It's incredibly easy for trucks; they're already licensed and weighed. Most trucks in the US already have something in the cab to handle signaling the driver to pull in to the weigh station, implying that adding tracking & billing wouldn't be too hard. We're also starting to see GPS & LTE in the trailers too, to fight theft, so that's an easy integration point.

It's a bit harder with cars, since there's a lot less infrastructure to track usage, and people would rightfully balk at the invasion of privacy.