> I think it's pretty widely acknowledged that speed limits are not effectively enforced on cars.

No it's not, unless you have some study to back it up because it's not my experience.

(edit: just to be clear, the spirit of this comment is not to dismiss anything that isn't backed by studies, sometimes simple observation of the world around us is enough to reach solid conclusions, but I disagree that speed limits aren't respected and enforced. Could depend on where you live as well.)