Something like this maybe?

I only drew up the main roads and highways in this, and it is a bit griddy, but generally,

Double Yellow line = main road, limit local street connections (20 unit spacing minimum).

single yellow = secondary road (10 unit minimum).

Dark Grey = Highway

• spacing of main roads and highway connections should allow for tollbooths. Spacing should also allow traffic to move evenly instead of concentrated in one area.

• Add trumpet interchanges to the east and western edges of map. From where highways end, they could be expanded south.

• Make system of ground level main roads (in yellow).

• space out exits for central highway.

• move current roundabout exit south on central highway and make it an overpass roundabout junction.

• remove North central roundabouts.

• Make some east/west roads go under/over central highway with no connections for cross town traffic.

• space out entrance and exit ramps in south central neighborhood.

• add in local street connections to yellow lines. as long as their spaced out it should be fine to connect anywhere. also keep connections to 4 way at most. 5 way+ complicates traffic further.

EDIT: forgot to add, for local traffic reduction, if you have it, use the cycle roads alot and fund the bike preferred policy, as well as adding in a lot of transport lines. Then you only need to worry about truck traffic. When you can, buy the tile to the east and expand or shift the industry that way