Excessive traffic congestion and the financial burden of removing lodged trucks and repairing overpasses is a good incentive to apply a relatively simple and inexpensive means of warning drivers if the load that they carry exceeds the maximum height for a bridge or overpass.
Purchase a set of industrial photoelectric sensors that use an amplifier with an internal manual relay. These photoelectric sensors should be positioned in an opposed-mode configuration (or through-beam) so that the transmitting eye is located on one side of the lane and the receiver is on the opposing side. The photo eyes should face one another so that the infrared beam projects across the roadway. The infrared beam should be positioned so that it is horizontal to the road surface and at a height that is just below the maximum allowable vehicle height of the overpass. When the beam is broken by a vehicle or by cargo, it will trigger a signal from the photoelectric amplifier. These photoelectric sensors should be located far enough from the overpass, that they will detect the over-sized vehicle in time to warn the driver to exit the roadway.
Connect the relay from the photoelectric amplifier to a set of flashing lights and use the proper signage to alert the driver and provide them with simple instructions, for example, "WARNING MAXIMUM HEIGHT EXCEEDED - TAKE EXIT WHEN LIGHTS ARE FLASHING!" Though, this solution is a simple one and very inexpensive, the benefits will be quickly realized.
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