Ostensibly, toll roads are enacted so that the expense in building the road to begin with can be repaid. Invariably, the politicians and the bureaucrats love that money rolling in so much that the money is used not just in paying the cost of the road, but the infrastructure that's needed to support the freakin' toll system. You know, the kickbacks, graft, corruption, cute little signs, I-Pass systems, computers, not to mention the poor bastard standing there in an unheated toll both in the middle of January.
The cost of that poor bastard freezing his balls off in the toll booth is a drop in the bucket compared to the union construction road workers, greased palms, and other niceties all provided by the schmuck who's dropping the money in the till.