If you go to the UI editor and get the AI app, you can set the speed limit for every car. It's usually set to around 40 MPH. This works if you want to do highway driving, but they might speed on side roads.
Dynamic speedlimits would be pretty cool. Right now when you enter a highway you can increase the speed limit in the traffic debug app accordingly, but it would be nice if traffic adjusts its speed to the type of road: 100/120 for highway; 80 for country road; 50 for city/village roads; 30 for dirt paths. Also, I think the AI risk parameter should be increased a bit, since the AI sometimes slams the brakes on a slightly curved highway. Back in the traffic tool mod, I changed the risk parameter in the main LUA from 0.6 to 1 to increase traffic flow
I would also be nice for AI to be able to accelerate and brake realistically (like using pedals vs the up and down arrows on the keyboard) and not just 100% all the time.