Serious safety concerns about new road surfaces being laid across the UK have been uncovered in a BBC investigation.
Stone mastic asphalt is used on some of the UK's busiest roads
The materials - stone mastic asphalt, or SMA - are approved by the Highways Agency for trunk roads and motorways.
File On 4 found the same surfaces are banned in Ireland on some roads because of fears about poor grip.
The Department for Transport said just because roads require further investigation, "it doesn't necessarily mean they are unsafe".
SMA surfacing systems are widely used because they are said to last a long time, are quick to lay and give a smooth, quiet ride.
But police crash investigators have become concerned because, in certain conditions, some do not offer much grip for up to two years until they have bedded in.
Sergeant Jim Allen said he experienced grave difficulties when conducting routine skid tests in optimum conditions on a Derbyshire road newly laid with SMA.
"It was a sunny day in August. I jumped on the brakes and the car just kept going and going.
"Instead of the scream of tyre on road and a cloud of smoke there was just a gentle hiss as I passed over the road, and I skidded far further than I ever expected to."
Concerns about grip have also been raised in Germany, where SMA was pioneeered in the 1960s, and in Holland.
Tests carried out by the National Roads Authorities (NRA) in the Irish Republic raised questions about the materials' ability to provide enough friction for tyres at higher speeds.
The NRA has decided to restrict its use to roads with a 30 mph speed limit, and has taken remedial action on other roads where they have put it down.
"When we found the skid resistance to be doubtful we simply had to go and surface dress all those roads to make them safe again," said NRA spokesman Sean Davitt.
"Basically, our attitude towards the material is that we still have to be fully convinced of its benefits."
In a statement, the Highways Agency said all new materials pass a rigorous testing procedure which includes examination of surface texture for skid resistance.
But File On 4 found that skid resistance tests were conducted in the wet and on surfaces that were worn down, but that tests in dry conditions were overlooked.
Significantly, critics of SMA say that slipperiness can be a problem on new, dry roads.
'Lives at risk'
The programme also reveals that one in five miles of existing main road are now potentially dangerous due to low skid resistance, according to the government's own most recent maintenance survey.
And the AA Motoring Trust warns that England's road network is broadly in the worst condition since records began in the 1970s.
Spokesman Paul Watters says: "I think road surfaces are a hidden menace to road users and I think perhaps we don't know the half of it, to be honest with you.
"In London it's as many as a third of main roads that have skid resistance at a level that needs looking at. Clearly this is an alarm bell. There could be lives at risk."
The Department for Transport would not be interviewed for File On 4.
But in a statement it said: "A number of factors would have to come together to make a road unsafe, including volumes of traffic, speeds and the nature of the road itself...Just because roads have reached a level requiring further investigation, it doesn't necessarily mean they are unsafe."
The government would not accept that road repairs are in crisis, saying it was committed to spending more than £31bn over a 10-year period.
File on 4: BBC Radio 4, Tuesday 22 February at 2000 GMT, and repeated on Sunday, 27 February at 1700 GMT.