I doubt very much the 15 to 20 mph suggested, it may look like that sort of speed to you stood watching but likely to be 5 to 10 mph. At the old venue I attended on a number of occasions with CET and M578, finding someone who was prepared to walk 1/2 a mile from the camping areas in front of a tracked vehicle was difficult, near imposible.
You clearly have not experienced driving or applying the brakes on any tracked vehicles... at these sort of speeds they will stop on a pinhead without skidding like a wheeled vehicle might.
What is the walker in hi vis supposed to do, he is looking forwards where he is going he cannot see what is happening behind him, it is the drivers responsibility to observe and react to what is happening around him.
I can see no problem with a quad bieng used instead of the walker.