Motorways are good for you...

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Those who ventured on to the M25's south-western section this week, will have noticed a new hi-tech installation aimed at making the road both safer and more free-flowing. The Controlled Motorway Pilot Scheme works by varying speed limits during busy times. It is just one of the ways used to improve safety on motorways in the face of continuing traffic growth. Elsewhere in Britain, there are sensors that automatically warn motorway drivers of bad weather and traffic hazards ahead, and on parts of the M1 near Wakefield a special asphalt surface has more or less solved the problem of lethal wet-weather spray.

Motorways aren't only our fastest roads, they're also our safest. In terms of deaths and serious injuries per mile driven, they are four times safer than rural roads, and six times safer than town streets. Some 200 people die on our motorways every year, which is shocking, but that equates to just three deaths per million million vehicle kilometres - so the risk on each journey is tiny.

The Department of Transport's safety target for the year 2000 is to cut overall road casualties by one third of the 1981-85 average. To meet their share of that target, the motorways will have to carry more traffic and yet be the scene of even fewer crashes. Indeed, the introduction of motorway tolls could be prevented if it encourages cars and trucks on to by-roads, harming both safety and the local environment.

Electronics figure heavily in this safety drive. On those parts of the M25 susceptible to quick-forming fog, automatic sensors switch on fog messages on roadside dot-matrix signs. On parts of the M4 and M1, sensors under the surface enable warnings to be given about queues ahead. Drivers should then slow down enough to avoid ploughing into the back of stationary traffic. Larger dot-matrix signs are suspended above parts of the M25, and the police can use these to inform drivers of a variety of hazards, and even suggest alternative routes.

Between the A3 and M4 junctions of the M25, this technology has been extended. The idea of the Controlled Motorway Pilot Scheme is to eliminate stressful stop-start driving at busy times, when it's not unusual to speed up to 70mph then encounter a seemingly inexplicable jam. "Flow breakdown", the engineers call it.

Because drivers are kept to 50mph when a build-up of traffic is sensed (and beware, speed cameras enforce these part-time limits, which are displayed on overhead gantries), the flow remains calm and uniform, and motorists can safely travel with less space between cars. People don't weave because all lanes move at the same speed, and overall traffic flow rises.

Because drivers aren't careering at 70mph towards stationary cars, a reduction in accidents should also increase the flow rate, since a crash always causes a jam. German experience of a similar scheme suggests a 10 per cent increase in traffic flow and a 20 per cent overall fall in casualties.

Fog isn't the only weather hazard to motorway drivers. Black ice and snow are worse, and sophisticated new detectors not only trigger warnings to drivers, but are used by the authorities when deciding whether to spread grit and salt. Too much salting is a waste of money, and it rusts your car, but new automatic roadside weather stations enable applications to be as timely as possible.

If you've driven on a busy, wet motorway in blinding spray, you'll appreciate porous asphalt, which allows water to drain away through its surface. Driving from a conventional surface into the porous one, your vision returns in an almost magical way. You won't aquaplane, either. Unfortunately, it's not widely used yet because it's not proven to be as hard-wearing as ordinary asphalt and it can crack up if there's any water trapped when it freezes.

Road engineers want a surface to last as long as possible before it's torn up and replaced. Road works and contra-flows are not only costly and universally hated by drivers, they're also the most dangerous stretches of the motorway network. These days there are smoother chicanes and better signs, but it's still disorienting to steer among a forest of cones. That's why it's here that you'll most likely be scrutinised by speed cameras.

When building or modernising other roads, engineers often use roundabouts or tight curves to force drivers to slow down, or at least make them more aware of their speed. But on motorways that isn't the tactic. They're designed to be as safe as possible at whatever speed people use.

However, driver behaviour is the likeliest cause of any accident. So the road engineers are taking simple steps to educate us. Those chevrons painted on certain motorways, for example, encourage us to travel further from the car in front. "Rumble strips", roughened white lines at the edge of the road, alert us with loud tyre noise if we dozily stray over the line. Among the cheapest effective safety aids are the big yellow signs on the western end of the M5. "Tiredness can kill," they admonish. "Take a break."