Indeed, the friction coefficient is different in case of static (stiction) and sliding contacts: https://en.wikipedia.org/wiki/Friction

I was curious about your statement, so I looked it up:

> ABS increases stopping distances on surfaces covered with gravel, snow, or other loose materials. In such situations, a locked tire digs into the snow or gravel, pushing it forward and forming a wedge in front of the tire, which brings the vehicle to a stop

Source: https://itstillruns.com/do-brakes-work-ice-snow-6162289.html