One of the few driving pleasures left for motorists comes from leaving congested urban streets, abandoning the grim and grinding motorways and hitting the open road. There you can drive at up to 60 miles an hour and without the ever-watchful and baleful presence of speed cameras.

Soon, however, if the government has its way, yet another of life’s little pleasures will be denied to motorists. The speed limit on ordinary roads – 150,000 miles of them – is to be cut from 60mph to 50mph and will be enforced by “average speed” cameras.

People will be familiar with these from motorway roadworks, usually where there is no evidence of anybody working. Traffic crawls along for mile after mile, drivers fearing three points on their licence if they deviate even slightly from the permitted speed.

Now that fate awaits drivers on ordinary roads. Not only will we be obliged to drive slowly, but it will be with one eye on the speedometer. Modern cars are safer than ever but apparently we cannot be trusted to drive them. The proposed cut is the first since 1978, when the speed limit was reduced from 70mph to 60mph.

Jim Fitzpatrick, the roads minister, thinks the “vast majority” of motorists will support the change. Nearly 3,000 people die on our roads each year, he says, and that is far too many. Of the 1,430 deaths in 2007 among car occupants, 984 or 69% were on roads outside built-up areas.

The government has also been influenced by evidence from Speed Check Services, the company that supplies average speed cameras, which shows (unsurprisingly) that the numbers killed or seriously injured has dropped by 60% at sites where they are used.

That may be true, but there is also more than one way of reducing the death toll. Many accidents occur at black spots; few drivers would argue with cutting speed limits at these points and policing those reductions with cameras. It is the blanket nature of the proposed changes that offends. Where does it stop – if lives are saved at 50mph, would more not be saved at 40mph or 30mph?

The principle of proportionality is increasingly going out of the window. The majority of people behave sensibly in the course of their daily lives, but they are being asked to constrain their behaviour because of a tiny minority of wrongdoers and careless people. The official mind needs to look at society’s problems through the other end of the telescope. The real question is how do we intervene to prevent a troublesome minority harming themselves and others. Universal punishment or restraint is a lazy, bad solution.