Why Forward Must Be Up

When signs need to indicate a forward direction, they typically use an upward pointing arrow. "Forward" is actually perpendicular to the face of the sign, so an up- or down-arrow is equally wrong. However, roads leading away from you rise in your field of vision as they get farther away, so an upward vector is the more sensible choice.
I've wondered for years about the apparently arbitrary "up equals forward" convention, and I just today realized this satisfying explanation.