That's a very good question indeed and a very difficult one. Personally, I think that saying that ехать is about a vehicle is a vast oversimplification, maybe even a disinformation. In my opinion, It's not really about a vehicle, it's more about moving without using your legs. E.G. you can very well ехать на коне, the horse will obviously use its legs there, but you will not, so you say ехать. Another example from my childhood: riding down a snowy icy slope on your buttocks - ехать на жопе, съехать с горки на жопе - forgive me my language here Just note that ехать is about ground movement obviously.

Coming back to your example, your legs разъезжаются because you don't personally move them and they slide on their own uncontrollably Ноги разошлись... I suppose you can say that too.

P.S. Same can be said about ходить. Claming that it's about moving using your feet would be an oversimplification as well. It's more about moving with intermittent "stages", steps so to speek. For this very reason: часы ходят/идут, its hands move intermittently, step by step. Same for время in general, of course, its movement is technically continuous, but people look at it through hours, minutes, seconds etc.