Signing Minimum Vertical Clearance
The signed clearance (the clearance shown on the sign) should be three inches less than the actual measured clearance. This establishes a clearance buffer to allow for future surface overlays. The following signing requirements are summarized in Figure 6-3.
Structures Not Requiring Signing.
Overhead obstructions with minimum clearances exceeding 20 feet do not require any clearance signs. The clearance signs should not be applied to overhead sign support structures.Structures Requiring Signing.
All overhead obstructions, except overhead sign structures, that have a minimum measured clearance of 20 feet or less require both advance CLEARANCE (W12-2) and structure mounted ( ) FT ( ) IN clearance (W12-2a) signs, unless the immediately preceding structure is lower and no access (entrance or exit ramps, driveways, or intersections) for traffic exists between the structures. When the advanced CLEARANCE sign legend is less than 14 feet, 6 inches, the LOW CLEARANCE plaque (W12-2TP) should be included on the advance sign assembly.The W12-2 sign must not be located between a MERGE sign and the entrance ramp or between an EXIT DIRECTION sign and the exit ramp. See the
for more sign placement information.
Figure 6-3. Initial determination of need for warning signs for overhead obstructions. (Signed clearance should be three inches less than the actual measured clearance.)
When the minimum measured clearance over the travelway is less than 14 feet, 9 inches, use the following CRITERIA to determine the necessary signing.
Criterion | If... | Then... |
|---|---|---|
1 | the minimum measured clearance over the travelway is located within the travel lanes, | only one clearance sign is required at the obstruction (see Figure 6-4 for example). |
2 | the difference between the minimum measured clearance above the travel lanes and the minimum measured clearance above the usable shoulder is 6 inches or greater, | the travel lane and shoulder clearances should be signed independently on the structure (see Figure 6-5 for example); otherwise, only one clearance sign is required. |
3 | the minimum measured clearance over the travel lanes requires a signed clearance greater than or equal to 14 feet, and if the minimum measured clearance over the usable shoulders requires a signed clearance of less than 14 feet, | the travel lane and shoulder clearances should be signed independently (see Figure 6-6 for example). |
Figure 6-4. Criterion 1. If the shoulder clearance is greater than that of the travel lane, then only one sign is required.
Figure 6-5. Criterion 2. If the difference between the minimum measured clearance above the travel lanes and the minimum measured clearance above the usable shoulder is six inches or greater, then the travel lane and shoulder clearances should be signed independently on the structure.
Figure 6-6. Criterion 3. If the minimum measured clearance over the travel lanes requires a signed clearance greater than or equal to 14 ft., and if the minimum measured clearance over the usable shoulders requires a signed clearance of less than 14 ft., then travel lane and shoulder clearances should be signed independently.
Shoulder Clearance Signing.
When an additional clearance sign is installed over a shoulder, the advanced CLEARANCE signs (W12-2) should indicate the same clearance as shown on the sign mounted over the travel lanes. Additional CLEARANCE signing for the shoulder must include a downward arrow plaque (W12-3TP). The downward arrow plaque should be located adjacent to the shoulder CLEARANCE sign and centered over the point for which the signed clearance applies, typically the outside edge of the usable shoulder.