What you say is true - city driving is complicated with too many decisions to make, but it does not need a very optimized controller to control the car through

What you say is true - city driving is complicated with too many decisions to make, but it does not need a very optimized controller to control the car through those scenarios. Changing a lane at 15-25MPH is far more easier than on a turning highway with merging traffic at 55-60MPH. At that speed the controller would not only need to safely execute the maneuver but also stabilize the vehicle in the event it needs to come to a halt or take an evasive action. Moreover if there is say some uncertainty in the sensor readings at 25mph, that is multiplied by many factors at higher speeds and becomes very significant. Control at higher speeds has more stability issues and can deal with less uncertainty and than at lower speeds.