“things” over highways before scales

I have been curious for a while what the devices over the highway before scales are for. At least in California they seem to always be set up exactly the same but I can only guess as to the use of them. If my memory serves me it is two boxes on a pole over the right two lanes and then one box on a pole over the right lanes and then two again. That may not be the exact pattern but its something like that. My thoughts were they are either for signaling truckers to enter the scales when they are open or they detect trucks passing illegally. Thanks in advance for your insight.

Pretty sure they are transponder receivers for transponder equiped trucks. I see signs instructing non-transponder equiped trucks to stop, where the info on equiped trucks is picked up by these devices.

Here is a Site with a little info on how it works.