The switch to a single European currency may cause changes in pricing behaviour, which will further muddy the waters.

ECONOMIST: European Central Bank

Before implementing road-pricing schemes, the anticipated effect on motorists' behaviour must be clearly understood.

ECONOMIST: Excessive

But it would also undermine the efficient pricing of risk by providing insurance, after the event, for risky behaviour.

ECONOMIST: Central banks have played a starring role

Because road systems are complex, and because the behaviour of individual drivers is largely unpredictable, the overall impact of a particular pricing scheme is anyone's guess.

ECONOMIST: A survey of commuting