The case for and against: Half of all London’s speed limits drop to 20mph

More than 400 people were killed on London ’s roads in 1990 .

More than 300 traffic-calmed 20mph zones were introduced in residential areas.

In these new zones, the numbers of serious and fatal casualties fell by 40 per cent .

But little changed on London 's main roads and town centres.

21 of 33 London boroughs have either introduced a default 20mph limit or plan to.