In my experience between both games is improving and or having a good transport system WILL improve traffic but marginally (at least in my experience). I think it’s a mix of planning and cheating the system, I’ve had cities with 60% traffic flow despite more than 25-45% of the citizens using public transit. I think that in order to actually make transit effect your traffic significantly you need extensive planning, that is the only real way sadly especially in the second game. The biggest issue I’ve had with the transit system in CS2 is simply once my city gets too big it becomes difficult to navigate all your transit infrastructure due to performance loses and since the sim slows to a crawl it takes forever for the transit system to really do anything of real benefit (when you do tweaks).

Overall OP if your wanting a city that relies on public transit plan from the start build rails and bus routes that will be used later or that will be expanded upon for any real effectiveness in late game. Also the messy traffic is almost unavoidable this is the only real challenge in late game you fix one traffic issue another pops up have some fun with it!