They generally don’t.
Roundabouts are hostile to pedestrians as a consequence of their design, as they are an attempt to keep traffic flowing without the interruptions that people use to cross the road. Drivers also accelerate away from them, which means that the natural place for pedestrians to cross the road isnow the least safe option. Combining lights with roundabouts just gives you the worst of both worlds, as you now have interrupted flow and drivers accelerating out of turns right into the crossing.
Really, roundabouts should only be used for medium density junctions on roads with no pedestrian access. They can handle fairly large volumes of traffic without having to build a grade-seperated junction.
