Do you have a source you can direct me to showing me the requirement that any publicly (taxpayer) funded road is only usable by taxpayers?
I never stated any such requirement.
What I did say was that taxpayers have a right to use public roads. That is not the same as saying anyone else is prohibited from using them. The latter is a matter of how we choose to make public policy.
Our current public policy is that anyone is, in practice, allowed to use public roads, whether they are taxpayers or not. We could choose a different, more restrictive public policy regarding non-taxpayers, but we don't. (That is probably, as I said before, because it's not worth the cost to check everyone using public roads to see their taxpayer status.)
But we cannot, at least not based on the legal principle I stated, choose a public policy that does not allow taxpayers to use public roads, since they have a right to do so.
If the right is not enumerated, then it probably isn't restricted to taxpayers. Right of mobility is a natural right.
Yes, but that doesn't mean they don't apply. It is simply not worth the cost to verify that you are a taxpayer before letting you use the road.