12 mph is an odd speed to choose given legal e-bikes in Europe are limited to 15.5 mph for electrical assist. Might as well either make it 15.5 mph if the road is open or say something like 5 mph if it's a pedestrian zone.

By the way with 5 mph you wouldn't enforce it accurately as there are no speedos or speed cameras for bikes but it makes it clear if you are going 15 mph you are breaking it. That's what the have for the shared walk in Regent's Park for example.