This is actually not true. The vehicles DO take traffic congestion into account when generating paths. They are calculating and figuring that that particular route is STILL their best option despite the congestion. The screenshot appears to show what is called "weaving". Something is causing the cars merging in to want to change immediately to the middle lane. To be able to tell what, I'd need to see more of what you've built.

Is there an on or off ramp just off the screen? Cars move over to avoid blocking them, which may cause this behavior.

@SimonW You click on the 3D view

Nope. Think of it like a "+" sign verses a "T" shape. You can turn left from any of the "+" sign's arms, but on the top of the "T", there's nowhere to turn left when coming from the left side.

Thinking about it some more, I'm actually not sure why this is happening... Under high crown authority any character whose inheritance would cause you to lose land to a foreign realm should be disqualified from succession. Do you happen to know the crown authorities of the realm you took land from/ took land from you in these situations?

Looking at that, I think its just that that article on the wiki is poorly worded and vague. Let me write up an answer that I think is relevant to what you're experiencing.

Could I have a link to where in the wiki you're looking?

Were your crown authority laws the same in both instances? What was your character's rank in each instance? If you were a king or emperor in the first instance, but a duke yourself in the second, that could explain this behavior.

Edited to clarify

How much in-game time has passed? I don't recall the exact amount of time it takes, but IIRC the game only recalculates succession every so often. You can also force it by typing "recalc_succ" into the console.

Yes. Once the law has been passed, changing religion will not nullify it, even if being of that religion was originally a requirement for passing it.

Yes, but you don't have to connect every small road to the larger road. The small roads can branch off each other without much hassle because they shouldn't be handling lots of traffic. So what I'd do is eliminate three out of those five intersections of two-lanes to the four-lane, then have those two lane roads branch off the other two lane roads.

The only thing I see that I would change there is, again, too many intersections too close to each other. There's nowhere for the vehicles to actually sit and wait for a light to change without clogging up another intersection.