So because the number of active vehicles is much lower than reality, the road network is actually a lot more efficient: it’s a nearly 1:1 scale city with less than a fifth of the real vehicles that would take up the streets. Mostly because of this, public transport in game is a lot better than our particular reality. In fact, the trains are completely underutilized - we can tweak transport modal share (and should) to get the numbers to balance better.

I suspect it's missing the traffic coming into the city from outside

No, that’s modeled in. Those lanes are always active.