This recipe was created by Todd Wilbur for his book "Top Secret Recipes Unlocked" (Penguin/Putnam)
Very simple, very tasty. I used Miracle Whip instead of mayo due to allergies. If you do that, you might want to taste it and see if it needs a bit more horseradish. Gets even better if it sits in the refrigerator for a day.
Excellent sauce, just like the stuff they serve at Arby's!