Beginning with the 1989 model year, some Jetta models are available with an anti-lock braking system, widely referred to as ABS. As the name implies, the purpose of this system is to prevent the wheels from locking up during hard braking. Speed sensors at each wheel sense when the wheel is about to lock, and an electronic system modulates the braking force to that wheel. Contrary to popular belief, research and testing have shown that the tires brake most effectively just before the point of locking up and skidding. Preventing the wheels from locking maintains directional control, and is especially helpful on slippery roads.