A Steering Wheel Warrior is the motoring equivalent of a keyboard warrior. Someone who acts real tough in their car but in reality they are actually a big yellow coward. A Steering Wheel Warrior will drive aggressively, beep their horn, wave their arms, flip the bird, scream and shout in their car and generally act like a complete pig towards other motorists, pedestrians, cyclists etc regardless of the traffic conditions.
They are usually very angry all the time and blame others for their problems.
Joe: "I was waiting at traffic lights today and even though the lights were red this guy kept beeping his horn at me to move."
Chris: "Ugh, what an asshole. Sounds like a typical Steering Wheel Warrior."