I was very surprised when re-reading "The roads must roll" not too long ago (it's been a while since I read it before that) and I came across the description of the tumble-bugs... except for them having only one center wheel vs. the two side-mounted ones of the Segways, they seem to be pretty much the same thing. Looking at those similarities, I was not sure if this was more of a prediction or an actual invention made by Heinlein - though I'm leaning towards "prediction" now: it's simply the smallest design you can come up with for a person to ride more or less safely - a platform to stand on, a handle to hold on, and some stuff underneath/around to make it go (to house the engine/power source). Personal mobility can not really get much smaller than that - imagine how today's cities would look if all trips not requiring a roof and/or storage were done on those little critters
