2. "Cops are cracking down"
Unless you live in a place with amazing cab/Uber access or a great bus or train system that keeps you out of cars more often than not, you're depending on a designated driver or yourself to get home.
Roll the deep-voiced "cops are cracking down" commercials. The Department of Transportation has an entire Traffic Safety Marketing branch dedicated to this sort of thing, including a whole campaign focused on St. Patrick's Day alone. Sure, it involves that terrible reboot of RoboCop from last month, but it has a point.
This is just about the last day of the year you'd want to be on the road for any length of time, never mind pulling out of the parking lot of Billy O'Ray's on Route 46. According to the National Highway Traffic Safety Administration, more than 269 people were killed by drunk drivers on St. Patrick's Day over the years 2007-11. There were 45 people killed in 2011 alone.
You don't get points for not being the drunken one in that equation. Even if you're stone sober and haven't been to a bar all day, the increased police presence makes it about the worst time to have a taillight out, have a car overdue for inspection, fail to signal when switching lanes or have an outstanding parking ticket when the cruiser that suddenly appears behind you decides to run your plate "just in case.' It's a hassle you don't need.