Calculating the subsidy associated with state maintenance of roads for private vehicles is difficult.
Some basic level of signage and hazard prevention (potholes, ice etc) is necessary for emergency vehicles and other government operations.
Thereafter, although there is some maintenance cost associated with the road wear from private vehicles and the additional infrastructure required for higher traffic volumes, we don’t have an easy way to calculate the cost.
Wouldn't the very patterns of development be different (and more amenable to rail) if the state didn't spend oodles of money building nice roads to make sprawl livable?
There is plenty of undeveloped land that people don't move to because there are no roads to get to.
We have more than a century of data showing that roads are subject to induced demand. If you build more roads, people move and sprawl (and take more trips in general) until traffic is once again unbearably bad.
If you build more lanes, more people move further out. Roads create sprawl.
I disagree that it's difficult: excepting toll roads and "unassumed" roads‡, the subsidy is 100% (and most toll roads are likely subsidized at a substantial fraction of their cost, since the tolls are rarely high enough to be self-sufficient).
It doesn't matter whether the money used to pay for road maintenance comes from property taxes (like in Toronto), income taxes (Ontario areas not maintained by cities / regions / counties), or gas taxes (much of the U.S.?). If you're not paying per use of the road, there is a 100% subsidy on the maintenance of the road.
There are definite costs to not maintaining those roads (people won't move where there aren't roads; unmaintained roads will result in more costs to the people using those roads as their vehicles are damaged more frequently—which may result in costs to the municipality for not properly maintaining those roads), but we should not pretend roads are self-sustaining or aren't entirely subsidized.
‡ Unassumed roads are those built privately by the landowners in common and are not maintained by the local government. When used for private dwellings, these roads usually fall under the sway of the local government after a period of time, even if they were built initially by a subdivision subcontractor.
Some basic level of signage and hazard prevention (potholes, ice etc) is necessary for emergency vehicles and other government operations.
Thereafter, although there is some maintenance cost associated with the road wear from private vehicles and the additional infrastructure required for higher traffic volumes, we don’t have an easy way to calculate the cost.