I don't know if waiting your turn is essentially a grown up characteristic when the wait might take forever. If everyone is waiting in line but the person at the front lets everyone coming up on the right in front of them, then you will wait forever. It is sometimes common sense to save yourself the time of waiting. True, if everyone has this mentality then many accidents will occur, but sometimes necessity warrants cutting in line. Maybe this person really had to pee or their babysitter called and there was a problem at home.
Like I said, even if there was no emergency, it's equally the fault of the person letting this driver in. Additionally, if everyone in a line of cars had to slam on their breaks then they are equally guilty for following too closely and not paying attention.
If I'm waiting in a line and I see people cutting ahead, sure I get a little pissed that I have to wait longer, but I get more angry at the people ahead of me letting them in.