1) brake as hard as it is possible to do safely, memorize as many details as possible about car and driver, and take the best evasive move available. 2) Draw my own weapon, and place at some kind of low ready position, still concealed from view of other drivers. 3) continue to observe the other guy as long as possible, pull to side of road quickly, call 911. 4) Wait for police, reholster handgun quickly and quietly as soon as I see them show up.