This is of those “gotcha!” questions that when asked libertarians are supposed to shrivel up and concede the point that in a free society we would all just lay in the mud and cry.
Road provision needs to be addressed from several angles.
The first point that needs to be brought up is that the central planning of transportation or anything else is inefficient due to what F.A. Hayek refers to as ‘the knowledge problem.‘ The individuals directly responsible and affected by projects should be the ones planning it, not a top-down and distant bureaucratic entity. The costs of acquiring all of the local information necessary to calculate such a complicated endeavor is insurmountable.
People who invest in developing infrastructure should not be allowed to force everyone inside of an arbitrary geographical area (like the United States of America) to subsidize its construction and maintenance either. Why should you have to pay for a road you will never see in St. Augustine, Florida? A port in Galveston, Texas? The people who want such development should bear the full cost o their actions and allow consumers to support or not support their plans at the point of consumption (i.e. voting with one’s dollar).
Kevin Carson and Noam Chomsky have both posited the extremely negative dislocating effects of state transportation infrastructure. By socializing the costs of the transportation of goods amongst all people, rather than amongst those who produce and consume the goods, there is far less of an incentive for consumers to consume locally. As a result, this series of policies artificially suppresses local industry and benefits distant producers. This is to some degree responsible for the unnatural centralization of major market players like Walmart, whom desperately needs the state to externalize the large costs of its goods’ transport. For more information, please read Kevin Carson’s The Distorting Effects of Transportation Subsidies.
Any alternative which involves indirect forced labor is not nor could it ever be anarchistic. There are many other practical arguments as well, and details about the ones mentioned, in Walter Block’s paper [pdf warning], Free Market Transportation: Denationalizing the Roads.