The need for traffic signs, roadway illumination, utility service, and postal delivery results in roadside features frequently placed within the roadway right-of-way. (Also see Chap. 6, Safety Systems.) The presence and location of these obstacles varies by roadway type and location. Rural freeways, for example, can be designed where traffic signs are the only obstacles that are added to the roadside. Signs, light pole standards, utility poles, and mailboxes are all frequently encountered on rural collectors. These obstacles, when present, perform a necessary function, but are also potential fixed objects for an errant vehicle. To reduce accident severity it is important that signs, roadway illumination supports, utility poles, and mailboxes be properly designed, located, and placed within the right-of-way. As a general rule, there are a number of options that can be used by design engineers to provide a safe design. In order of preference these options are
• Do not install the obstacle.
• Install it on existing overhead structures, where it does not become an additional fixed object hazard.
• Locate the feature away from the traveled way or behind existing barriers where it will be less likely to be struck.
• Reduce impact severity by using appropriate breakaway or yielding design.
• Shield the feature with a properly designed longitudinal barrier or crash cushion if it cannot be eliminated, relocated, or redesigned.
• Delineate an existing feature if other measures are not practical. Putting up hazard markers is a cost-effective method for alerting motorists to an existing hazard. Obviously, delineators will not make any difference if a driver hits the object, but they might help a driver avoid running off the road at that spot.
Yielding or breakaway supports should be used on all types of sign, luminaire, and mailbox supports that are located within the clear zone. The clear zone is the total roadside area, starting at the edge of the traveled way, that is available for safe use by a vehicle. The desirable width of the clear zone is dependent upon traffic volume, speed, and the roadside geometry. The traversable area is the roadside border area that permits a motorist to maintain vehicle control including being able to slow and stop safely. The traversable area can exceed the desirable clear zone called for in the Roadway Design Guide [10]. Only yielding or breakaway supports should be permitted in the traversable roadside, even if it is located beyond the clear zone. In those instances where yielding or breakaway supports are not possible, such as large cantilever sign installations, shielding with crash cushions or guardrail should be used.
Yielding supports refer to those supports that are designed to remain in one piece and bend at the base upon vehicle impact. The anchor portion remains in the ground and the upper assembly passes under the vehicle. The term breakaway support refers to support systems that are designed to break into two parts upon vehicle impact. The release mechanism for a breakaway support can be a slip plane, plastic hinges, fracture elements, or a combination of these.
The technology of yielding and breakaway support systems has experienced dramatic improvements. These improvements were prompted by an increased emphasis on roadside safety and by the large reduction that has occurred in the weights of automobiles. Many foreign and domestic automobiles on our roadways weigh less than 2250 lb (1020 kg), which was at the bottom of the domestic weight range in 1975. By 1983 the trend to more fuel-efficient automobiles had resulted in approximately 40 percent of auto sales being vehicles weighing less than 2250 lb (1020 kg). Automobiles of 1600 lb (725 kg) and less are now operating on U. S. highways. The typical family automobile weighs somewhere between 2000 and 4000 lb (900 and 1800 kg), with only the luxury and a few other types weighing more. A survey of high-level automotive industry leaders, conducted by the University of Michigan, indicates that the total vehicle weight will remain fairly constant [11].
The evolving safety feature environment and the change to the vehicle fleet weights have resulted in a number of revised standard specifications for the testing and acceptance of yielding and breakaway support systems. Requirements for yielding and breakaway support systems were introduced by AASHTO in 1975 and revised in 1985 to keep abreast of new research and development. Two of the most significant changes in the 1975 and 1985 specifications are the reduction in weight of the design vehicle from 2250 lb (1020 kg) to 1800 lb (820 kg) and the change from measures of momentum to measures of change in velocity. These changes, however, do not imply that safety features that satisfied the old specifications do not satisfy revised specifications. For example, the 1985 standard testing guidelines require that supports should impart a preferred vehicle change in velocity of 10 ft/s (3.1 m/s) or less, but not more than 15.4 ft/s (5 m/s). A support that would cause a 2250-lb (1020-kg) vehicle (i. e., 1975 design vehicle weight) to experience an 11-ft/s (3.4-m/s) change in vehicle velocity at a test speed of 20 mi/h (32 km/h) would likely result in 15.4-ft/s (5-m/s) change in velocity when tested under the same conditions with an 1800-lb (820-kg) vehicle (i. e., 1985 design vehicle weight) [12]. These values compare favorably with the change in momentum requirements cited in the 1975 specifications. Supports that had acceptance test numbers near the preferred values for the old specification can, therefore, be expected to meet the new specification requirement.
Some of the changes in the 1985 AASHTO standard specifications were due to testing guidelines contained in NCHRP Report 230 [13]. NCHRP Report 350 establishes current testing guidelines for vehicular tests to evaluate the impact performance of permanent and temporary highway features, and supersedes those contained in NCHRP Report 230 [13, 14]. These guidelines include a range of test vehicles, impact speeds, impact angles, points of impact on the vehicle, and surrounding terrain features for use in evaluating impact performance. Acceptance testing of yielding and breakaway supports requires evaluation in terms of the degree of hazard to which occupants of the impacting vehicle are exposed, the structural adequacy of the support, the hazard to workers and pedestrians who may be in the path of debris from the impact, and the behavior of the vehicle after impact. The guidelines include requirements for
• The structural adequacy of the device to determine if detached elements, fragments, or other debris from the assembly penetrate, or show potential for penetrating, the passenger compartment or present undue hazard to other traffic.
• A range of preferable and maximum vehicle changes in velocity resulting from impact with the support system. The preferable change in vehicle velocity is 10 ft/s (3.0 m/s) or less. The maximum acceptable change in vehicle velocity is 16 ft/s (5.0 m/s). Note that due to conversion to the SI system the limiting velocity changes were rounded and consequently are not precisely the same as those in NCHRP Report 230 [13].
• The impacting vehicle to remain upright during and after the collision.
• The vehicle trajectory and final stopping position after impact to intrude a minimum distance, if at all, into adjacent or opposing lanes.
It is important to use only those support assemblies that have been tested, using the standard specifications, and subsequently approved for use by the FHWA. This is true for city and county jurisdictions where roadway speeds are generally less than what can be expected on state and rural roadways. Impacts with supports can be hazardous even at lower speeds, especially for occupants of a small vehicle. It should be noted that many supports can be more hazardous at low speeds, say 15 to 20 mi/h (25 to 40 km/h), than at high speeds, say 55 to 60 mi/h (90 to 100 km/h). For example, sign supports that fracture or break away can be more hazardous at low speeds, where the energy imparted to the support is not sufficiently large to make the device swing up and over the vehicle. The result can be intrusion of the lower portion of the support into the passenger compartment. Similarly, devices designed to yield are generally more hazardous at high speed, due to the reduced time available for deformation and subsequent passage beneath the vehicle.
The acceptance testing guidelines are intended to enhance experimental precision while maintaining cost within acceptable bounds. The wide range of vehicle speed, impact angle, vehicle type, vehicle condition, and dynamic behavior with which vehicles can impact the support cannot be economically replicated in a limited number of standardized tests. The use of an approved device does not, therefore, guarantee that it will function as planned under all impact conditions. However, the failure or adverse performance of a highway safety feature can often be attributed to improper design or construction details. The incorrect orientation of a unidirectional breakaway support, or something as simple as a substandard washer, are major contributors to improper function. It is important for proper device function that the safety feature has been properly selected, assembled, and erected and that the critical materials have the specified design properties.
When possible, and appropriate, the placement of traffic signs, luminaires, and utility and mailbox supports should take advantage of existing guiderail, overhead structures, and other features that will reduce their exposure to traffic. Care should be taken to ensure that supports placed behind existing, otherwise required barriers are outside the maximum design deflection standards of the barrier. This will prevent damage to the support structure and help ensure that the barrier functions properly if impacted. The design deflections are based on crash tests using a 4400-lb (2000-kg) vehicle impacting the barrier at 60 mi/h (100 km/h) and an angle of 25°. The crash tests are conducted under optimum conditions. Other conditions such as wet, frozen, rocky, or sandy soil may result in deflections greater or less than the design values. Typical anticipated deflections are presented in Table 7.1. A summary of FHWA letters of acceptance for sign support types and hardware may be found in the AASHTO Roadside Design Guide [10].
