I ran into several of these sites on I-71 and I-90 in Ohio when I went up to Erie, PA earlier this month. I did a little research via Google to see if I could find how it works. Although I didn't see much technical information, I found out the following:
Prepass is a system offered by Help Inc. to States where it has signed contracts. The system basically allows Trucks belonging to companies with good safety and regulatory histories the chance to bypass Weigh Stations. Each truck that is signed up received a little tranceiver device that is mounted inside the truck at the corner of the windshield. The device transmits a code that identifies the truck and receives a signal telling the driver if he can bypass the weigh station or if he must stop (green LED to bypass, red LED to stop). Help Inc. makes $4 and change each time a truck bypasses an open weigh station.
The system is set up (some of this is educated guessing) as follows:
There is a post with a transceiver on it over the road about 1 mile from the Weigh Station. There is also some kind of in-highway scale at this point. Netstumbler identifies its AP as "aaaa". I assume this is the AP that communicates with the device in the truck.
There is also an antenna on the pole at the side of the road that is pointed towards the weigh station. On I-71 NB North of Cincinnati, the AP was named "1701" (SB was "1702"). At the weigh station, there is another AP with the same name. So what looks like what happens is
1. The truck reaches the Prepass point on the highway one mile from the weigh station.
2. The "aaaa" AP polls the truck device, which sends its ID code. The ID code, some weight and height information is sent from AP "1701" at the pole to AP "1701" at the station. The system looks up the truck by Prepass ID. If everything is in order (truck info, weight, and height) the station AP sends a message back to the pole (and the truck) giving it a green light to bypass the weigh station. If something is wrong (or the station is closed) a red light is returned, telling the Truck Driver that he must stop at the Weigh Station (if it is open). The trucking firm is billed for each green light.
There are several companies and State run systems, go to http://www.prepass.com
for information on Prepass, or Google prepass and you will find links to prepass and other similar systems.