I don't see the point in them. If you can get a family bike (be that a trailer, a tandem or longtail, whatever) or something adapted for disability like a handcycle though them, it would be easy to get a motorbike through too.
It seems perverse to block access for the less able against a largely nebulous fear of motorbikes. For instance, I've been using NEPN for over five years and there are numerous points where you can just drive right on. Never seen a single motorbike.
There's a footbridge over the canal just before the bypass which has brutal gates (I got through by grabbing both rails and using my SPDs to lift the bike up and swing it around, and even so it was tight).
About a hundred yards along from this the towpath has level access for motorbikes. Madness.