Knowing the height of your TC can prevent a very costly disaster. Understand that every state is different as to whether they mark the height of bridges, tunnels or not, or whether that sign posted is accurate.
Upon approaching a state/town bridge or tunnel, don’t always trust the post height sign. Local and state bridges don’t have to abide by the federal regulations. So, there are many bridge or tunnel that will be significantly lower, particularly the further you get away from major commercial areas.
MEASURING YOUR TC
Start by placing your TC on a hard level surface such as a driveway. Measure from the ground to the top of the highest object on your TC. The air conditioner and/or TV antenna/satellite dish could be the highest objects.
Once you have the height, consider adding a safety margin – add two or three inches to the height and post this new height on your dashboard where you and/or passenger will see it. The extra safety margin of those extra added inches is in case the road’s original surface has been paved over. Repaving can lower bridge and tunnel clearances by 2 inches each time it is repaved. Those signs may not have been updated after the last repaving job.
Another safety measure to consider when using on a GPS unit, is to overstate the height of your TC in the GPS height settings. By adding two inches from the recorded height and then inputting the higher height, the GPS should provide advance warning on your intended route.
If the height of your TC is 11 feet, 6 inches, set the GPS height setting to 11 feet, 8 inches in the GPS unit.
When you input your itinerary into the GPS, the unit should provide a warning that a bridge or tunnel with low clearance is on the route. This will allow you to alter your route. Confirm the height with the local authority such as the police department and/or Department of Public Works (DPW).
WORDS OF WISDOM FROM THE US DEPARTMENT OF TRANSPORTATION, FEDERAL HIGHWAY ADMINISTRATION:
Chapter 2C Warning Signs and Object Marker, Section 2C.02 Application of Warning Signs:
The use of warning signs should be kept to a minimum as the unnecessary use of warning signs tends to breed disrespect for all signs.