You can add
Low Clearance Alert Warnings to any Garmin Nuvi, with over 3600 warning locations across the U.S.
You can also add this to my favorite Laptop based GPS, Streets & Trips. And DeLorme, along with TomTom, Google, and Navman.
And all low clearance underpasses/bridges are well marked with big warning signs, which usually still give you some wiggle room. There are not many around anymore unless you are in the inner city or way out on side roads. I learned a long time ago to read highway warning signs!