They already stay in lane, and adhere to speed limits. What else could you want? Give ways (yields) and roundabouts will be a challenge for AI.. however stock maps don't really have many of these. Perhaps just stopping at red lights will do the trick?
You forgot to mention the AI can now indicate(poorly). Stopping at red lights would be quite nice and would add some realism to driving in Beam, but I'm not sure how to do it.
It would be nice to have them obey traffic lights in particular now some maps have working ones, maybe stop signs too. A bigger annoyance for me is the messed up ai paths in a number of places on the built in maps and that fact that the ai really can’t cope with narrow roads. The paths on West Coast are messed up all over the place, causing the ai to drive on the wrong side or randomly u-turn. The ends of the main highway in Italy also have some weirdness causing ai to swerve onto the wrong side. These should be relatively simple fixes for them to implement. The narrow road issue is trickier, but often despite two cars being able to fit the ai instead drive at each other, stop, then back up into something.
I also think the traffic ai shouldn't spawn so many cars on narrow roads, you can use only the main roads in Italy when traffic is enabled