Indeed it is true on gravel, sand, and other loose material.  In my experience loose gravel stopping distances are more like 2x not +20%.  That experience is mostly on dry desert gravel and sand.

An off-road vehicle should have a manual or automatic means of disabling the ABS system.  For example, locking any of the differentials on my truck automatically turns off the ABS.

...Alan

-----Original Message-----
From: piclist-bounces@MIT.EDU [mailto:piclist-bounces@MIT.EDU] On Behalf Of Apptech
Sent: Friday, January 25, 2008 11:08 PM
To: PIC List
Subject: [EE]:: ABS increases stopping distance by typically 20%+ on gravel surfaces

A competent authority has assessed that ABS brakes INCREASE stopping
distances on gravel (metal) surfaces by typically 20+ percent.

        http://en.wikipedia.org/wiki/Anti-lock_braking_system
A 2003 Australian study[2] by Monash University Accident Research Centre
found that ABS:

  a.. Reduced the risk of multiple vehicle crashes by 18 percent.
  b.. Reduced the risk of run-off-road crashes by 35 percent.
On high-traction surfaces such as bitumen, or concrete many (though not all)
ABS-equipped cars are able to attain braking distances better (i.e. shorter)
than those that would be easily possible without the benefit of ABS. Even an
alert, skilled driver without ABS would find it difficult, even through the
use of techniques like threshold braking, to match or improve on the
performance of a typical driver with an ABS-equipped vehicle, in realworld
conditions. ABS reduces chances of crashing, and/or the severity of impact.
The recommended technique for non-expert drivers in an ABS-equipped car, in
a typical full-braking emergency, is to press the brake pedal as firmly as
possible and, where appropriate, to steer around obstructions. In such
situations, ABS will significantly reduce the chances of a skid and
subsequent loss of control.

In gravel and deep snow, ABS tends to increase braking distances.

On these surfaces, locked wheels dig in and stop the vehicle more quickly.

ABS prevents this from occurring.

Some ABS calibrations reduce this problem by slowing the cycling time, thus
letting the wheels repeatedly briefly lock and unlock. The primary benefit
of ABS on such surfaces is to increase the ability of the driver to maintain
control of the car rather than go into a skid - though loss of control
remains more likely on soft surfaces like gravel or slippery surfaces like
snow or ice. On a very slippery surface such as sheet ice or gravel it is
possible to lock multiple wheels at once, and this can defeat ABS (which
relies on detecting individual wheels skidding). Availability of ABS
relieves most drivers from learning threshold braking.

But part of the answer is that on heavy snow, locked wheels can be useful
because they gather up a "wedge" of snow which helps to slow the vehicle.
ABS allows this wedge to clear every time the wheels are unlocked. The same
can apply on sand in some conditions.

A June 1999 NHTSA study found that ABS increased stopping distances on loose
gravel by an average of 22 percent [1].



_________



Very interesting paper on ABS brake accident stats.

No great relationship to gravel roads.






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