Hunter's point that those who build the roads have the right to set the rules for their use has some resonance, but I cannot agree that building roads gives the government open-ended regulatory power over everything that happens on them. In particular, I should simply point out again that regardless of who paid for the roads, U.S. citizens have a right to be free of unjustified searches and seizures by their government, wherever they happen to be. A citizen has a right to enter a government building, for example, and if he is not making a disturbance and is not in a place restricted to authorized personnel, the government has no right to detain or otherwise search him, regardless of how much wine he had with his dinner. Just so for the roads.
Hunter argues that the government's confiscation of property and subsequent building and maintenance of roads overcomes this right, but it appears to me that the only one who can overcome a right is he who gave it. And if that Particular Source has spoken on the matter of drinking and driving, I have not yet heard it.