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20 curb-tripped and the soil-tripped vehicles. The higher roll Allen et al., 1997 (36 ) rate of the dolly-rolled vehicle was attributed to the 48-degree initial roll angle of the dolly when it contacted the ground. Researchers at STI and JPC Engineering further improved This caused a greater moment arm from the point of impact the Slip Tire Model (STIREMOD) for use in the vehicle to the center of gravity of the vehicle. dynamics computer simulation program, VDANL. STIRE- The analytical model developed in the study was based on MOD was expanded to include the full-range of operating con- the assumption that a constant tripping force acts on the vehi- ditions for both on- and off-road surfaces, including unlevel cle during the rollover initiation phase. Although the model terrain, changing surface conditions, and tires plowing through did not account for the effects of tire and suspension system soil. They discussed in some detail the input parameters for compliance, the results compared well with the test data. the model and the means for establishing typical model It was found that the kinematics of the tripped vehicle parameters. The model would be useful for the analysis of varied significantly, depending on the tripping mechanism vehicle encroachments onto the road shoulders and (i.e., curb, soil and dolly). Curb impacts produced very high sideslopes. The model could also be used for analyzing vehi- decelerations, usually in excess of 10 Gs. Some curb-tripped cle tire interaction with curbs, where the curb would be vehicles, however, did not rollover because critical structural modeled as an abrupt change in surface shape and surface components (e.g., the wheel assembly) failed during impact, properties (e.g., asphalt pavement to a concrete curb). providing an alternate path for the unbalanced forces. When components of a vehicle collapse or break during these types Allen et al., 2000 (37 ) of impact, the duration force may not be sufficient to initiate a rollover. Allen and other researchers at STI wrote a paper summa- rizing the development and application of the vehicle dynam- ics computer simulation model, VDANL. The subsystem models of VDANL are described (e.g., tires/wheels, brakes, Allen et al., 1991 (35 ) steering, power train, roadway inputs, driver model, steering Researchers at STI conducted a study to determine the control, and speed control). Discontinuities in the roadway, directional and rollover stability of a wide range of vehi- such as potholes, speed bumps, and curbs, can be modeled in cles using the computer simulation program VDANL. They VDANL with additional inputs to the surface profile. showed that rollover stability and directional stability are VDANL models the inertial component of the vehicle as a related to center of gravity location and track width, as well six-degree-of-freedom sprung mass connected by springs and as the other characteristics that influence these variables dampers to the axles, which are supported by pneumatic tires. under hard maneuvering conditions. Vehicle dynamics and According to Allen et al., "Communications services have tire-ground interaction under such conditions are nonlinear also been added to VDANL so that it can provide commands and can be quite complex; therefore, computer simulation is for display image generators, feel and motion systems, sound essential in analyzing stability problems. cuing, and miscellaneous controls and displays"(37). The Forty-one vehicles were used in the study for parameter program runs in real time on Pentium-class computers run- and field testing. Spinout occurs when rear tire adhesion lim- ning Windows 95/98/NT network. its are exceeded while the front tires still have side force A specialized version of the software was developed for capacity available. Computer simulation results were vali- the FHWA as part of the IHSDM, which allows new road- dated with the field test results, and it was found that in many way designs to be assessed using a driver model. Two case cases the dynamic behavior of the vehicle was largely depen- studies were presented in their study using VDANLIHSDM dent upon the tire model and tire-ground interaction. Thus, to determine (1) if a truck-climbing lane was necessary for a detailed information about the tire properties and friction proposed roadway alignment and (2) if a loaded tractor- coefficients are necessary for valid model development. trailer would be able to maintain a specified speed traveling One conclusion from their study was that load transfer dis- downgrade on the roadway without losing control. tribution among the tires should be near to, or greater than, the vehicle weight distribution, although there are several SYNTHESIS OF LITERATURE REVIEW other factors that influence limit performance maneuvering. As the center of gravity of a vehicle is raised or the track Both sloping and vertical curbs are regularly used in urban width is narrowed, wheel lift off becomes more likely and areas along low-speed roadways for drainage purposes, walk- balancing load transfer distribution becomes a critical issue. way edge support, pavement edge delineation, to discourage The computer simulation program, VDANL, was validated vehicles from leaving the roadway, and to provide limited for both stable and unstable vehicle maneuvering conditions redirection of encroaching vehicles. Vertical curbs have a and was considered to be a practical and effective means of vertical or nearly vertical face and are recommended for use analyzing vehicle stability problems. only on low-speed roads. Sloping curbs have a sloping face

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21 and are configured such that a vehicle can ride up and over dynamics of the vehicle may cause the vehicle to impact a the curb, in order to reduce the likelihood of causing tire secondary object in such a manner that will cause the object blowout or suspension damage. Sloping curbs are used pri- to not function properly. marily for drainage purposes, but are also used on median A curb located in front of a guardrail may cause an impact- islands and along shoulders of high-speed roadways for delin- ing vehicle to strike the guardrail at a point higher or lower eation and other reasons. than normal. Under certain impact conditions, the curb can Curbs along low-speed roadways are not likely to result in cause the vehicle to ramp high enough to vault over the bar- serious injuries and are commonly used in urban areas where rier, or, in some cases, underride and snag on the barrier (22, speed limits are in the range of 40 to 48 km/h. Curbs along 24, 25, 29, 32). Another example of possible adverse effects high-speed roadways have been discouraged by AASHTO of a curb on the performance of a device is the placement of for many years because of the potential hazard caused by a curb in front of a breakaway pole. The breakaway features high-speed impact with curbs (1). In the intermediate range at the bases of the poles are designed to work when the pole of speed (between 60 and 80 km/h), however, there are no is struck near the base. If a vehicle is airborne when it hits a standards for the use of curbs. Highway engineers must, breakaway pole, the impact point may be well above the base; therefore, determine if a curb is warranted based on individ- thus the breakaway feature may not work as it is intended. ual roadway conditions and location. In urban areas, curbs In some studies, the lateral displacement of the vehicle at are often considered acceptable; whereas in rural areas curbs maximum rise height has been considered an important fac- are discouraged at intermediate speeds (1). tor for determining the potential for vehicle underriding or There have been a limited number of studies performed to vaulting a barrier (22, 24, 38). Design parameters defined by determine the effects of impact with curbs on the dynamic AASHTO for curb impacts are shown in Figure 16. It was stability of vehicles and on the performance of barriers placed reported that underride and vaulting of a standard strong-post in combination with curbs. The studies have involved full- guardrail were possible when the barrier was placed within scale crash testing (22, 2430, 32) and computer simulation some critical range behind the curb, usually within 0.76 m for using the HVOSM (2123). A summary of full-scale crash underride and between 0.01 and 3.66 m for vaulting. These tests involving curbguardrail combinations is presented in data were obtained through measuring vehicle trajectory dur- Table 4. Although it has been found that sloping curbs do ing impact with curbs. not significantly redirect a vehicle during tracking impact, It was assumed for many years by design engineers that if they do affect the vertical trajectory of the vehicle. Thus, while the curb is placed behind the face of the W-beam that the the curb itself presents very little threat of harm when hit curb-guardrail system would perform adequately in safely con- by a vehicle, when a vehicle impacts and mounts a curb, the taining and redirecting an impacting vehicle. Previous crash TABLE 4 Summary of full-scale crash tests of curbguardrail combinations with curb located behind face of guardrail Literature Testing Test no. Vehicle type Speed and Curb type Guardrail Result Comment reference agency angle type Bryden and Dodge Station 100 km/hr 152-mm Thrie-Beam Passed smoothly redirected Phillips (26) NYDOT Wagon 26 degrees vertical curb Bridge Rail (2041 kg) 1862-1-88 3/4-ton Pickup 100 km/hr 203-mm G4(1S) Failed vehicle vaulted over rail Truck (2449 kg) 20 degrees AASHTO A 1862-4-89 Small Car 100 km/hr 152-mm G4(1S) Passed smoothly redirected (820 kg) 20 degrees Asphalt Dike FHWA 1862-5-89 Large Car Sedan 100 km/hr 152-mm G4(1S) Failed vehicle vaulted over rail Memorandum (2041 kg) 25 degrees Asphalt Dike ENSCO Feb 1992 (27) 1862-12- Large Car Sedan 100 km/hr 100-mm G4(1S) Passed vehicle was airborne but did not 90 (2449 kg) 25 degrees AASHTO G vault 1862-13- Large Car Sedan 100 km/hr 152-mm G4(1S) stiffened Passed smoothly redirected 91 (2041 kg) 25 degrees Asphalt Dike with W-beam 1862-14- Large Car Sedan 100 km/hr 152-mm G4(1S) stiffened Failed vehicle speed change at 91 (2041 kg) 25 degrees Asphalt Dike with rub rail redirection was too high Holloway MwRSF M06C-1 1985 Ford LTD 96.1 km/hr 152-mm G4(1S) Passed smoothly redirected & Rossen (28) (2041 kg) 25.1 degrees vertical curb Polivka et al. MwRSF NEC-1 1991 GMC 103.2 km/hr 102-mm G4(1S)-mod with Failed excessive anchor movement / (29) 3/4-ton Pickup 24.5 degrees AASHTO G wood blockout guardrail ruptured (2,000 kg) Bullard and TTI 404201-1 1995 Chevrolet 101.8 km/hr 100-mm G4(2W) Passed significant guardrail damage and Menges (30) 3/4-ton Pickup 25.2 degrees CDOT curb anchor movement (2000 kg) Polivka et al. MwRSF NEC-2 1994 GMC 100.3 km/hr 102-mm G4(1S)-mod with Passed vehicle experienced extreme (32) 3/4-ton Pickup 28.6 degrees AASHTO G wood blockout trajectory but did not vault over (2,000 kg) nested W-beam rail

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22 Figure 16. Design parameters for curb impacts as defined by AASHTO (38). tests, involving large sedans and pickup trucks impacting var- Some curb types are more likely to cause vaulting of a ious curb-guardrail combinations, have provided researchers vehicle than others. The FHWA memorandum in February with mixed results regarding the performance of such sys- 1992 (27) reported that, in the case of curbs 150 mm high or tems (24, 25, 2730, 32). higher, if a guardrail deflects enough for the wheels to mount In full-scale crash tests performed by ENSCO with full- the curb, the vehicle could vault over the guardrail. It was size cars, it was shown that vaulting is possible even when also reported in the FHWA memorandum that crash tests the curb is located flush with the face of a W-beam guardrail. involving the AASHTO Type G curb (a 100-mm curb height If guardrail deflections during impact are sufficient to allow with slanted face) placed behind the face of the W-beam the wheel of the vehicle to contact and mount the curb, the resulted in the vehicle becoming airborne when guardrail vehicle may vault over the barrier (28). Even though the deflection permitted the wheels to mount the curb; however, vehicle contacts the barrier prior to reaching the critical tra- the vehicle did not vault the guardrail. A similar conclusion jectory height that would signify override, the vehicle will was found in other studies, which showed that vehicle impact continue to rise while it is in contact with the barrier and may with low curbs would result in very little change in the ver- result in vaulting during redirection. Crash tests with pickup tical trajectory of the vehicle (50-mm maximum), regardless trucks performed at Midwest Roadside Safety Facility, on of the vehicle's speed and angle of impact (22, 24). the other hand, have demonstrated that similar curb/W-beam A W-beam guardrail is sufficiently stiff that the lateral guardrail combinations do not degrade the performance of deflections of the barrier are minimal during impact with a the barrier systems (23, 28). small car; thus for curbguardrail combinations in which the