I have 21 million pop in 1 of my regions, most cities between 1.2 and 1.7 million pop, and just use normal grids. Easy to build a 1 million pop self-sustaining balanced RCI without traffic issues, using grid with all normal intersections, if pattern and balance of RCI works well with the agent system.
Streetcars and mixing RC will keep traffic from causing issues, even with 1 million pop. Using streetcars, just have to realize sims are randomly dropped off, hence patterns must
allow random drops to work.
At 600k pop, most traffic is green with some yellow, with below layout of single city, see pic (only difference is at 600k-1Mk I still had balanced RCI, along a street grid was 2R 2C 2R, instead of now see 5R, and industrial along edge where megatowers are). At 1 million pop, I get some red traffic at rush hours, but all my buildings are maxed green (which is 82-85% happiness high density, and 95-99% low/med density).
T sections, few intersections vs regular are just different tools, what matters is keeping majority of sims from driving by having good patterns for RCI mix, which is vital.
Here is pic of my city, was self sustaining 1 million pop mixed RCI, til I got bored and destroyed CI and replace with R in all my cities to see how many sims could cram in a region, now its 1.7 million mostly R, and region pic with 21 million. But have to design a 1 million pop self sustaining happy city, to be able to get to 1.7 million mostly R.
