An article by Sewell Chan in today's online New York Times discusses an interesting concept for regulating traffic: charging an extra fee or toll to enter a congested area. This plan has apparently been adopted in other cities, including London. It is policed by a "big brother" series of cameras that allow license plates to be monitored and matched against those who pay the daily fees, which in London run $14/day. This has apparently cut down traffic and resulting air pollution.
Though we have had toll roads for years, this approach is different in that the collection of the toll is diffused and placed in the responsibility of the driver. The in terrorem effect of the subsequent ticket for nonpayment, if the "big brother" camera discovers you, serves to reinforce the civic duty of payment. Query, however, whether this system could sustain a challenge under the commerce clause, particularly if local residents are exempt.