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What’s Under Your Wheels??

July 11, 2009
Slippery When Dry!!!

Slippery When Dry!!!

Hunstanton, England – Have you ever mused when riding as to what exactly that black stuff that makes up the road surface you are riding on is? Have you assumed that the ‘authority’ responsible for its laying and maintenance has taken its duty of care to you as a road user seriously? Have you wondered why England appears to be the home of nearly all the ‘slippery road’ signs in the known universe? If yes to any of the above read on or just read on anyway!

 That nice black apparently benign surface you are riding on is almost certainly Stone Mastic Asphalt (SMA) or a generic development of the material. Now SMA has a lot of advantages. It’s particularly resistant to deformation, i.e. rutting, by heavy traffic at high temperatures, it’s far cheaper to lay the material than conventional hot rolled asphalt and above it can laid at an ambient temperature thus it’s a nice ‘cost effective’ way of surfacing our roads. Well ‘cost effective’ for those paying and those laying the stuff that is. Cost effective = cheap.

Now for the geeks amongst us – SMA is a dense, gap-graded bituminous mixture with high contents of stone, filler and bitumen, modified with a suitable binder carrier such as cellulose fibre. The essential elements of mixture design comprise the formation of an interlocking stone skeleton that provides high resistance to deformation and the filling of the skeleton voids with a rich bituminous mortar to provide high durability.

OK road surface lecture over back to the real would. SMA has a problem. Following laying its surface needs to be ‘conditioned’ by traffic moving over it. This ‘conditioning’ can take up to six months and while its taking place the road surface is considerable less skid resistant than it should be. In a nutshell the surface when dry may well have the skid resistance characteristics of the same surface when wet, and when wet the skid resistance may well be below the current UK national requirements. Not exactly confidence inspiring for users of two wheels and explains why, even on brand new roads, the ‘slippery road’ signs are installed as a matter of course. Nice cop out for the road authority in the event of a claim being made. We had the signs up ‘sir’ so obviously your manner of riding was inappropriate for the conditions – sort of thing.

So what does all this add up to? Well the road authorities in the UK are using a cheap and nasty road surfacing material, often palmed off as ‘environmentally friendly, that they know is unsafe when first laid and for a considerable time afterwards. A material that could be a factor in the high number of single vehicle motorcycle accidents that take place on corners. Why are they doing this, because we let them.

Be careful out there.

© Back Roads Rider 2009


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