Motorway rain drainage

[fly258]

While driving on the M1 the other day during torrential rain, we noticed certain stretches of the motorway appeared nearly dry with very little spray being kicked up. Other stretches had quite a bit of surface water and the spray was quite bad.
It would appear the road surface was absorbing the water some how on the drier bits, but how, and why only certain stretches?

[sddsddean]

Probably areas of 'Low fines asphalt'. This has been tried out initially on stretches of the M25, but after extensive tests has become more prevalent when sections are re-surfaced(thats why its in 'patches').Were you between J19 and 21, as I know this was recently re surfaced? Basically, standard asphalt is made from stone (for wearing), sand(to fill in the gaps) and bitumen (to hold it all together). I'm sure you have noticed that tarmac is laid in layers. The top (or wearing) course is typically 40 - 60mm thick. The layer beneath(base course) is 100 - 200mm thick. In 'Low fines asphalt', the base course is laid as usual (so it presents a 'solid' surface) and the wearing course is laid on top, but as it is low fines(thats the sand element) there are 'holes' in it. So, when it rains, instead of the water having to track all the way across the top of the wearing course to the drains at the edge of the road, it percolates through the wearing course to the top of the base course and then drains away to the side. As the water is effectively removed from the running surface, there is no spray which is safer for everyone. Of course there are 2 major down sides. 1) It doesn't last as long and needs replacing about every 20 years, and 2) it needs maintainence. The surface has to be regularly swept by a road sweeper to stop the voids getting bunged up. Hope you have been taking notes; I will be asking questions later!!

[fly258]

sddsddean,
That is a fantastic answer.
I can't honestly remember where it was, all I can remember was the awful driving conditions and I was a long way from home.
I don't need to take notes you explanation was so good.
I think it's about time all motorways were resurfaced in this stuff .