A lot of roads have been resurfaced in SMA - Smooth or Stone Mastic Ashphalt.
This causes problems to vulnerable road users, such as motor cyclists and horses. 
There have been campaigns since 2004 to stop it being used on roads where it could cause a hazard to such road users
Extract from an article in Horse & Hound in 2004:
"First developed in Germany in the 1960s, stone mastic asphalt was tested in the UK in the mid-Nineties and quickly became a popular road surface because it is very durable and reduces noise. Until recently, it was only used on major carriageways but it has now been rolled out to all roads at the behest of the Highways Agency.
However, stone mastic asphalt is slippery and can prove dangerous to horses. “It is so slippery it’s like glass,” says Sheila Hardy, the BHS (British Horse Society) Head of Safety.
Alerted by a growing number of members reporting incidents, the BHS started looking into the problem and discovered that the material, which is made to EU standard, has commonly been used on the Continent where it hasn’t given any headache to riders and horses.
The problem, Hardy thinks, is that the mixture used in Britain is slightly different and it is laid out differently.
“If it works in Europe, why doesn’t it work here?” she asks. “It’s something the UK are doing at the time of the final coat dressing which is wrong. If it was put down properly so it gave adhesion to horse hooves, it would be fine.”
I guess if it’s slippery to horses and motorcycles, it would also give less traction to other vehicles, or pedistrians trying to legitimately cross a road.
So it may not just be your imagination, or the weather! 