> You and I do NOT benefit from private passenger vehicles other than our own.
That's not entirely true. If I drive to McDonalds but there's nobody working there then it's pretty useless. I suppose in theory they could take public transit, but that's spotty at best in many areas of the country. If I'm disabled to the point of being effectively unable to drive then I do not directly use roads at all, but am far more dependent on them. And lets not forget the joys of pizza delivery!
In general, there are lot of benefits to having a good infrastructure regardless of whether I use it directly. I'm not sure it's worth even trying to determine who uses it the most.