Yes when Millennium Force initially opened in 2000 you had to be 54 inches tall to ride. I think they changed that rule in 2002, because I have "The World's Largest Amusement Park" video which was made in 2001 and the ride op. says you must be 54 in. tall to ride. I don't know about TTD, it's a possibility I guess. But they don't just go by height. They say "ok, the average person that's 48" tall is so and so weight and so and so dimensions." The only thing I don't get is why some parks have identical or near identical rides yet won't let the same people ride them. I remeber when I was little I could ride the Wild Mouse at Dorney but not at Hershey (might be the other way around i'm not sure...) Another one that gets me is that you have to be 54 or 52" to ride S:RoS at SFA, but only 48" to ride SFDL's version. But I'm sure that'll all change now....