Low bridge Duck.

I use that on my Garmin... you can adjust how far in advance you want it to warn you... make sure you set it it warn you with enough time to exit the road you're on so you can find an alternate road before you have no way out.

I use the Low Clearances POI on Delorme Street Atlas. It's very helpful in avoiding roads with headbangers. Updates seem to come every few months.

The database includes low clearances under 16 ft, and as our rig is under 12 ft, there are a lot of alerts I don't need, so I edit the file.

Doesn't cost all that much, either.

al