Expanded use of targeted tolling would most likely occur first on freeways and major highways and may encourage traffic diversion onto unpriced arterials, causing congestion and associated problems on those networks. But the extent of diversion appears to depend on the price levels and availability of alternative routes. For example, research into how drivers in Spain reacted to toll increases on their national system of toll roads found that the more alternative routes there were, the greater was the diversion to avoid tolls.59 Similarly, a study on the effects of toll rate increases on Ohio's turnpikes found that, as expected, the higher the toll rate, the greater the diversion. The problem was especially severe with truck traffic, where one-third to two-thirds of trucks chose to take congested alternate routes rather than pay a toll to use uncongested turnpikes.60 Not only is this a problem of increased congestion on alternate routes, but diversion of heavy vehicles may create an additional problem since the tolled roads are typically built to higher standards than alternative routes. A heavy vehicle does less damage per mile on such roads than it does on roads built to lower standards. Hence, the diverted truck traffic may actually increase the amount of road damage. Diversion also could create operational problems due to the lack of width, clearance, or geometric characteristics (such as turning radii) on lower-order roads to accommodate heavy vehicles.

The federal government is uniquely positioned to understand and place appropriate importance on the impact of transportation management and investment decisions on interstate commerce, goods movement, and nationwide mobility.