Automatic gear change for bicycle. Rotation sensor (magnetic or optical, counts teeth) on chain ring. Optimum pedal speed = 90 r.p.m. tired/fresh, athlete/slob, makes no difference. Gear changes to keep pedal speed optimum. Rear (and front maybe) mech has actuator (motor and screw on parallelogram) and position sensor. Box for electronics, battery, control. Action inhibited under back-pedal or no-pedal. Pedal slowly under free-wheel when approaching junction to set to bottom gear.
One less thing to think about while avoiding death by automobile.