Length of Need

Tangent length of need per RDG: angle-line method with deflection and runout checks.

Reference only. Use current Roadside Design Guide and agency practice for final design.

Design/posted speed (mph)

Context

Hazard offset from edge of travel lane (ft)

Barrier offset from edge of travel lane (ft)

Hazard length along roadway (ft)

Barrier deflection to plan for (ft)

Departure angle (deg, optional)

Effective offset = barrier offset – deflection. Length of need = hazard length + (lateral gap / tan(θ)). Runout length check based on RDG.

Defaults: Departure angle 15° for speeds ≥50 mph, 20° for speeds <50 mph. Rural runout lengths are longer than urban/low-speed.

If computed LON exceeds the runout length for the speed/context, extend upstream or reassess placement/offset.

Runout length table (ft)

Approximate RDG values used for the check.