Road user pricing would encourage people to take non-essential trips at a different time, or not at all.
Charging people to drive has been the dream of policy wonks – serving politicians tend to see it as political poison. So when federal minister Paul Fletcher raises it, that's a step forward.
By persuading some drivers to travel a different route or at a different time, congestion charges can dramatically improve the flow of traffic.
Bigger cities increase wages, output and innovation, but also problems of congestion and pollution. Congestion charges can minimise these problems by dramatically improving traffic flows.
If the choice is between waiting in their cars and long waits on inefficient public transport, many people prefer to drive.
Once a new road opens, people switch back to cars and congestion increases back to a steady-state point of gridlock. For lasting effectiveness, policy needs to include congestion charges and better rail services.