Hmm, so it only affects transit transport? Hmm, interesting... Though, my jams are not in commercial or residential anyway. I have jams mostly near train stations and on highways (because of the traffic going to train stations).
Well, the jams in my game happened when the commercial delivery traffic met the industrial traffic that hauled basic products to manufacturing industry or city entry/exit points (freight stations, cargo harbor). The logging trucks were easy to see, but basically all semi trucks belong into this category. They don't have any business in commercial districts, but they will drive through those if they lie on their way.
Placement of freight stations can be tricky. I will try next to place them into the industrial district in a way that the traffic avoids left turns.
Up to now, I managed to solve all traffic problems. I know I have a problem in the very center of downtown, because I wanted to have this big boulevard there. It's a deep red in the traffic view, but at the moment, it's still flowing. I hope it won't get worse when I extend the city to the other side. I'm still mostly just using 6 to 7 tiles. In the end, there's always the possibility to just replace some commercial areas with offices and move the commercial stuff somewhere else.
My first use of the "heavy traffic ban" was this area:
I know you are not supposed to build roundabouts like this, but I liked the look and wanted to keep it. Semi trucks were driving from left to right and completely clogging up that roundabout. The ban solved that. The small delivery trucks like the one at the bottom are still allowed. There is some risk of an economic dip if that happens to be the busiest freight route, as it was in my case
. It evened itself out after a while.
Keep in mind that you can also make very small special purpose districts like this one:
It's just that short stretch of road and is supposed to force the trucks on the elevated road.