Raising the speed limit to 80mph on one of Kent's busiest roads would cause more crashes, according to a road safety charity.

A report by the Road Safety Foundation has found many UK motorways are not safe enough to support faster speeds.

They include a stetch of the M20 between junctions four (West Malling) and seven (Detling), which the government could use to trial the 80mph limit.

The road already has a variable speed limit – one of only three in the country where police can alter the speeds at which drivers travel, by using 19 overhead gantries.

But it scores just three out of a possible four stars in the latest European Road Assessment Programme (EuroRAP), which measures safety on England's A-roads and motorways.

There were 34 fatal or serious crashes on the M20 last year, of which almost a third were rear-end shunts.

Caroline Moore, a research analyst at the Road Safety Foundation, said: "You get lots of congestion on busy motorways and when one car brakes, the driver behind has to react quickly.

"If people are driving faster, their reaction times need to be much quicker.

"On roads like the M20, which carry more than 100,000 vehicles every day, we're going to see more shunt crashes as a result of higher speeds."

She added: "It's all well and good having these controlled motorways, but are there adequate crash barriers lining the roads, for example?

"We want five-star drivers in five-star vehicles, using four-star roads."

No decision has yet been taken on where and when to start the 80mph trial, although roads on which a variable speed limit is already in place are expected to be used first.

The new speed limit could be introduced in early 2013.

Dozens of drivers every year would avoid fines if an 80mph speed limit was introduced between junctions four and seven of the M20.

Figures show 84 drivers were given fixed penalty notices for speeding at between 80 and 90mph along the six-mile stretch in the last three years.

At present, drivers are unlikely to be pulled over unless they are caught speeding at 80mph or above – 10% plus 3mph above the national speed limit.
That could increase to 91mph, if the speed limit is raised.

In total, Kent Police issued around 1,250 penalty points and collected nearly £25,000 in speeding fines on the same stretch of road in the last five years.

The figures do not include motorists who were sent on speed awareness courses.