I once heard a man tell a story of what happened to him while driving, a woman in a red sports car tried to cut in front of him as he was driving to work (he worked at a local hospital), he got mad, and he veered, heading her off and almost causing an accident, she got on the shoulder and maneuvered her way forcefully in front of him and got away. He fumed all the way to work. When he got there, he spotted the red car in the parking lot and went into the hospital looking for her to give her a piece of his mind, but stopped cold when he found out that on the back seat she had her little boy having a very dangerous asthma attack that couldn't be controlled at home. She was driving him to the ER.
Not everyone who cuts in front of you wants to take advantage or ruin your day, just ignore them, chill out!