Traffic circles where I live are a complete boondoggle. They are seen by city planners as a wonderful, progressive, very forward-thinking concept. And I have no problem with them in theory. But when they convert a normal "four-way" intersection to to a traffic circle it always seems to take about a year, is built far larger than necessary, requires annexation of pieces of private property around the perimeter, and features a large center island with trees and ornate landscaping.
I can't see a reason it should take more than a few weeks to round out an intersection and remove the stoplights/stop signs.
There are reasons to make roundabouts with large radii. Ironically, some of them involve traffic-light controlled roundabouts, where traffic queued at a red light queues back around the roundabout. The consideration here is that one makes the circumference of the roundabout greater in order to provide more queueing room before the queue for one junction backs up beyond the preceding one.
The government just upgraded a traffic-light-controlled roundabout where I live, and enlarging the circumference in order to provide more queueing space was one of the considerations mentioned in the public consultation doco.
Also bear in mind how large, possibly articulated, vehicles have to navigate roundabouts versus how they can navigate box junctions, especially since the approaches to roundabouts by design slew approaching traffic to one side. (This is a consideration that for smaller vehicles brought about the idea of the mini-roundabout, and is also behind having overrun areas around reduced central islands.)
I can't see a reason it should take more than a few weeks to round out an intersection and remove the stoplights/stop signs.