Breakaway supports are designed and evaluated to operate safely on the basis of the characteristics of the vehicle fleet. One of the primary characteristics included in discussions of the impacting vehicle is its weight. While weight is very important, the bumper height is equally important, since it establishes where the vehicle weight is first concentrated on the breakaway support. The majority of the safety evaluation tests are conducted on level terrain. This implies that the impacting design vehicles are striking the breakaway supports at a known height—typically, about 20 in (500 mm) above the ground. Roadside safety could, therefore, be enhanced if wide, level areas are provided along the roadside.
Providing this level roadside is not practical or possible in the majority of roadside situations. Side slopes, ditches, cross-slopes, curbs, and other drainage and terrain
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FIGURE 7.3 Breakaway support stub height measurement. Conversions: 100 mm = 4 in, 1.5 m = 5 ft. |
features are necessary roadside design features. How these features can interact with and influence vehicle trajectory and device performance must be considered prior to device installation.
Breakaway support devices are designed to function properly when the slip base is subjected to shear forces. If the point of impact is at a significantly higher point than the design height of 20 in (500 mm), then sufficient shearing forces may not be transmitted to the base. The result can be binding of the mechanism and nonactivation of the breakaway device. It is critical, therefore, that breakaway supports not be located near abrupt changes in elevation, superelevation transitions, changes in slope, or curbs that will cause vehicles to become partially airborne at the time of impact. As a general rule, if negative side slopes are limited to 6:1 or flatter between the roadway and the breakaway support, then vehicles will usually strike the support at an acceptable height.
Supports should not be placed in locations where the terrain features can possibly impede their proper operation. Placing supports in drainage ditches can result in erosion and freezing, which can affect the operation of the breakaway support. In addition, vehicles entering the ditch can be inadvertently guided into the support.
Supports should not be installed closer than 7 ft (2100 mm) to other fixed objects. If the supports are placed closer than 7 ft (2100 mm) to other objects that by themselves are considered acceptable, such as a 3-in-diameter (76-mm) tree, then a vehicle will be able to strike both the support and the object simultaneously. The combined effect of both the tree and the support on the change of velocity can be much higher when impacting both objects simultaneously.
Terrain in the vicinity of the support base must be graded to allow vehicles to pass over portions of the support that remain in the ground or that are rigidly attached to a foundation. Remaining portions of the support that protrude more than 4 in (100 mm) above the ground line over a horizontal span of 5 ft (1.5 m), as presented in Fig. 7.3, can snag the vehicle undercarriage.
