As for the problem of motorcycles on cycle paths, if these are really a problem* then why not adopt the design used on rural bridleways of the muddy, horsey type: a padlocked gate which can be swung open for maintenance access, but in its closed position has a large cutout with a lip a few inches high. Just high enough to stop a motorbike (or other wheeled vehicle) crossing it, but easy for a horse to step over. Obviously, you can't cycle over this - but it's quite easy for most people to lift most cycles over, even a tandem. It would, of course, still be a problem for many disabled people unless they had an able-bodied companion, and might require unhitching a trailer. It's far from perfect, but it's got to be a lot better for cyclists (and cheaper to install) than those spirals (or even lots of bollards and chicanes currently in use).
*Having used the Bristol-Bath path off and on since 1986, I've only once seen a 'live' motorcycle on the path, and that was chugging along slowly and respectfully with a fisherman on its back (might have been a bit lost, actually). OTOH, I've seen lots of scooter-shaped charred tar areas - always in the same districts, too.