Car a is doing 30mph. Car b is doing 60mph because that's around the norm for that road and that's what the driver is expecting. Car b is going to catch up on other car a very quickly, especially if there's any sort of obstruction, like a bend.
Erm - where to start with this one? 
OK here goes:
It isn't driver a's fault if driver b catches up with them so quickly it is likely to cause an accident. Driver b should bone up on their stopping distances. My instructor taught me to think of the car ahead as a potential brick wall. If they suddenly stopped dead, would you be able to stop in time?
If it is a straight bit of road, then driver b should be able to see car a from quite a long way away, giving plenty of time to judge car a's speed and to slow down.
If there is any kind of obstruction visible ahead (a bend is an 'obstruction' - really???), then driver b should have seen it and slowed down already. Then, coming upon car a after the bend, they would already have anticipated a possible obstruction and wouldn't have to suddenly react to it.
Driver b shouldn't be driving round a bend too fast. If they can't see far enough round the bend to know that the road ahead is clear, then they need to slow down.
Driver b shouldn't be expecting other drivers to behave in a certain way. Never assume anything, especially not when you are driving at speed.
You can't either expect or assume the road in front of you to remain clear. You need to anticipate potential problems and give yourself time to respond.
(And yes, I do often drive at 60 on the country roads round here. I did today. But only some of them, and not all the time - I know them too well for that).