>>But what if a car standing still starts to slide? Will the ABS-system
>>be able to release the brakes?
>
>Why would you want to?  The best way to keep a car still would be to lock
>the brakes.  If the tires don't have enough static friction on the road
>to keep the car from moving, releasing the brakes would only make it move
>faster.
>
>
I don't think this is entirely correct.  The coeffient of static friction
is greater than the coefficient of kinetic friction.  If the car is parked
and not sliding then obviously this is static friction.  Once the car
begins to slide, you now switch to kinetic friction because the tire's
surface is moving relative to the road.  Theoretically, to best try to
regain control, the brakes should release so that the tires can grip the
road and convert back to static friction (the spinning wheel that is
rotating at the same speed as the road it is transversing is working with
static friction whereas the wheel that is rotating at a different speed
than the road is working with kinetic friction).  Then once the wheels are
rotating at the same speed as the road, it should try to brake without
locking up.

Of course practically speaking if the brakes can't hold the car when it is
parked, such as an icy road that is sloped, there won't be much the braking
system can do to save the day.