Boone County Public Works Department

Traffic Safety Manual - Boone County, Missouri



The County may use guardrails to shield motorists from natural or man-made obstacles located near the road.   The primary purpose of the guardrail is to prevent the vehicle, if it inadvertently leaves the road, to strike a fixed object or travel a terrain feature that is considered more dangerous than hitting the guardrail barrier.   A guardrail is warranted if it reduces the severity of the potential accident.   In other words, if the consequences of a vehicle striking a fixed object or running off the road are believed to be more serious than hitting a traffic barrier, than a guardrail is considered warranted.   Guardrails are used typically to provide protection from steep embankment heights, fixed objects, and pedestrians and bicyclists.   Table 8 outlines the guardrail warrants for non-traversable terrain and roadside obstacles.

Table 8. Warrant Considerations for Guardrail Placement

Terrain or Obstacle 1,2


Bridge Piers, Abutments, and Railing Ends

Shielding generally required.


A judgment decision based on nature of fixed object and likelihood of impact.

Culverts, Pipes, Headwalls

A judgment decision based on size, shape, and location of obstacle.

Cut Slopes (smooth)

Shielding not generally required.

Cut Slopes (rough)

A judgment decision based on likelihood of impact.

Ditches (transverse)

Shielding generally required if likelihood of head-on impact is high.


A judgment decision based on fill height and slope.

Retaining Walls

A judgment decision based on relative smoothness of wall and anticipated maximum angle of impact.

Signs/Luminaire Supports 3

Shielding generally required for non-breakaway supports.


A judgment decision based on site-specific circumstances.

Utility Poles

Shielding may be warranted on a case-by case basis.

Permanent Water Bodies

A judgment decision based on location and depth of water and likelihood of encroachment.

•  Shielding non-traversable 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.

•  Marginal situations, with respect to placement or omission of a barrier, will usually be decided by accident experience, with 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 road if there is reasonable likelihood of their being hit by an errant motorist.

Source:Roadside Design Guide, 1996 (p.5-5)

Embankment height and side slope are basic factors considered in determining if a guardrail is warranted.   Table 9 lists the embankment height/side slope situations that may warrant a guardrail.   Encroachment occurring due to the placement of the guardrail and the costs associated with the guardrail will also be considered when using this table.

Table 9. Embankment/Side Slope Warrants for Guardrail

Side Slope (H:V)

Embankment Fill Height


3 ft or steeper


6 ft or steeper


9 ft or steeper


16 ft or steeper

Note: 1. Encroachment occurring due to the placement of the guardrail and the costs associated with the guardrail will also be considered when using this table.
Source: Roadside Design Guide, 1996 (Figure 5.1, p. 5-3)


If it is not immediately obvious whether the barrier or the unshielded condition presents the greater risk, an engineering evaluation may be used.   The warrant will be established by using a benefit to cost analyses.   The evolution will take into account design speed, traffic volume, and costs associated with the barrier or improvements.   The evaluation will typically look at three options.

•  Remove or reduce the area of concern se that it no longer requires shielding,

•  Install an appropriate barrier, and

•  Leave the area of concern unshielded.

The use of guardrails to shield or protect drivers from roadside obstructions is generally not cost-effective for roads with an average daily traffic (ADT) less than 400 vehicles per day.   A guardrail itself is a roadside obstacle and a significant proportion of vehicles impacts with guardrails produce injuries.   The low frequency of collisions with guardrails and the cost associated with maintaining guardrails makes it impractical for using guardrails on low volume roads.   Engineering judgment should be used in determining locations where guardrail should be used due to the potential of a severe accident due to departure from the road.

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Last Updated: August 3, 2009