[botro]

And to take a historic analogy, cars today are as wide as they are because that's about how wide a single lane roadway is. And a single lane roadway is as wide as it is because that's about the width of two horses drawing a carriage.

[falcor84]

The story goes that this two horses width also limited the size of the space shuttle's boosters (SRB), so we ended up taking this sort of path-dependence off to space.