yup most ppl dont enjoy getting honked at.
Also who said being impatient is not an aveirah.
I would think that impatience is a form of ka’as (anger).
When you have feelings of impatience you are annoyed, you are angry.
Chazal have very disparaging things to say about ppl who become angry.
Even if you disagree with my above speculation, most of the time the guy doing the honking is pretty angry at the person in front of him.