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A comb attached to a small motor and sensor which works like this. Attach the comb to the root of you hair, and start it and leave it in the hair. This moves down a clump of hair using the motor and when it reaches the hair ends, it pulls itself up using the clump of hair like a rope (think of a mountain winch), and move to another location probably an inch away and starts going down the hair. This way you can use it while driving, and probably have to reposition it once or twice to cover the whole head.
As a bonus it could also sprinkle some gel or hairsetting fluid while combing.