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57 Curbs were very seldom the MHE in fatal crashes (5%) mation available on key issues, such as vehicle impact but much more often the MHE in total curb-related speed or angle for minor fixed objects like curbs; the crashes (53%). This implies that curb impacts can cause need to weight such small samples resulted in interpre- enough property damage to result in a reportable crash, tation problems. but that a fatality is more likely to result from a rollover. Based on an enhanced sample of 21 curb-related cases SV curb-related crashes were clearly no more severe on higher-speed roads, it appeared that there was a wide than crashes involving other roadside objects. Indeed, range of impact speeds (i.e., 15 to 61 mph), with approx- among crashes under similar conditions and controlling imately 50% between 35 and 45 mph; that there was a for rollover occurrence, curb impacts were slightly less wide range of impact angles (6 to 31 degrees) with a sig- severe than crashes with other objects. nificant proportion falling between 11 and 15 degrees; Severe curb crashes were often related to ramps. and that 77% (unweighted) or 97% (weighted) of the Severe curb crashes on Interstates and higher-speed vehicles in these crashes were tracking, depending on non-Interstates were often in urban areas. whether the unweighted or weighted data are used. It did not appear that environmental or driver factors played a major role in curb crashes. Severe curb crashes were likely to occur in clear weather on dry roads. There VEHICLE CURB TRAVERSAL SIMULATIONS AND TESTS was little difference between the curb and noncurb groups with respect to violations cited or whether the The kinematic behavior of a vehicle traversing a roadside crash was considered "speed related." As might be curb is the primary focus of this section. The modified NCAC expected from other research on alcohol use and crash finite-element model of the C2500 pickup truck was used to severity, the data from Michigan did appear to hint at investigate the vehicle's response when crossing a number of more alcohol use in the severe curb crashes than in the different curb types at various impact conditions. The results less severe crashes. of the simulations were verified through a series of live-driver Severe curb impacts involved passenger cars and motor- curb traversal tests with a C2500. cycles more often than SUVs or pickups. Rollover is a factor of interest in improving curb design and placement, since it clearly differentiated between a Curb Traversal Simulations severe and nonsevere crash. Rollover after impacts with curbs appeared to be a relatively low-frequency occur- The case of a vehicle impacting a curb in a tracking man- rence, but it remains a problem worthy of design atten- ner was investigated using LS-DYNA. This section is a brief tion due to the severity of crashes involving rollovers, overview of the methods and results of the FEAs; much more as demonstrated by the higher rollover percentages in detail can be found in a dissertation by Plaxico (52). fatal curb-related crashes. On urban freeways, segments with both guardrails and curbs were likely to have both more SV curbguardrail Parametric Study crashes and more SV curbguardrail injury crashes than segments with only guardrails. Assuming this relation- A parametric study was conducted using different curb ship holds for other roadway classes, which it should for types, impact speeds, and impact angles, as shown in Table 19. at least rural Interstates, it clearly supports attention to The curb types included in the study were AASHTO Types the design and placement of curbs in combination with A, B, C, D, and G and the 100-mm New York curb, which is guardrails. referred to in the tables and figures as "New York" or "NY." Even though a relatively large number of crash recon- These curbs are illustrated in Figure 29. Two impact speeds structions are developed each year in the NASS-CDS were used: 70 km/h, which corresponds to the intermediate system (i.e., 5,000 per year), there was very little infor- speed range of interest (i.e., 60 to 80 km/h), and 100 km/h, TABLE 19 Simulation matrix of impact speed, angle of impact, and curb type for the 2000-kg pickup impacting in a tracking manner Impact speed = 70 km/h Impact speed = 100 km/h Curb Angle of impact: Angle of impact: type 5 15 25 5 15 25 A T T T B T T T T T T C T T T T T T D T T T T T T G T T T T T T New York T T T T T T

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58 150 mm 150 mm 10 mm m 50 mm m 150 mm 0 R1 125 mm 150 mm m m 75 mm 100 mm 5 R7 R75 mm m 5m R2 (A) (B) (C) 50 mm 130 mm 300 mm 100 mm 305 mm m 150 mm 0m 25 mm m 100 mm 100 mm R5 44 m (D) (G) (NY) Figure 29. Curb types used in this study. which corresponds to the high speed range (i.e., greater than crete surface with its wheels locked and the force required to 80 km/h). Three angles of impact were investigated: 5, 15, pull the truck was measured using a hydraulic scale. and 25 degrees. Impact angles of 5 and 15 degrees repre- Data collected from the simulations included bumper tra- sented the more probable range, while the 25-degree impact jectories and vehicle paths; acceleration-time histories; yaw, was consistent with NCHRP Report 350 impact conditions pitch and roll-time histories; yaw, pitch, and roll angle rate- for longitudinal barriers. The vehicle used in the simulations histories; sequential snapshots; and TRAP results (i.e., occu- was the 2000-kg C2500 pickup truck model developed by pant risk values). NCAC with modifications made by WPI. Only one roadway cross-section, shown in Figure 30, was used in the parametric study. A typical two-lane cross-section Results in which a curb and a barrier may be installed together was chosen: a road surface with a 2% cross slope and 1.1-m wide Bumper trajectories and vehicle paths. Figure 31 shows gutter section with a 4.5% slope. Although a 4% backfill the bumper height of the pickup truck during impact with the would have been typical, a level roadside (i.e., 0% slope) was different curbs for each impact angle studied. From the results used in this study since it was more conservative in terms of of the simulations, the following observations were made the potential for the vehicle to override a barrier. The road- regarding the potential for barrier override. way and curb were modeled using shell elements with rigid material properties, and the tire-ground interaction was sim- Influence of lateral offset. For the 150-mm curbs (i.e., ulated by using a surface-to-surface contact definition that AASHTO types A, B, and D), there was a potential for bar- included friction. The friction values used were based on rier override if the barrier was positioned within 8 m behind results from physical tests in which a C2500 pickup truck the curb. For the 100-mm curbs (i.e., AASHTO types C and was pulled at low speed over an asphalt surface and a con- G and the NY curb), the potential for barrier override appeared Figure 30. Roadway cross-section used in this study.

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59 Figure 31a. Bumper height with respect to lateral Figure 31b. Bumper height with respect to lateral distance behind curb and curb type for C2500 pickup distance behind curb and curb type for C2500 pickup crossing the curb at an angle of 5 degrees at 70 km/h (top) crossing the curb at an angle of 15 degrees at 70 km/h (top) and 100 km/h (bottom). and 100 km/h (bottom). to be less if the barrier was positioned between 2 m and 3 m from the results of the FEAs and input into TRAP. From or more than 8 m behind the curb. However, in the case of these data, occupant risk factors were computed based on the 25-degree impacts, the trajectory of the front bumper OIVs and ridedown accelerations and were found to be minor, continuously increased over a lateral distance of approxi- as expected. The primary purpose for using TRAP, however, mately 4 m behind the curbs. was to obtain information that would aid in quantifying vehi- cle stability regarding the various curb types. Influence of impact conditions. The trajectory of the front The stability of the vehicle during and after interaction with bumper was nearly independent of vehicle speed but slightly curbs may be adversely affected by wheel interaction with the dependent on impact angle (it increased with increase in curbs. For example, the front wheels of a vehicle may undergo impact angle; less so for the 150-mm curbs). For 100-mm abrupt steering during impact with a curb which may eventu- curbs the potential for barrier override was minimal for impact ally lead to spin-out or overturn of the vehicle. TRAP pro- angles of 5 and 15 degrees. For a given impact speed and vides information based on maximum accelerations, maximum angle, the mode of vehicle trajectory was similar for all curb angle displacements, and maximum displacements rates that types (for a given impact speed and angle, the maximum may be useful in discerning vehicle instability. The results bumper trajectory occurs at approximately the same point, from TRAP for each analysis in the study matrix are pre- regardless of curb type). sented in Table 20. Influence of curb type. The maximum value of bumper tra- Acceleration-Time histories. An Acceleration Severity Index jectory was dependent on curb height (i.e., increased with (ASI) value was computed from each analysis. These values increase in curb height). It was also slightly dependent on were relatively low concerning occupant risk during impact, slope of curb face, with some discrepancy in the results from but they did give an indication of the overall acceleration the AASHTO curb type A analyses. response of the vehicle during vehicle-curb interaction, which The accelerations and angle-displacement rates computed may be regarded as some measure of difficulty for a driver to at the center of gravity of the vehicle model were extracted maintain control of the vehicle. For example, a higher ASI

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60 increased as curb height increased and decreased as impact angle increased. Roll rates were also independent of the slope of the curb face, and they were minimally affected by impact speed. The roll rate increased as curb height increased and as impact angle increased. The influence of impact angle on roll rates was much more pronounced for cases involving 150-mm curbs than for cases involving 100-mm curbs. Pitch angles were minimal in all cases (i.e., less than 3.5 degrees). They appeared to be independent of impact speed and slope of the curb. The pitch angles increased slightly as curb height increased. Pitch rates were independent of the impact speed and slope of the curb face. For the 150-mm curbs, pitch rates varied significantly with respect to impact angle; for the 100-mm curbs, the pitch rate was much less influenced by impact angle. Yaw angles were primarily affected by the steer angle of the front wheels after impact with the curb. As the front wheels steered out, usually to the right, during wheel- curb interaction, the yaw angle increased and was typi- cally greatest at the end of the analysis. As the vehicle tra- versed the curb, the resulting yaw angle of the vehicle could lead to an impact with the barrier at a higher or lower impact angle than the original encroachment angle. Yaw angles were independent of curb height and impact Figure 31c. Bumper height with respect to lateral distance behind curb and curb type for C2500 pickup speed but increased as the slope of the curb face crossing the curb at an angle of 25 degrees at 70 km/h (top) increased. For the 150-mm curbs, the yaw angle ranged and 100 km/h (bottom). from 8 to 28 degrees and varied erratically with respect to the impact angle. For the Type C curb, the yaw angle ranged between 9 and 24 degrees; for the G curb, the angles were very low (3 to 10 degrees) except for the high-speed, high-angle impact for which the maximum value indicates that the vehicle experienced higher accelera- yaw angle was 22 degrees; for the NY curb, the angles tions, which could affect the driver's ability to maintain con- were also very low (i.e., 3 to 6 degrees, and negative trol of the steering and braking of the vehicle during impact. 8 degrees in one case) except for the high-speed, high- Figures 32 and 33 show a comparison of the ASI for each angle impact for which the maximum yaw angle was analysis; note that curb types A, B, and D are 150-mm curbs 18 degrees. and curb types C, G, and NY are 100-mm curbs. Yaw rates increased as the height of curb increased and Figures 32 and 33 point to the following conclusions about as the slope of the curb face increased. For the 100-mm ASI values: curbs, the yaw rate was independent of impact speed but increased slightly as the impact angle increased. For the They increased as impact velocity increased. A curb, there was no discernable effect of the impact They increased as impact angle increased. angle on the yaw rate; for the B curb, the yaw rate var- They increased as the curb height increased. ied significantly and erratically with respect to impact They increased as the slope of the curb face increased. speed, while the impact angle had minimal influence except for the high-speed, high-angle impact; for the D Yaw, pitch, and roll. Figure 34 shows the maximum angu- Curb, the yaw rate increased as impact angle increased lar displacements and maximum angular displacement rates and increased slightly as impact speed increased. from each analysis case. The following observations were made from the analyses: Roll angles were minimal in all cases (i.e., less than Summary 8 degrees). They appeared to be unaffected by the slope of the curb face, especially at higher impact speeds, The FEA program LS-DYNA was used in a parametric and almost unaffected by impact speed. The roll angle study to investigate the influence of several factors regarding

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61 TABLE 20 Summary of results from TRAP for each analysis in the curb study matrix Impact Max. vertical Max. Max. angle Max. angle disp. rates Curb conditions acceleration (G's) vertical displacements (degrees) (deg/s) ASI type Speed Angle 60 Hz 10 ms 50 ms impulse Roll Pitch Yaw Roll Pitch Yaw (km/h) (deg) filter average average (N*s) rate rate rate 5 6.31 3.21 1.05 1618 0.18 -6.4 3.0 22.6 68.2 36.2 35.2 A 70 7 73 4 01 1 36 2170 0 20 65 31 27 4 77 0 50 3 40 9 15 25 9 41 4 11 1 77 3519 0 22 54 24 12 0 82 3 69 0 38 0 5 5 08 1 00 2 57 1785 0 11 69 24 20 2 46 1 28 3 47 2 70 7 38 3 88 1 42 5238 0 19 66 33 25 2 67 5 54 4 47 2 15 150-mm curbs B 25 6 59 5 47 1 92 3453 0 25 54 28 26 9 116 0 34 1 44 3 5 4 44 2 72 1 07 2523 0 19 76 23 21 4 62 4 27 8 34 6 100 6 79 4 65 1 39 2517 0 22 50 26 20 0 80 7 39 9 37 2 15 25 14 93 10 00 2 84 4284 0 29 42 24 23 1 97 5 71 6 57 1 5 1 50 1 25 0 90 1506 0 10 74 22 11 1 45 8 17 9 22 7 70 3 58 2 63 1 31 1990 0 14 54 26 84 63 4 34 4 32 7 15 D 25 7 56 5 57 1 75 3349 0 21 52 27 28 1 100 7 38 8 39 7 5 2 55 1 40 0 87 2115 0 14 71 18 78 45 3 13 4 26 9 100 5 78 4 51 1 17 2443 0 19 53 28 24 6 72 9 37 7 37 6 15 25 11 41 7 19 2 54 3772 0 26 42 24 23 8 95 8 57 5 51 1 70 5 1 30 0 97 0 63 1318 0 09 60 16 12 6 37 9 12 2 18 9 15 2.86 1.73 0.95 1557 0.11 -4.2 2.1 11.4 36.4 22.9 21.7 C 25 3.93 2.42 1.20 2411 0.16 -3.9 2.5 23.7 48.2 23.0 28.7 5 1.50 1.03 0.70 1594 0.09 -5.7 1.4 9.1 36.8 12.1 17.9 100 15 3.20 2.06 1.00 1990 0.15 -3.8 2.3 22.8 50.1 22.8 23.5 25 5.86 4.54 1.25 2500 0.18 -3.4 2.0 23.7 61.1 28.0 27.8 100-mm curbs 5 0.83 0.77 0.61 1097 0.07 -5.9 1.6 6.4 35.9 12.3 15.8 70 15 2.17 1.41 0.85 1811 0.09 -4.0 2.2 4.1 36.1 17.9 17.5 G 25 4.20 2.73 1.14 2319 0.14 -4.1 2.7 9.8 54.7 24.6 20.1 5 0.99 0.89 0.74 1589 0.08 -5.4 1.3 3.3 37.7 12.1 13.1 100 15 2.59 1.81 1.06 1973 0.12 -4.0 2.4 6.7 47.8 26.6 16.8 25 7.81 5.87 1.81 2585 0.19 -3.6 2.2 21.7 66.2 26.9 29.0 5 0.43 0.34 0.26 796 0.05 -4.9 1.2 4.0 27.8 12.2 15.8 70 15 1.30 0.84 0.63 1188 0.07 -3.8 2.1 3.0 32.3 14.5 12.5 NY 25 3.10 2.00 1.11 2017 0.14 -3.7 2.3 -7.8 35.0 25.1 16.2 5 0.96 0.88 0.71 1477 0.08 -5.5 1.2 4.1 37.6 12.1 10.6 100 15 1.72 1.43 0.97 1626 0.10 -3.7 2.0 5.9 29.2 19.0 13.3 25 5.23 4.45 1.59 2313 0.17 -3.4 2.1 18.4 57.7 22.3 19.3 vehicle stability and trajectory when traversing curbs. The cient to override a standard strong-post guardrail placed at variables used in the study included curb height and shape, 0.5 to 8 m behind the curb. impact speed, and impact angle. Acceleration and angular rate data collected at the center The results of the study indicated that the trajectory of the of gravity of the vehicle model during analysis were used as front bumper was only slightly affected by impact speed, inputs to TRAP. The results indicate that ASI values were impact angle, or the slope of the curb face. The most signif- proportional to impact speed, impact angle, curb height, and icant factor influencing trajectory was the height of the curb. the slope of the curb face. This suggests that a driver was Based on the range of impact conditions considered in this much less likely to lose control while traversing a lower curb study, the trajectory of a 2000-kg pickup truck traversing with a more mild, sloping face (e.g., the New York curb) than curbs with a height of 100 or 150-mm was considered suffi- while traversing a taller, steep-faced curb such as the AASHTO Type A or B. The analysis showed that vehicle impacts with roadside curbs could often result in the driver losing control of the vehi- cle. There were many factors that influenced vehicle behavior during curb traversal, such as abrupt steering caused by the Speed = 70 interaction of the front wheels with the curb, loss of contact between the tires and the ground, excessive vehicle accelera- ASI Speed = 100 tions, and excessive roll, pitch, and yaw during impact. While each of these factors may lead to loss of control of the vehicle, total loss of control is unlikely except in extreme cases. A more A B D G important issue may be the effects that these factors precipi- C NY tate when curbs are placed in combination with roadside hard- Curb ware (e.g., guardrail, crash cushions, or breakaway poles). Figure 32. ASI of C2500 pickup truck by curb type and Vehicle behavior in impacts with curbguardrail combinations speed at impact, based on FEA. are discussed later in this section.