Approximately 15 percent of all fixed-object fatalities involve sign and luminaire supports or utility poles. The options available to the highway engineer to improve on this record
are similar to those presented earlier: remove or redesign, relocate, use a breakaway device, shield, or delineate. Although it is desirable to have an unobstructed roadside, it is not always possible to relocate appurtenances such as signing and lighting supports, because they must remain near the roadway to fulfill their intended purpose. Thus, emphasis is given to the use of breakaway hardware—selection of the most appropriate device and installing it to ensure acceptable performance. (See Chap. 7.) Supports should be designed in accordance with AASHTO’s Standard Specification for Structural Supports for Highway Signs, Luminaires, and Traffic Signals.
Breakaway supports include all types of sign, luminaire, and traffic signal supports designed to yield when hit by a vehicle. Typical release mechanisms include slip planes, plastic hinges, and fracture elements. Criteria for breakaway supports are given in National Cooperative Highway Research Program (NCHRP) Report 350, Recommended Procedures for the Safety Performance Evaluation of Highway Features. The criteria require that a breakaway support fail in a predictable manner when struck head on by a 1800-lb (820-kg) vehicle, or its equivalent, at speeds of 20 and 60 mi/h (35 and 100 km/h). It is desirable to limit the occupant impact velocity to 10 ft/s (3.0 m/s), but values as high as 16 ft/s (5.0 m/s) are acceptable. Also, the maximum stub height is set at 4 in (100 mm) to avoid snagging the undercarriage after impact. The crash vehicle must remain upright with no significant deformation or intrusion of the passenger compartment.
Full-scale crash tests, tests with bogie vehicles (reusable, adjustable surrogate vehicle), and tests with pendulums (having special nose sections to model vehicles) are used for acceptance. Pendulum tests are the least expensive, but are used mostly for luminaire support hardware and are mainly limited to 20 mi/h (35 km/h). NCHRP Report 350 discusses acceptance testing. Tests are run in a standard soil, but weak soil should be used in addition for any feature whose impact performance is sensitive to soil-structure interaction.
Many general practices are similar to those previously discussed. Supports should not be placed in drainage ditches, because vehicles may be channeled into the obstacle and freezing might interfere with proper functioning of the breakaway device. Also, breakaway supports must not be located near ditches or on steep slopes where a vehicle is likely to be partially airborne at impact, because breakaway devices may bind and not function properly when hit in this manner. They have been developed to be struck about 20 in (500 mm) above the ground.
Locate supports where they are least likely to be hit, such as behind roadway barriers (beyond design deflections of the barriers) or on existing structures. In general, only when the use of breakaway supports is not feasible should a traffic barrier or crash cushion be used for shielding. Generally, breakaway supports should be used unless an engineering study indicates otherwise. Concern for pedestrians being struck by falling supports after a crash has led to the use of fixed supports in some urban areas such as near bus shelters or where there are extensive pedestrian concentrations.
