People wrongly assume that 20mph limits delay journey times. Yet, average city speeds are generally well below 20mph owing to congestion and queues. And traffic flows more freely at 20mph than 30mph: drivers make better use of road space by packing closer and junctions work more efficiently and at a higher capacity as its easier to merge. Because drivers feel safer, some leave their cars at home, further reducing congestion. 20mph limits mean quicker journeys.