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23 curb is placed underneath a strong-post W-beam guardrail, DeLeys and Brinkman used crash data analysis and com- there is little chance of vehicle contact with the curb (24, 27). puter simulation to investigate rollover tendencies of vehi- It has also been found that stiffening the guardrail system by cles traversing various kinds of roadside terrain. They con- installing a W-beam rail to the back of the posts or installing cluded that the data bases lacked the comprehensive and a rub-rail will enhance the safety performance of a curb detailed information necessary to define conditions that lead guardrail system (28). The installation of a rub-rail may pro- to rollover. A modified version of HVOSM with improved vide the most safety benefit, since it both stiffens the system application for rollover situations was used in their study to avoid vehicle-to-curb contact and shields the posts from (33). Full-scale tests were used to validate the computer potential wheel snag. models and, subsequently, over 200 simulations were con- There have been three tests performed on curbguardrail ducted to investigate the rollover tendencies of vehicles tra- systems under NCHRP Report 350 Test 3-11 impact condi- versing various sideslopes, fill embankments, and ditch con- tions: MwRSF tests NEC-1, NEC-2 and TTI test 404201-1 figurations. They did not investigate vehicle-curb interaction; (29, 30, 32). These tests involved 100-mm-high curbs placed however, the models that were used in their study may have in combination with strong-post guardrails. Both test NEC-1 been applicable for such analysis. and test TTI 404201-1 resulted in significant tensile forces in Cooperrider et al. carried out a series of full-scale crash the W-beam rail and excessive movement of the anchor sys- tests to determine the potential for rollover of various vehicle tem. In test NEC-1, the two upstream anchor posts for the types tripped by a curb, sliding in soil, and rolled off a dolly G4(1S) guardrail with wood blockouts ruptured causing the (34). A steel 152-mm-square tube section rigidly affixed to vehicle to pocket (29). This ultimately resulted in rupture of the roadway was used to represent a curb in their tests. In five the W-beam rail element, and the vehicle penetrated the guard- of the eight tests that they conducted, the vehicles rolled rail. The poor performance of this system was not directly over. In the cases where rollover did not occur, the wheel attributed to the effects of the curb, but rather to a loss of ten- assembly failed during impact with the curb due to the high sile capacity of the guardrail during impact when the anchor forces that were developed. The failure of the wheel assem- system failed. bly, consequently, removed the overturning force that was In TTI test 404201-1, the foundation of the anchor posts being applied to the vehicle. If the wheel assembly had not of the G4(2W) guardrail moved in excess of 70 mm at the failed in those cases, it is possible that all the tests would ground line, and there was considerable damage to the guard- have resulted in a rollover. rail system; however the system did meet all safety require- The vehicle dynamics code, VDANL, has been used to ments of NCHRP Report 350 (30). Also, the extent of dam- study vehicle rollover as a function of unstable maneuvering age to the system in test TTI 404201-1 was much greater than conditions and also to investigate vehicle rollover because that of previous crash tests on the G4(2W) guardrail system of impact with various vehicle tripping mechanisms such as curbs, soil, ditches, and so forth (3537). The results of the without a curb present (31). computer models developed in those studies were validated In test NEC-2, the G4(1S) guardrail with wood blockouts with full-scale tests. VDANL was chosen by the FHWA to be was modified and retested (32). The guardrail was modified incorporated into the IHSDM, which is used to assess new by nesting 12-gauge W-beam rails along the length of the sys- highway designs. tem. This test resulted in excessive vertical trajectory of the vehicle during impact, but the vehicle remained upright and successfully met all safety criteria of NCHRP Report 350. SUMMARY Vehicle tripping on curbs was addressed in a very limited number of studies. The studies that were identified in the lit- While there has been some work performed on the safety erature used a variety of techniques for analysis including effectiveness of curbs and the use of curbs in conjunction analytical methods, computer simulation, full-scale crash test- with traffic barriers, the literature review shows that there are ing, and accident data analysis (24, 33, 34). Vehicle tripping many limitations, such as the age of the tests, the lack of on curbs was addressed in Holloway et al. using HVOSM to sophistication in early computer models, and changing full- simulate nontracking impacts of large passenger sedans (24). scale crash testing guidelines. The following are the major Based on the results of their simulations, they concluded that findings of the literature review: sloping curbs may not be a significant cause of vehicle roll- overs; however, it should be noted that the models used in Curbs should not be used in combination with W-beam their study were not validated for nontracking impacts. It guardrail systems on high-speed roadways due to the was not reported whether or not friction between the tires potential safety hazard of vaulting or underriding the and ground surface was included in the simulations. Fric- barrier. In cases where design engineers include curbs tion between the tires and ground will affect the initial roll along high-speed roadways for drainage reasons or to angle and roll rate of the vehicle prior to impact, which may improve delineation, other methods should be sought to increase the vehicle's tendency to rollover. achieve those purposes.

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24 Neither the large and small cars crossing 150-mm-high Nontracking impacts with curbs may result in vehicle or smaller curbs in a tracking manner are likely to result instability and rollover, especially impacts involving in loss of vehicle control or cause serious injuries. The vehicles with high centers of gravity. From the literature response of the 2000-kg pickup truck crossing curbs, study it seems that the most likely methods for analyz- however, was not known. The large passenger car used ing nontracking impacts will be vehicle dynamics codes, in the previous crash testing procedures was replaced in such as VDANL. There has been a great deal of advance- the current testing procedures (NCHRP Report 350) ment in computation power and in code development with the 2000-kg pickup truck. The dynamic response over the past few years that has enabled computer sim- of this particular vehicle type crossing over curbs (not ulation programs to become a very efficient means of in conjunction with a roadside safety barrier) has never analysis. Both tracking and nontracking impact on curbs been evaluated with either full-scale tests or computer may be investigated using vehicle dynamics codes, such simulation. as VDANL, and finite element analysis (FEA) using Most of the curb impacts that were found in the literature LS-DYNA. involved vehicles encroaching the curb in a tracking A small number of tests have been performed in which manner. It was concluded in every case that a vehicle a curb was placed behind the face of guardrail barriers. encroaching onto a sloping curb in a tracking manner is The idea was to locate the curb such that minimal inter- not likely to cause the driver to lose control of the vehi- action between the vehicle and curb occurred. This cle or cause the vehicle to become unstable unless a sec- worked well with lighter vehicles, such as the 820-kg ondary impact occurs. Another aspect of collisions with small car, but did not prevent vehicle-curb interaction curbs involves an out-of-control vehicle impacting the with the heavier vehicles, such as the 2000-kg pickup curb in a nontracking position. In these situations, vehi- cle tripping may be highly probable during impact. truck, unless the guardrail was retrofit in some man- Errant vehicles leave the roadway in a variety of orien- ner to strengthen it and minimize guardrail deflection. tations; however, it is assumed that the majority of these To circumvent the problem, one option considered vehicles encroach onto the roadside in a semicontrolled was to use a low-profile curb underneath the guardrail. tracking manner. In such cases, the left or right front This was expected to minimize the effects that the bumper would be the first point of contact with a road- curb would have on vehicle trajectory when the wheels side object in an impact event. The position of the bumper of the vehicle were able to contact the curb during upon impact has, therefore, been a primary concern impact; however, full-scale tests conducted by various involving impacts with longitudinal traffic barriers, where organizations provided mixed results. In some cases it has been assumed that the position of the bumper dur- the crash test was successful, while in others it was not. ing impact is a reasonable indicator of vehicle vaulting In cases where the test was a failure, it was not clear or underriding the barrier. Due to pitching of the mov- whether the failure was induced by vehicle-curb inter- ing vehicle, the bumper height at impact may be higher action or if it was simply caused by inadequate barrier or lower than the static position of the bumper. performance.