20 -8 2.5 21 6 11 45 12.5 -11 2 23 5 12 45 20 -8 2.2 25 8 11 60 5 -11.2 2 23 3 17 60 12.5 -12 2 25 6 13 60 20 -9.5 2.5 31 10 11 75 5 -12 1.5 23 4 16 75 10 -13 2 25 6 13 75 15 -11 2 31 9 12 Type G (4-in.) 30 12.5 -5 1 18 5 13 30 20 -3 1 18 9 12 45 5 -7 1 20 3 15 45 20 -4 1 20 10 14 60 5 -7 1 20 4 15 60 12.5 -5 1 20 8 13 60 20 -3 1 20 10 13 vehicles vaulting or underriding the barrier. They also showed 1" that problems with barriers on raised curb-medians or curb 2" guardrail configurations could be reduced in certain situations by sloping the median or the roadside to the top of the curb. 1/4" 8- 2 /16 "R 4" 6" R Holloway et al., 1994 (24 ) 6" Three types of sloping curbs commonly used by the Nebraska Department of Roads (NDOR) were investigated 2% for safety performance through a combination of full-scale 18" testing and computer simulation using HVOSM. The curb types investigated included a 100-mm lip curb (13 slope on curb face), a 150-mm lip curb (13 slope on curb face) and a 150-mm AASHTO type I curb. The AASHTO type I curb, shown in Figure 6, is the curb type most widely used by NDOR. The test matrix in the study included 23 full-scale Figure 6. AASHTO Type I Curb.

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11 tests: 13 tests on the 100-mm lip curb, 2 tests on the 150-mm impacted the curb at an encroachment angle of 20 degrees lip curb, and 8 tests on the AASHTO type I curb. and at impact speeds of 72.4 and 86.9 km/h. The maximum The three curbs tested were found to have little potential roll and pitch angles were 7.8 degrees and 2.6 degrees, respec- for causing a vehicle to lose control during tracking impacts, tively. The tests indicated that there was a slight potential and, thus, the researchers concluded that the curbs would not for the vehicle to underride a standard W-beam guardrail, if pose a significant hazard to vehicles impacting in a tracking the guardrail was placed within 1.22 m of the curb; how- mode. Although the 100-mm curb performed better than the ever, the tests also indicated that there was very little potential 150-mm curbs in all impact conditions, the safety benefit was for the vehicle to vault over the barrier. not considered significant. It was also concluded that the per- Tests conducted on the AASHTO type I curb resulted in formance of W-beam guardrails could be adversely affected maximum roll and pitch angles of 9.7 degrees and 3.1 degrees, when installed behind curbs and that, when curbguardrail respectively. Although the angular displacements of the vehi- combinations are necessary, the curb should be placed behind cle during impact with this curb were somewhat higher than the face of the guardrail to minimize the potential for vehicle those produced in impacts with the lip curbs, the potential for ramping. loss of control of the vehicle was again considered very low. The testing area was on a negative grade that may have had The driver of the vehicle in the study reported that the sus- some effect on the vehicle kinematics during impact. Tests pension system fully compressed and bottomed out against were conducted using two types of test vehicles: a small car the suspension bumper stops during impact with the 150-mm with a mass of 817 kg (1984 Dodge Colt) and a large car with curbs and a small jolt was felt. The trajectory of the vehicle a mass of 2,043 kg (1986 Ford LTD). The center of gravity during the tests indicated there was a potential for underride of the test vehicles were 533 mm and 572 mm for the 817-kg of a standard W-beam guardrail if the barrier is located within and 2043-kg vehicles, respectively. 1.22 m of the curb; however, there did not appear to be any The impact speeds used in the full-scale tests were 64.4, significant risk of the vehicles vaulting over such a barrier. 72.4, 80.5, and 88.5 km/h at encroachment angles of 5, 12.5, The HVOSM was also used to investigate alternate impact and 20 degrees. Vehicle decelerations were very low, indi- conditions. Simulation models of the 23 full-scale tests were cating that there is little risk of occupant injury as a direct developed, and the results were compared to the full-scale result of curb impact. The yaw rate and yaw angle were also tests to validate the model. An additional 55 simulations were very low, indicating that there was minimal redirection of the then performed. Thirty-one simulations were performed to vehicles as they impacted and mounted the curbs. supplement the original 23 impact scenarios, including 5 sim- Thirteen full-scale tests were conducted on a 100-mm lip ulations with the 100-mm lip curb, 16 simulations with the curb, and two full-scale tests were conducted on a 150-mm 150-mm lip curb, and 10 simulations with the 150-mm type I lip curb. For low-angle impacts on the 100-mm curbs with curb. Another 24 simulations were performed to evaluate the the 817-kg vehicle, the maximum roll and pitch angles effects of curb impact with the curb placed on flat grade. increased as the impact velocity increased; values ranged The simulations with the lip curbs were performed with from 5.6 to 9.0 degrees and 0.7 to 1.4 degrees for roll and pitch vehicle velocities of 72.4 and 88.5 km/h at encroachment angles, respectively. For the moderate- and high-angle impact angles of 5 and 20 degrees. The results of the simulations tests, the maximum roll angle increased as the impact speed with the 100-mm lip curb showed no potential for either under- increased, while the maximum pitch angle decreased with an riding or vaulting a W-beam guardrail installed behind the increase in impact speed. The maximum roll angle in the tests curb. The results of the simulations with the 150-mm lip curb was 9.3 degrees, and the maximum pitch angle was 2.6 degrees. indicated that the small vehicle (817 kg) may underride a Thus, the pitch and roll angles were considered to be relatively W-beam guardrail if the guardrail is placed within 1 m of the insignificant in terms of producing loss of vehicle control. curb, and it is likely to vault over a guardrail placed 0.46 to It was also concluded in the study that there was only a 3.7 m behind the curb. The simulations with the large vehi- slight potential for an 817-kg vehicle to underride a standard cle (2043 kg) indicated a slight potential for underriding a 686-mm W-beam guardrail when the 100-mm lip curb is W-beam guardrail located within 1 m of the curb, and vault- placed in combination with the guardrail. The greatest poten- ing of the guardrail was likely if the barrier was placed in a tial of the vehicle vaulting over the barrier would be when the region of 0.61 to 3 m behind the curb. barrier is located in a region 0.76 m to 2.74 m behind the curb. The simulations with the AASHTO type I curb indicated Similarly, for low-angle impacts with the 2043-kg vehi- that impact with the curb could cause underride of a W-beam cle impacting the 100-mm lip curb, the roll and pitch angle guardrail placed within 0.61 m of the curb. For small car increased as the impact speed increased. The maximum roll impact, a potential for vaulting existed if the guardrail was and pitch angles were 7.2 degrees and 1.1 degrees, respec- placed 0.46 m to 3.0 m behind the curb. For large car impact, tively, for the low-angle impacts. The maximum roll and a potential for vaulting existed if the guardrail was placed pitch angles for the high-angle impacts were 7.2 degrees and 0.46 m to 3.7 m behind the curb. 2.0 degrees, respectively. There were only two tests conducted The additional 24 simulations were performed on all three on the 150-mm lip curb. In these two tests, a 2043-kg vehicle curb types to investigate the effects of impact with the curbs