When the car lanes are too narrow -as here, on Whiteladies Road, where two southbound lanes try and squeeze alongside the bike lane, the bike lane suffers.
It was too narrow a lane anyway, you shouldn't really use as it is too dangerous. Locals know that; it's documented on Bristol Streets. But what about visitors -how will they know it's dangerous?
That's where the self-degrading paint comes into its own. If you paint a bike lane in a paint that is strong enough to withstand bike use, the lane stays red -bike lane. But where cars, buses and here, ambulances, go down the lane, their accelerating and braking tyres scrape off the red, leaving black underneath.
This provides a valuable cue to visitors that this is not really a bike lane, and should be avoided unless they are tired of life.