Leading article: A hi-tech spoke in the speeder's wheels

You don't have to be a parent or a teenager to empathise with the gleeful relief of the former and the surly resignation of the latter at this week's announcement from Ford.

The car maker is offering a dual key system that will allow parents to set a top speed for their offspring's excursions and even, if they are feeling really mean, a maximum volume for the sound system. It adds a whole new dimension to locking it up and throwing away the key.

There are plans, apparently, to market the key to others, such as car hire companies and fleet managers, and one can imagine that the courts and insurers might like to make it obligatory for cars used by persistent speeders. But the development also invites a question: why, given the safety and environmental consequences of high speed, are cars today built with so much spare capacity? Most countries now have speed limits, even on motorways, of 70 or 80mph, yet the dial on most private cars goes up to 120mph or even higher. Why should the perils of the German Autobahn be visited upon us all?