Tolls are constitutional, it falls right along with the taxing authority, even more so than the income tax actually which required the 16th amendment to end direct taxation prohibition. Now, I think that toll roads have their place to an extent, but only with a very concise explanation to the public. I'll give you an example of where I think there is a good explanation, in New Orleans there are two main highways over Lake Pontchartrain, the original I-10 bridge and the new causeway. The causeway was built specifically to alleviate traffic on the lake but at a premium, it was explained from the beginning that the new avenue would be a toll road.
For a typical roadway, there should exist no tolls because that should have been funded by already collected taxes, something that serves as a convenience but not absolutely crucial like an "express" road could be justified with a building/maintenance fee collected as a user based tax, it's a convenience, so you have the right to use/pay or not use/avoid the additional fee. Where I think a toll is not proper is a roadway that serves as the only avenue to transportation. In that case it is already funded by tax dollars, additionally, fuel taxes go to the funding for maintenance, and there is no choice to travel it. In that particular case a toll is excessive.
