You don't need a GPS jammer.
If your bridge is 8 feet high, you simply need a metal arch 9 feet high, and a 'low bridge' sign suspended from it by two one-foot pieces of chain.
Hence, any driver approaching the bridge who should fail to notice the 'low bridge' sign will have their attention drawn to it when it collides with their vehicle, causing a loud noise but less danger than a vehicle-bridge collision.
I think there are reasons why they built that bridge with an 8 feet tall arch and not a 9 feet high arch(eg. structural strength, etc..). Physics doesn't and shouldn't make allowances for idiotic drivers blindly following directions.
I do agree, gps jammers need to be located in such areas to disrupt gps signals and force drivers to pay attention to the signs and warnings posted alongside the road.
In every hierarchy the cream rises until it sours. -- Dr. Laurence J. Peter
