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FHWA Home / Safety / Geometric Design / Publications / Mitigation Strategies For Design Exceptions

11. Vertical Clearance

Target areas:  Any highway with a structure that has low vertical clearance.

Strategy:  Signing.

Signing is the most common mitigation strategy for vertical clearance (Figures 65 and 66).  Whenever vertical clearance criteria are not met, advance warning should be placed at the nearest intersecting road or wide point in the road at which a vehicle can detour or turn around.  The MUTCD provides guidance on the size of warning signs for various highway types but notes that larger signs may be used when appropriate.  Larger warning signs should be considered for design exception locations.  In some locations, electronic message signs have been used to provide enhanced warning.

An innovative strategy for providing additional warning is to combine the sign with chimes that are hung from a sign truss at the same height as the vertical clearance of the structure (Figure 67.  If a truck hits the chimes, the driver is alerted that the truck will not clear the structure.

Target areas:  Highways with a nearby detour route that is designed to carry heavy vehicles.

Strategy:  Detours.

Target areas:  Highways where an alternate route for large vehicles exists&mdash  Non-interstate highways.

Strategy:  Prohibiting large vehicles.

Figure 65.  Vertical clearance signing.

FIGURE 65

Vertical clearance signing.

Figure 65 is a photo showing a post-mounted diamond-shaped sign with a yellow background. It shows an upward-pointing black arrow above the notation 14'-9" , which is above a downward-pointing black arrow.

Figure 66.  Vertical clearance signing.

FIGURE 66  

Vertical clearance signing.

Figure 66 is a photo showing a horizontal rectangular sign with a yellow background and the words LOW CLEARANCE  14&rsquo -2"   in black affixed to the bridge span over a road.

Figure 67.  Warning sign with hanging chimes installed at the same height as the vertical clearance of the structure.

FIGURE 67

Warning sign with hanging chimes installed at the same height as the vertical clearance of the structure.

Figure 67 is a photo showing a set of two horizontal rectangular signs, one immediately above the other. The signs are mounted on a sign truss cantilevered over the travel lanes of a road.

Both signs are yellow with black borders and legends. The top sign contains the word WARNING, and the bottom sign&rsquo s words are TRUCKS THAT HIT THIS WILL HIT BRIDGE.  Underneath the bottom sign, vertical rectangular chimes that are striped yellow and black are suspended from the metal pole.  The bridge referred to in the sign is visible in the background of the photo.

In some locations, it may be appropriate to provide marked detours for trucks and other large vehicles that allow them to bypass the low structure.  Similarly, it may be appropriate to prohibit large vehicles on certain routes to prevent impacts with low structures.

 

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Page last modified on April 1, 2019
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