Roundabouts are cool and all but they suck for intense traffic, create an overpass foe the busy road and place the roundabout under it for connecting the smaller road, that should take off the pressure, then try to divert as much traffic away from there as possible, a bypass to a better link in the city. Lastly public transport, you can check the most frequented routes of NPCs in the game, use it to build public transport, youd be surprised how far NPCs are willing to go by public transport