21st Century Smart Cars Act This bill directs the Department of Transportation (DOT) to prescribe federal motor vehicle safety standards that (1) establish minimum performance requirements for crash avoidance technologies; and (2) require all new passenger motor vehicles manufactured for sale in the United States, introduced or delivered for introduction in interstate commerce, or imported into the United States, to be equipped with the crash avoidance technologies. DOT must also conduct advanced crash systems research into (1) direct driver monitoring systems that will minimize driver disengagement, driver distraction, prevent automation complacency, and account for foreseeable misuse of vehicle automation; (2) lane keeping assistance that assists with steering to keep a vehicle within its driving lane; (3) automatic collision notification systems that notify emergency responders that a crash has occurred and provide the geographical location of the vehicle and crash data in a manner that allows for assessment of potential injuries and emergency response, and that transfer anonymized automatic crash data for the purposes of safety research and statistical analysis; and (4) intelligent speed assist that determines the applicable speed limit where a vehicle is operating and alerts the driver to the current speed limit and discourages exceeding that limit.