Man, come on. That zipper merge thing only works if everyone knows the rules and actually lets people in, which they don't. You got flipped off because from that grandma's view you just cut the whole line like a jerk, and honestly she's not wrong. Think about it from her side, she's been sitting in that lane for ten minutes while you flew past everybody. Most places don't even teach the zipper thing, so it just looks like cheating. The "trick" only saves the rest of us time if the whole road is in on it, and it never is.