Ken Livingstone's campaign stressed the need to ease traffic congestion in central London by persuading people to switch from private cars to public transport. The congestion-charging scheme was at the heart of a larger transport strategy designed from the outset to tackle four key transport priorities for London: reducing congestion. The congestion charge was commissioned in February 2003; generally positive reviews allowed Livingstone to be re-elected in May 2004. Planners often gloss over traffic congestion and queuing as "engineering problems." Road pricing assumes a number of forms. By varying the tolls according to the time of day, such facility-specific tolls can achieve, at least locally, some of the benefits of congestion pricing. Policies that add to pollution by having non-polluting cars run at free-flow conditions while polluting cars idle in congestion have, at least, short-run costs that should be considered. The measurement and standards of congestion were critical issues.