According to Portsmouth City Council: "the 20mph limit is for roads where the average speed is already 24mph or less. We have installed prominent 20mph signs where drivers enter the new speed restrictions, as well as 'repeater' signs as reminders. It has been found elsewhere that this method reduces speeds by 3-4mph. Road humps are not part of the scheme, although if speeds do not drop on particular roads, then residents will be consulted again to see if they want additional measures. In most cases the 20mph limit will be self-enforcing and further speed enforcement measures will not be needed."
On most of our residential roads, it's not safe or appropriate to drive at more than 20mph, because they're narrow and lined with parked cars. What we want to do is target the small number of drivers who drive at inappropriate speeds without regard for road safety or respect for anyone else. A pedestrian hit by a car travelling at 20mph is likely to suffer slight injuries, but at 30mph they are likely to be severely hurt. At 40mph or above they are likely to be killed.
This scheme has won Portsmouth lots of attention for its boldness in improving safety for our residents. I'm sure it could save lives, particularly those of children and elderly people, and get more people cycling. We believe the scheme will be mainly self-policing. If the new limit isn't respected on some roads, we will look at other measures or get police involved. We want to change the culture in the city so that speeding in residential areas is seen to be what it is - dangerous and anti-social. - Cllr Alex Bentley, Executive Member for Environment and Transportation
The majority of residential roads are covered by the 20mph limit, apart from major north-south and east-west routes - which will keep their current limits.
The scheme was implemented for a cost of around £550,000 in a programme of 6 'Traffic Regulation Orders'. This approach, using signs only, made the programme much cheaper than 20 mph zones, which require some sort of speed reducing feature (hump, chicane or road marking) every 100 metres.
Other cities and towns are eager to follow Portsmouth's lead. Although 20 mph is very common in many European towns and cities, it is still relatively infrequently used in Britain. Many London boroughs have implemented 20 mph zones (using traffic calming) which have shown huge casualty reductions of around 42% post implementation.