This has worked really well for me. On my last city I started playing around with a design and was still running smooth up to 100k people so redid it in the center of my map on my new city.
The roads are one way in and one out to a highway.
The bottom rows are for bus stops that go all over the city, and all pull off on their own sideroad to pick up people to get out of the way of other colored buses.
4 subway lines that go to each part of town, and 4 train stations that do the same.
People using mass transit all pass through this 'hub'. And have one bus loop that does nothing but trolley people around the hub stopping at the stations and the subways.
It seems to work really well. on the 100k people map I had so many packs of cims walking all over that areas zooming in all the way my FPS would drop