The 20 metre distance came from the fact that a horse would not back up more than that distance so it is a quite arbitary figure. You should be able to risk assess that away.
Many people, probably much more qualified to make these decisions than you or I, contributed to the 2006 ADB. Why did they choose to keep the turning circle in if it is not actually required or helpful and it can be just simply be 'risk assessed away'?
I dislike the idea of simply risk assessing something away. If the process of risk assessment
proves that something is not required then that is fair enough. But to intend to get rid of something that would generally be required just by being a bit of a smart arse with a few excuses and a bit of 'management' on paper, (That is not directed at anyone in particular, just a generalisation) is not a good solution in my eyes.
Wee Brians and John Webbs suggestions seem much more in keeping with meeting the functional requirements.