Congestion charging in Stockholm has become more successful over time, according to a study by Swedish researchers. Although the total cost of a journey that enters the congestion charge zone has fallen in real terms since the charges were first introduced in 2006, there has consistently been around 29% less traffic within the zone, compared with levels in 2005.

Congestion charges are an effective means of reducing road traffic, but are often strongly opposed by the public. This study combined quantitative and qualitative methods to explore attitudes towards congestion charging in Spain, finding that opposition is reduced when revenues are spent on environmental improvements.