Source on those 3 trillion passenger miles being on exclusively federally-funded highways? That sounds horribly off, pretty sure you're referring to total passenger miles for cars across all road types, which would then have to take into account subsidies at the local and state levels.

This has total vehicle miles at 1.4 trillion in 2017, having around 2 occupants per vehicle on average sounds about right: https://www.statista.com/statistics/193007/vehicle-miles-tra...