Whilst cycling over the years, I've had quite a few close calls with manhole covers. When wet, or polished to a mirror-finish by thousands of passing cars, these can be as slippery as... well, I can't think of a colourful simile: as slippery as something really quite slippery that definitely shouldn't be in the middle of the road, is what I'm getting at.
One of the big problems is that because all manner of pipelines and conduits follow the same courses as the roads, manhole covers are particularly prevalent at junctions: they meet one another underground just as the roads meet one another on the surface. This is a nuisance, as junctions are already a threat to vulnerable road users and we don't need them to be made any worse. Slippery bits at a junction are a particular hazard as the turning forces make cycles and motorcycles ever more likely to fall.
In 2005 I carried out the Oxford and Cambridge Cycling Survey, and the responses we received were full of reports of people coming a cropper on manholes, so I'm very excited to learn that slippery manhole covers could soon be a thing of the past across Europe. But it's really interesting to consider why they have existed as long as they have. The clear explanation is that small slippery spots on the road just don't pose a threat to cars - if one wheel is on a slippery spot, the other three will compensate - especially on modern cars with clever traction controls that will quite literally compensate for the lost grip under one tyre. Since cars aren't affected by little slippery spots, Mr Average Public has never seen them as an issue.
When you ride a single-track vehicle, however, such as a bicycle or motorcycle, I can assure you that small slippery spots on the road are really quite a big deal. It's pretty difficult to escapable the conclusion that the problem would have been sorted decades earlier except that it was a problem only affecting minority road-users. It's one more reason why everybody should have to spend a few months cycling before they are allowed to drive. Yes, my friends: that's the master plan and I'll explain it another time.