TP,
Basing low vis ops on ACAS use is another example of people trying to use it for purposes its not designed for.
Display filtering means it does NOT reliably display all nearby traffic. Also, Display smoothing leads to significant lag, which means azimuth plotting can lag badly, this can be a real problem during turn rates associated with ground operations rather than flight operation. So, if you have a high turn rate, e.g. turning the corner on a taxy way (perhaps just before a holding point) the horizontal picture can be misleading. Specifically, an aircraft on finals can look like downwind traffic for a few seconds.
Basically, ACAS can tell you an aircraft is somewhere, but it can not reliably tell you an area is vacant. In fact, the busier the area, e.g. a holding point with a dozen aircraft, all mode S-ing and ACASing, packed into a tight space, the more likely it is the ACAS will go into interference limiting mode and stop full time tracking all the blips, which leads to them being removed from the traffic display.
Hence, in ground ops, ACAS can be used to avoid danger, but it can not be used to confirm safety. Certainly no panacea.
CPB