To expand on your answer, judging by the IR led's in the 'floodlight' arrangements on top, I'd say 100% that they're scanning plates, and it's highly likely they're doing face scans too(i.e.
http://iomniscient.com/) and other imaging.. For instance in Australia they sometimes check for people not wearing seat belts with similar camera arrangements. I'd also bet that they take speed readings.
They likely house a level of chemical-sniffing technology too, like police cars can be equipped with (http://www.inficon.com/).
These sniffers would likely be actively looking at emission levels etc (being near tunnels would provide a reason for that), however can also detect explosives etc.
