Hey I ´m glad you´re considering a CAR AI solution for UE4!!
My personal take on would be, as you mentioned, a solution for general traffic management with:
- Habillity to have vehicles go from point A to point B and loop on that circuit;
- The option for vehicles to randomly choose diferent directions based on a array of options;
- The option to manage how many vehicles we want to have on the level via UMG buttons/slider;
- The solution should consider killing/ destroying the vehicles+AI as soon as ther´re not needed or visible anymore, thus keeping frame rates/memory civilized;
- Stop/go on traffic lights;
- Give way to pedestrians on crossings;
- Blinking when changing direction/lanes;
- Turn on head/tail lights (not too AI related I know…)
- Avoid obstacles such as other vehicles and have the hability to wait the a car in front of them is turning/ changing directions;
- And like pointed out: *The the AI logic when a turn is near, and decide if it should overtake a car when a turn is near, taking into account the it could be danger.
A lot of the above I had going in UDK using the AI nodes in kismet right out of the box. IN UE4 since Epic hasn´t done anything with vehicles since 4.2 and with zero AI support for cars…I crawl into my little corner and cry… 
So if You decide to take it into your own hands, l Newton…I´ll be your 1st customer Yehhaawww