Depending on their deflection characteristics upon impact, roadside barriers can be classified as flexible, semirigid, or rigid. Table 6.2 lists the most widely used barriers in each classification. Details of most of these operational barriers are presented along with other available information in Figs. 6.13 through 6.22. The dynamic deflection listed is that observed during the standard test defined by NCHRP Report 350 for the test levels listed in Table 6.2 as defined in Art. 6.4. Other characteristics of the barriers are discussed below.
TABLE 6.1 Barrier Warrants for Nontraversable Terrain and Roadside Obstacles*f
Shielding generally required A judgment decision based on nature of fixed object and likelihood of impact A judgment decision based on size, shape, and location of obstacle Shielding not generally required A judgment decision based on likelihood of impact
Refer to Figs. 6.5 and 6.6 Shielding generally required if likelihood of head-on impact is high A judgment decision based on fill height and slope (see Fig. 6.12)
A judgment decision based on relative smoothness of wall and anticipated maximum angle of impact
Shielding generally required for nonbreakaway supports
Isolated traffic signals within clear zone on
high-speed rural facilities may warrant shielding A judgment decision based on site-specific circumstances
Shielding may be warranted on a case-by-case basis
A judgment decision based on location and depth of water and likelihood of encroachment
*Shielding nontranversable terrain or a roadside obstacle is usually warranted only when it is within the clear zone and cannot practically or economically be removed, relocated, or made breakaway and it is determined that the barrier provides a safety improvement over the unshielded condition.
fMarginal situations, with respect to placement or omission of a barrier, will usually be decided by accident experience, either at the site or at a comparable site.
$Where feasible, all sign and luminaire supports should be a breakaway design regardless of their distance from the roadway if there is reasonable likelihood of their being hit by an errant motorist. The placement and locations for breakaway supports should also consider the safety of pedestrians from potential debris resulting from impacted systems.
§In practice, relatively few traffic signal supports, including flashing light signals and gates used at railroad crossings, are shielded. If shielding is deemed necessary, however, crash cushions are sometimes used in lieu of a longitudinal barrier installation.
Source: From Roadside Design Guide, AASHTO, Washington, D. C., 2002 and 2006, with permission.
