There are a number of problems with legalism: it often confuses means and ends; it insists on black/white when a matter is gray; it majors in the minors; it usually leads to self-righteousness, judgmentalism, and nasty comments; and so on-- and if it's a lifestyle, it will take you to a small life.
Another tough thing about legalism on a topic (or being a legalist in general). If you get dogmatic, you're vulnerable to the realities of an "X - 1" argument. Here, if driving 56 is unsafe and sinful, then you should drive 55. But really you should drive 54 or 53 or 50...or just stay at home.
The first time I remember something like this: For a time, my brother had become convinced that kissing a girl before marriage was always a sin. After trying various angles, I settled on pretending that holding hands with a girl before marriage was always a sin-- and that I would judge him for holding hands.
One prominent aspect of the ministry of Jesus was dealing with the professional legalists. Think of all of the healings on the Sabbath. They provoked him to righteous anger. Think of Jesus noting that the Pharisees tithed their spices but missed justice. And so on.) If you find yourself hanging out in L territory, be really careful!