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12 placed on flat grade. Impact conditions included vehicle on February 28, 1992. The tests involved various types and speeds of 72.4 and 88.5 km/h and encroachment angles of sizes of vehicles impacting W-beam guardrails with curbs 5 and 20 degrees. The results of these simulations showed placed behind the face of the W-beam rail element. In the only minor differences in angular displacements of the vehi- cases involving curbs 150 mm high or higher, it was found cle, compared with the simulations with the curb placed on a that the vehicle would vault over the guardrail, if the guard- negative grade (i.e., the test area was on a negative grade). rail deflected enough for the wheels to mount the curb. In Nontracking impacts of vehicles with the three curb types crash tests in which the 100-mm AASHTO Type G curb was were also investigated using computer simulation; however, placed behind the face of the W-beam, the vehicle became no test data were available for validating the results. Impact airborne when guardrail deflection permitted the wheels to conditions used in the study were based on Appendix G of mount the curb; however, the vehicle did not vault the rail. NCHRP Report 350 and from accident data analysis stud- The best alternative for reducing the safety hazards associated ies. All simulations were performed with vehicle speed of with guardrail-curb systems is to stiffen the guardrail. Stiff- 80.5 km/h and impact angle of 20 degrees. Three initial posi- ening the guardrail reduces guardrail deflection and reduces tions of the vehicle were investigated: (1) 150 degree yaw the potential of the vehicle contacting the curb. In tests where angle with 50 deg/sec yaw rate, (2) negative 30 degree yaw the guardrail was sufficiently stiff, the tires of the vehicle did angle with a negative 25 deg/sec yaw rate, and (3) 180 degree not contact the curb, and the vehicle was redirected in a much yaw angle with 50 deg/sec yaw rate. They found that these more stable manner. Below is a summary of the ENSCO tests. curbs may be traversable over a wide range of vehicle orien- tations and impact conditions, and the curbs pose little threat of vehicle rollovers during impact. Test Number 1862-1-88 A 2452-kg pickup truck impacted a G4(1S) guardrail sys- EFFECT OF CURBS INSTALLED IN CONJUNCTION WITH GUARDRAILS tem with a 203-mm-high concrete curb (AASHTO type A) installed behind the face of the W-beam. The impact speed Buth et al., 1984 (25 ) was 100 km/h and the impact angle was 20 degrees. There was significant deflection of the guardrail, and the wheels of During the 1980s, the FHWA sponsored the testing of the vehicle contacted the curb. The vehicle vaulted over the numerous bridge railings, some of which included curbs. In guardrail. particular, TTI tested a New Hampshire bridge rail system with a curb protruding in front of the barrier face, and a Col- orado Type 5 bridge rail system with a curb flush with the face Test Number 1862-4-89 of the barrier. In both tests, the front impact-side wheel was damaged during impact with the curb, and the wheel wedged An 817-kg car impacted a G4(1S) guardrail system with a between the curb and the bottom rail of the traffic barrier. 150-mm-high asphalt dike. The impact speed was 100 km/h The performance of both bridge railings was considered unsat- and the impact angle was 20 degrees. The wheels of the vehi- isfactory, but it should also be noted that both railings had cle did not contact the curb during the crash event, and the other poorly designed features that may have contributed to vehicle was smoothly redirected. the poor performance. Bryden and Phillips, 1985 (26 ) Test Number 1862-5-89 Bryden and Phillips performed 12 full-scale crash tests for A 2043-kg sedan impacted a G4(1S) guardrail system with the New York Department of Transportation to evaluate the a 150-mm-high asphalt dike. The impact speed was 100 km/h performance of a thrie-beam bridge-rail system. Two tests and the impact angle was 25 degrees. There was significant were conducted with a 150-mm curb placed flush with the face deflection of the guardrail, and the wheels of the vehicle con- of the thrie-beam rail. The tests involved a 2043-kg Dodge sta- tacted the curb. The vehicle vaulted over the guardrail. tion wagon impacting the system at approximately 100 km/h at an impact angle of 26 degrees. The vehicle remained stable and was smoothly redirected in both tests. Test Number 1862-12-90 FHWA Memorandum, February 28, 1992 (27 ) A 2452-kg sedan impacted a G4(1S) guardrail system with a 100-mm-high concrete curb (AASHTO type G). The impact The results of a series of crash tests conducted by ENSCO, speed was 100 km/h and the impact angle was 25 degrees. Inc., were reported in an FHWA Memorandum distributed The vehicle became airborne but did not vault the guardrail.

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13 Test Number 1862-13-91 Holloway and Rossen, 1994 (28 ) A 2043-kg sedan impacted a G4(1S) guardrail system A study was conducted by Holloway and Rossen at Mid- stiffened with a W-beam bolted to the back of the steel posts. west Roadside Safety Facility at the University of Nebraska A 150-mm-high asphalt dike was placed behind the front Lincoln that involved a full-scale crash test on Missouri's face of the W-beam. The impact speed was 100 km/h and 150-mm-high vertical curb placed behind the face of a strong- the impact angle was 25 degrees. The guardrail system was post W-beam guardrail (i.e., G4(1S)). Missouri's 150-mm- sufficiently stiff to prevent the wheels of the vehicle from high vertical curb is very similar to the AASHTO type B curb, impacting the curb. The vehicle was successfully redirected. except that the Missouri vertical curb is on a flat grade and has very little rounding on the top and bottom edges of the curb. The impact conditions for the test was in accordance with NCHRP Report 230 specifications; a 2043-kg test vehi- Test Number 1862-14-91 cle (1985 Ford LTD) impacted the system at 96 km/h at 25.1 degrees. The center of gravity of the test vehicle was A 2043-kg sedan impacted a G4(1S) guardrail system stiff- 597 mm above ground. A summary of test M06C-1 is shown ened with a C6x8.2 hot-rolled channel rub rail. A 150-mm- in Figure 7. high asphalt dike was placed behind the face of the W-beam. During the test, the right front tire contacted the curb 20 mil- Again the guardrail system was sufficiently stiff to prevent the liseconds after initial contact with the guardrail and mounted wheels of the vehicle from impacting the curb and the vehicle the curb soon after. The maximum roll angle was negative was successfully redirected. The vehicle speed change at redi- 14 degrees (the roll angle was away from the system). The rection, however, was greater than the allowable (24 km/h) vehicle exited the rail at 706 milliseconds at a speed of 64 km/h according to NCHRP Report 230; thus the system did not meet and an angle of 6.2-degrees. Vehicle decelerations and tra- all required safety criteria. jectory were well within the recommended limits of NCHRP Figure 7. Summary of results for MwRSF Test M06C-1 (28).

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14 Report 230. As a result of the test, the researchers concluded that the system performed satisfactorily and the Missouri Department of Transportation should continue to use the guardrail-curb system where warranted. Polivka et al., 1999 (29 ) A study was conducted by researchers at the Midwest Roadside Safety Facility (MwRSF) at the University of Nebraska-Lincoln to evaluate the effects of an AASHTO type G curb (i.e., 102 mm high and 203 mm wide) placed flush behind the face of a G4(1S) guardrail system. Test NEC-1 was conducted with impact conditions recommended by NCHRP Report 350 TL-3, which involves a 2000-kg pickup truck (1991 GMC 2500) impacting at a speed of 100 km/h at an impact angle of 25 degrees (19). Sequential photographs of the crash test are shown in Figure 8. The center of gravity of the test vehicle was 737 mm. The test installation was a standard 53.34-m-long G4(1S) guardrail system anchored on both the upstream and down stream ends of the system by an inline breakaway cable ter- minal with a strut between the two end posts. The guardrail ruptured at a splice connection, thus the test was a failure. There was little vertical displacement of the vehi- cle as it crossed the curb in the full-scale test, and there seemed to be very little potential for underride or vaulting of the bar- rier. The anchor posts split during the collision, as shown in Figure 9, and there was a loss of tension in the W-beam, which resulted in pocketing and rupture of the W-beam rail at a splice connection. The splice failure was attributed to contact and snagging of the post blockout against the W-beam rail splice. The post twisted as it was pushed back in the soil, causing the bottom corner of the blockout to push up against the cor- ner of the W-beam rail splice. This resulted in a tear in the W-beam at the lower downstream bolt location. It was sug- gested that the guardrail-curb combination could be signifi- cantly improved by increasing the capacity of the W-beam rail. Figure 9. Guardrail terminal damage during Test NEC-1 (29). Bullard and Menges, 2000 (30 ) This study was conducted by researchers at the TTI and involved the evaluation of a 100-mm-high asphaltic curb, set out 25 mm from the face of the rail of a G4(2W) strong-post guardrail system, as shown in Figure 10. TTI test 404201-1 was conducted at the TTI on May 23, Figure 8. Sequential video frames from Test NEC-1 (29). 2000, and involved a Chevrolet C2500 pickup impacting