One possibility is the car has run flats and the tyre has no air left in it, the speed needs to be kept low and the hazards are a way of advising people to be wary. If this was the case then it seems quite sensible to me.
I agree Shamus, ordinarily I would have over taken it. Trouble was, somebody at the hire company left the oil cap off my brand new car with oil being blown over the engine so I didn't want to kick it down (it's an auto) for such a short distance to the exit. I was on my way back to drop it off. There was a fair distance to begin with and I slowed down to match but he kept going slower and slower until we got down to 40 and I was getting closet to him. I didn't get angry with him but he didn't know my reason for not overtaking and I didn't know his for progressively slowing down.
Same van, same idiot with fogs on - must be an inbred bumpkin