Cars making a last-minute dash for the ferry to France could be a thing of the past in the Hampshire city of Portsmouth. As of yesterday, the city was on its way to becoming the first in Britain to have a 20mph safety limit on almost all residential roads.

Starting with streets in the south-east of the town, Portsmouth City Council is rolling out black, white and red signs all over, in a bid to make the streets safer for pedestrians and cyclists, and to cut environmental pollution.

The ever-present danger that road vehicles pose to pedestrians was tragically underlined this week when four-year-old Olivia Donachie was killed by a hit-and-run driver in a south Edinburgh street.

The introduction of lower speed limits to reduce accidents is a policy that is based on sound research. As Councillor Alex Bentley, executive member for environment and transportation at Portsmouth City Council, said: "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. 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."

The national road safety charity Brake warmly welcomed the news and is calling for compulsory 20mph zones in residential areas and around schools for the whole country.

Still, there are those who question the wisdom of a blanket 20mph zone. Motoring organisations have suggested that a blanket limit could lead motorists to pay less attention to the 20mph sign than they would if they came across it only occasionally. Others argue it will prove to be unenforceable.

On the issue of enforcement, Portsmouth City Council hopes the scheme will be "self-policing". It hopes that, far from drivers disregarding the limit, its wider application will "change the culture in the city so that speeding in residential areas is seen to be what it is - dangerous and anti-social". If there are compliance problems, though, they will involve the police or look at other measures.

Currently in Scotland, 20mph limits exist around many schools and in some residential areas. Any council can use its discretion to put a lower limit on a local road, while changes in the speed limit on a trunk road or motorway is the jurisdiction of Transport Scotland. No road in the UK may be given a speed limit of more than 70mph, but Transport Scotland does have the power to reduce that level on trunk roads (such as has been done on sections of the M8).

Not all 20mph limits are mandatory. Those that are are ringed in red; they may apply permanently or at certain times of day. For instance, 20mph beacons outside some schools make the 20mph limit mandatory when the lights are flashing. This is typically when children are around, at the start and end of school and at break times.

The Twenty's Plenty signage on roads in many areas is typically advisory, although there could be mandatory limits within a Twenty's Plenty area. North Lanarkshire Council was the first in Scotland to introduce advisory 20mph limits and has since brought in some mandatory ones, too. Glasgow City Council has also mooted a city-centre-wide 20mph limit.

Some countries, though, have gone further. In Dutch "homezones", the speed limit is 10mph. Many motorists who might be inclined to grumble about such areas actually enjoy living in them, so the zones have become popular.