Alan Mitchell in the Australian Financial Review today (regrettably subscription only but an alternative account is here) discusses the cordon around Stockholm used to deal with traffic congestion. It is only being trialled but is popular.
The toll is applied to all roads entering the city and varies with time-of-day in accord with traffic congestion. There are no incentives to evade the toll by taking backstreets since all roads are tolled but strong incentives to shift your travel away from peak periods or to take public transport. What a contrast to the inept approaches to tolling in Sydney and Melbourne!
Overall traffic volumes on weekdays has fallen 20% - a huge reduction that substantially improves the efficiency of travel in Stockholm.
Further economics discussion is at Knowledge Problem while technical issues are discussed here.