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April 10, 2026 10 min read Highway Design
Guardrail Placement and Safety Barrier Design: Offset, Spacing, and Terminal Component Guidelines
An overview of AASHTO and ERA safety guidelines for the layout, offset, and spacing of road safety barriers.
Road safety barriers (guardrails) protect vehicles from roadside hazards (e.g. steep slopes, utility poles, bridge piers).
Proper barrier placement involves calculating the required offset and runout lengths to ensure maximum effectiveness.
1. Placement Guidelines
- Lateral Offset: Barriers should be placed as far from the travel lane as possible to provide a recovery area.
- Post Spacing: Standard spacing is 1.905m (6.25 ft). Spacing can be reduced (e.g. 0.95m) to increase stiffness.
- Terminal Ends: Ends must be flared or fitted with impact-absorbing terminals to prevent vehicle impalement.
2. Runout Length Calculations
The required guardrail length is calculated based on hazard offset and lateral distance parameters. These parameters ensure the barrier is long enough to intercept out-of-control vehicles.