I know this cop and he's a decent guy. I think I caught him off guard with this question, but he was having a little fun with it, as well as I was. But it really is a question which has an answer (somewhere). I assume those signs have user manuals, and there must be a mention of using them for speeds over 99. Normal drivers wont experience that speed, but if those same signs were used at a race track, they would not work, since many race cars exceed 99mph. Since truck and tractor pulls uses these signs, I would think that race tracks would use a similar device, but obviously need one that allows for 3 digits. (I assume they make a 3 digit one for such uses).
I wish I knew the manufacturer of the sign, so i could see if there is a user manual online.....
Till then, I'll go along with the "roll over to 00" !!!