On a large proportion of roads in England that you'd normally cycle on, cat's eye reflectors are usually like this:-
OK, they're not often blue, but they're usually low, square, smallish and made of rubberised plastic. Quite cute really. A bit of a bugger if you hit one at speed, but probably recoverable for most people.
This is what you get on older motorways, major multi-lane roads and the road I crashed on near Galway:-
Tall, mean metal and glass buggers, with a blimmin' huge metal housing around it and a ridge of tarmac. Not so cute. Yes, the bit in the middle is designed to compress downwards when a car goes over it, with the effect being that wipes the glass bits and keeps them cleaner than they might otherwise be. Can you guess what happens when you're cycling at a reasonable speed (around 35km/h, say) and your front (or back) wheel strikes that ridge in the metal housing? I'll give you a hint...
That is all.