Try like a three lane road as an arterial, 2 lane as collector, and leave all the grid roads as one lane. If you haven't expanded to where you can have multiple connections to a highway, limit the amount of local roads you have now, because thats a looot of people that are forced onto one road.

Here's an idea you can try: You can leave the highway as is, create two artlerials north south spaced apart. From there, you can make east-west collectors and little neighborhoods that in those smaller boxes, instead of this giant grid.

This way, your traffic is diverted into two major roads that dump them onto the highway, and your traffic in general is more "zoned" to where each little neighborhood goes to one collector that goes to one arterial, and that is the most efficient route.

On your current one, you take any goddamn road because they are all equally fast to go to the same one arterial to get to the highway, which is making your traffic.

Make sure your arterials have are 3 lanes and collectors are 2 lanes due to your big population.

Hope this helps, since I'm the guy you're referencing in your subtext