I could also claim that it’s unsafe to go that slow on a motorcycle due to instability, but really that’s just as true as saying 10km/h is too slow for stability on a bicycle - there is an element of truth in it but we both know it’s perfectly doable for anyone who’s been riding for a while.

To get a motorcycle license in QLD you are required to demonstrate your ability to ride safely at 5km/h. It's not easy, you often need to apply the brakes and clutch and throttle all at the same time and oh yeah counter-steering? It doesn't work at 5km/h. But, every motorcycle rider with a license can do it. And trust me it's a lot easier on a bicycle.

I'd argue if you can't safely ride slow on a bicycle then maybe you should get off the bike and walk across the bridge. Being able to ride slowly is an essential skill especially when you're sharing the path with pedestrians, which as we all know happens all the time.