Roundabouts are great on back streets where you'd naturally be going slowly, and okay on the meeting point between two major roads, but in my extremely limited

Roundabouts are great on back streets where you'd naturally be going slowly, and okay on the meeting point between two major roads, but in my extremely limited experience they don't work so well in the case where you have a major road (light would usually be green to let large amounts of traffic through) crossed by a street (light would usually be red, intermittently green for a few cars). Perhaps in that case you're meant to use overpasses, or maybe the data says that roundabouts work fine, but they don't seem to.