alexb:
Could well by bullshit, or there might be a byelaw saying "cycle with consideration to..." and the sign is showing what would be regarded as considerate. The implication being that over 8 mph might be regarded as inconsiderate, so not a speed limit as such, perhaps a "consideration limit"?
BTW, speed limits on roads can't be enforced on cycles because the law uses the words "motor vehicle" - there's no get-out for a motor vehicle without a speedo either.
clarion:
Yes, ridiculously slow, you would be overtaken by the average jogger.
Set limits on a cycle facility according to a formula or measurements? You jest, shirley. Like cycle paths have turning radii set by the design speed, have line of sight considered, camber, passing width, etc etc.