A private firm operating a single highway may not consider the network effects of its road pricing. Its toll schedule may be set up to maximize profits, but this can move traffic to other roads, costing municipal and state governments more in the long run as a result of increased local congestion and damage done by trucks to local roads (Regional Plan Association 2007). Also, given that the toll setting rights are transferred to the private sector, the public sector is restricted from controlling the effect of traffic diversions into public roads, and would not have the power to reduce tolls to restore "normalcy" in other parts of the highway network. Past experience shows that significant toll increases will divert traffic, as was the case in New Jersey and Ohio, where toll increases were eased because of significant truck traffic diversions into local routes.
Several attempts have been made to quantitatively study the relationships between toll increases and traffic diversion that might come about from PPP projects. Belzer and Swan (2008) construct a regression model to demonstrate the diversion effects of private companies setting tolls based on profit maximization policies. Using historic data along the Ohio Turnpike, the research suggests the existence of toll rates that would simultaneously maximize private profit and shift a significant number of cars and trucks to alternate competing routes. Diversion to these competing routes, many of which are non-limited-access, could pose significant safety hazards and maintenance costs to the road system overall. Although not necessarily questioning the wisdom of pricing, the authors suggest allowing private operators to control individual roads will erode system performance overall, create economic inefficiencies (deadweight), and curtail inter-state commerce.
In Oklahoma, opposition to a toll bridge PPP led residents near the proposed location for the bridge to take the case to court on the grounds that the public did not vote on the proposal and there were no open bids. One of the main concerns of this group was that the surrounding infrastructure could not handle potential traffic growth. The court struck down the project, although not for these reasons, but because the alignment for one of the bridge approaches fell outside the toll authority jurisdiction ("Municipal Toll Roads Become Likely Path" 2008).
States are aware and recognize the importance of this concern, as expressed through the state DOT survey. All respondents indicated that the impact of PPPs on the overall transportation networks was important.