Motorists still tend to blow through roundabouts at speed if they can't see anything coming. From personal experience you can easily navigate a roundabout at 35+ mph.
If a roundabout is small enough, you can't. I have seen many small roundabouts throughout Europe that you cannot navigate without swerving faster than 20-30 km/h (which is assumed safe speed for pedestrian).
https://goo.gl/maps/cmLVxuhViyF2
The space it requires is not much bigger than the traffic light. That town replaced many of its intersections with roundabouts even in central and older parts of the city.
(You might say they went a little overboard with it, looking at the southern limits near that hospital.)