Roundabouts are a separate component of the NAM. Different variations exist for Roads, Streets, One-Way Roads and Avenues which can be selected individually in the installer.
To trigger the initial Road roundabout, you must draw a Road circle in a 2x2 tile area using the Road network tool. There currently exist many variations with one, two, three or four orthogonal or diagonal Roads stemming from this initial 2x2 tile area.
Street roundabouts are triggered in the same way as Road roundabouts, but using the Street network tool.
One-Way Road Roundabouts
To trigger the initial One-Way Road roundabout, you must draw a One-Way Road circle in a 3x3 tile area. There currently exist many variations with one, two, three or four orthogonal or diagonal One-Way Roads stemming from this initial 3x3 tile area.
Avenue roundabouts are FLEX puzzle pieces which can be found under the Avenue Roundabouts and Interchanges button. After placing down one of these puzzle pieces, you can drag orthogonal and diagonal Avenues into the roundabout in order to connect it with the rest of your network. By default, the Avenue roundabout comes with FLUP support. If this functionality is not required, you can bulldoze the inner 2x2 tile fountain prop after building the roundabout and replace it by any other items. There are also a few Avenue roundabout variations including crossings with Elevated Highway and GLR.
Note that the same button also contains the Avenue Y-Stack interchange and turbo roundabouts.
Turbo roundabouts are static puzzle pieces that can be found under the Avenue Roundabouts and Interchanges button and serve as circular intersections between a double-tile and a single-tile network, such as Avenue/RD-4/RD-6 and ARD-3/TuLEPs. In order to make sure they can be properly connected to your network, it is advisable to bulldoze a 2x1 tile intersection between two suitable networks and plop the turbo roundabout onto the resulting gap.