'cul de sacs' are dead end streets with a circular turnaround.
'roundabouts' are circular roads with several entrances/exits - depending on the size, they cam be more or less effective at 'controlling traffic'. The small ones, such that buses can just drive straight of the centre bit - tend to be not only useless but killers, so that many of them locally which replaced traffic lights, have been replaced by traffic lights again!
"I like traffic lights I like traffic lights I like traffic lights I like traffic lights"
Sorry...
The very large ones are actually useful - but basically can be 'Dodgem City' cause Idiots Rule!