Travelers will make adjustments to congestion pricing in ways that cannot be fully predicted. Assuredly, however, some will seek alternatives to avoid the fee. Underestimating the size of these shifts could result in substantial dislocations. In Singapore, for example, it appears that the high price for entering the central area diverted a substantial portion of through traffic to a ring road that could not handle the capacity. Planners also underestimated the shift to transit. The large number of new transit patrons apparently increased bus boarding times and therefore slowed the travel times of the buses (Gomez-Ibanez and Small forthcoming). On the other hand, planners overestimated the demand for park-and-ride facilities on the edge of the central business district, which resulted in construction of some expensive and unnecessary parking facilities and purchase of small buses to operate between the lots and the city center. In the aggregate, the congestion on the ring road and slower travel times for bus patrons negated many of the time savings enjoyed by those who paid the fee to travel into downtown. Over time, the net benefits became more positive with the expansion of the ring road and bus fleet (Gomez-Ibanez and Small forthcoming), but the dislocations caused early in the Singapore experience suggest that considerable care should be paid to providing alternative modes or routes. In the example of the A-1 motorway in France, considerable care was taken to minimize any tendency to divert travelers to adjacent, signalized arterial routes.

When congestion pricing is applied to a single facility, there can also be the problem of traffic diverting from a tolled route to untolled routes. This diversion may slow traffic on the untolled routes. Motorists may also begin using neighborhood streets to avoid tolled routes, and fear of such effects by neighborhood groups could lead to rejection of proposals for congestion pricing. Any proposal for congestion pricing should make explicit the plans for handling traffic diversion and incorporate design features to minimize its impact. Avoiding the creation of congestion on unpriced routes would be particularly important for avoiding localized increases in automotive emissions.

Several key policy questions emerge from the previous chapters of this report, the answers to which will only be gained through experience. The major questions about congestion pricing can be summarized as follows: