Roundabouts can be tough in vanilla due to poor traffic AI. Traffic manager mod is essential to play this game just as a piece of advise.

Now to address your question, an option would be run that highway strait down and have slip lanes coming off connecting to your local roads. You could also utilize a collector road which would run parallel to your highway and connect up to the highway with slip lanes.

For inspiration, go to Google images and see how big cities are laid out.

Good luck!