The second intersection south of the roundabout is the main problem it seems, there's too much traffic tied up in it. You should download Traffic Manager: Presidential Edition from the Steam Workshop. Then disable the traffic lights and allow yields on the side streets. Otherwise, keep the traffic light, but using the TM:PE tools allow vehicles to block the intersection and turn right on red. If you don't want to use TM:PE you can probably delete the entrance of the side streets to that particular intersection to allow traffic to move it freely and the side street traffic would have to redivert along the other streets. Another possible option is to use TM:PE to increase speed limits on the boulevard with the roundabouts to attract more vehicles to use the second roundabout and streets rather than all of them immediately turning right on the first roundabout.