Longitudinal roadside barriers are used to shield motorists from natural or human — made obstacles located along either side of the traveled way, and sometimes to protect pedestrians and bicyclists. Median barriers and barrier end treatments are discussed separately in Arts. 6.9 and 6.12.
Barriers must contain and redirect vehicles. Because of the complicated dynamic behavior involved, the most effective way to ensure performance of new designs is through full-scale crash testing. Standard crash tests are presented in NCHRP Report 350, “Recommended Procedures for the Safety Performance Evaluation of Highway Features.” To match barrier performance to service needs, a series of six test levels are recommended to evaluate occupant risk, structural integrity, and postimpact vehicle behavior. Various vehicle masses, velocities, and impact angles are included. To view acceptance letters for longitudinal barriers under NCHRP 350, visit the FHWA web site, http://safety. fhwa. dot. gov/fourthlevel/hardware/longbarriers. htm.
NCHRP Report 350 establishes six test levels (TLs) for longitudinal barriers to evaluate risk, structural integrity of the barrier, and vehicle postimpact behavior. A range of vehicle weights (masses), speeds, and impact angles are addressed. The AASHTO Roadside Design Guide provides the following description:
TL-1, TL-2, and TL-3 require successful tests of an 820 kg (1800 lb) car impacting a barrier at an angle of 20 degrees and a 2000 kg (4400 lb) pickup truck impacting a barrier at an angle of 25 degrees, at speeds of 50 km/h, 70 km/h and 100 km/h (30 mph, 45 mph, and 60 mph), respectively. TL-4 adds an 8000 kg (17,600 lb) single-unit truck at an impact angle of 15 degrees and 80 km/h (50 mph) to the TL-3 matrix. TL-5 substitutes a 36,000 kg (80,000 lb) tractor-trailer (van) for the single-unit truck and TL-6 substitutes a 36,000 kg (80,000 lb) tractor-trailer (tanker). (p. 5-1)
Barriers typically go through an experimental phase in which a barrier that has passed crash test evaluation is subjected to an in-service evaluation, and an operational phase in which a barrier that has proven acceptable in the in-service evaluation is used while its performance is further monitored. Barriers are also considered operational if they are used for extended periods and demonstrate satisfactory performance in construction, maintenance, and accident experience.
The criteria by which the need for a safety treatment or improvement can be determined are termed warrants. Barrier warrants are based on the premise that traffic barriers should be installed only where they reduce the probability or frequency of potential accidents. Warrants may be based on a subjective analysis of roadside conditions or a benefit-cost study (life-cycle cost analysis). The latter can be used to rationally analyze factors such as design velocity and traffic volume in relation to barrier needs and associated costs and accident costs.. Three options may be evaluated:
• Remove or reduce the area of concern so that it does not require shielding.
• Install an appropriate barrier.
• Leave the area unshielded.
The last of these options would usually be cost-effective only where the accident probability is low.
The main uses of roadside barriers are to shield either embankments or obstacles, as discussed below. Barriers may also be used to protect pedestrians, school yards, or bicyclists. There are no firm criteria for these applications, and each must be evaluated on its own merits.
