That sign is meant for pickups hauling reportable quantities. You should have also noticed plenty of scales saying "all trucks enter scales" -- prepass is exempt for all but Arizona & Arkansas (provided the carrier has a current hazmat license): http://www.google.com/url?sa=t&sour...pYHQCA&usg=AFQjCNGXQBqvMtl9iSoVRZ2sKBoQaU5NJA
Yes, they have the signs posted and if you would do further research, you would find that those signs do not apply to trucks equipped with Prepass transponders.
Ohio and West Virginia: An Alliance for Uniform Hazmat Transportation license is required if hauling hazardous materials/waste. Follow in-cab signal. Disregard state signs to pull into weigh station.