This study, although about cycling, I think describes why people are prone to road rage. Drivers don’t match their style, and not all drivers are very confident, so they’ll react to others who don’t drive in a way they feel fits in.
EVERYONE IS A SCOFFLAW. Drivers, pedestrians, and cyclists are all breaking traffic laws. However, it is important to note that in many cases, cyclists are making rational choices to break the law given the existing bicycling infrastructure and their level of experience.
For cyclists that stop at all stop signs while cycling, they are more likely to be angered when they see a cyclist who does not. However, if a cyclist engages in an illegal behavior they are less likely to be angered by seeing a cyclist do the same.
Increased cycling frequency reduces the likelihood of an angry response, as does increased confidence as a cyclist. The anger in the response is associated with how one would behave in the same situation.
Then again, some people get “road rage” just because their natural state is perpetual irritability or ‘rage’.