Off-road ABS
Some off-roaders turn ABS off to improve braking, for example by removing the ABS fuse. Is this reasonable? Not on hard surfaces. But on surfaces with moveable material, e.g. snow, gravel, lose dirt, a locked wheel can build a wedge in front of it and thus increase grip and reduce stopping distances. A typical traction coefficient on a lose surface, with ABS modulated wheels, is 0.2 and it can rise up to 0.3 with complete lock.
How does the M-Class off-road ABS program work?
On road surfaces with good traction it works in the same manner as Mercedes passenger cars. But when off-road in low range, the ML can change into an off-road ABS mode if it detects rough road conditions by sensing uneven wheel rotation and the speed is below 30 km/h - 18.5 MPH. The brake apply phase is lengthened so the front wheels can "dig in". At higher speeds the off-road situation is also monitored, but it wont change into the off-road ABS mode until the speed drops below the limit.
Australian off-road ABS mode
Australian gravel roads turned out to be so smooth that the system would not reliably detect offroad conditions from jerky wheel motions. So in 2004 Bosch engineers in Australia developed a new optional algorithm especially for these conditions. Wheel slip is allowed to lengthen from the usual 10% to 40% on gravel, also depending on whether the car is in a curve. Why didn't ABS work when I slowly slid down my plowed driveway?
ABS doesn't work under 5 MPH / 8 km/h and allows wheel lock, similar to the off-road ABS mode.