Newton's first law says that without external forces, your velocity doesn't change. Remember that velocity is both how fast you're going but also what direction you're going.
Normally, a car turns because the tires on the ground have traction so they have friction which allows them to change direction and turn. But on an icy highway, tires can't get much friction so it's really hard to turn.
Because the ice stops friction, a car can't get enough force to change direction and follow the curve, meaning it will try and stay going in the same direction, which is dangerous.
