James, the longer the debate continues the more I think that the OHS/Safety advocates may need to step away. Regardless of whether a CPD increases or decreases risks and to what degree, here is what I (a motorcycle/scooter but never a quad bike rider) have learned about using quad bikes after reporting on the hazard for a looong time:

Wear a suitable helmet, and other protective equipment suitable for the work task,
Think about what you will be using the quad bike for, before you get anywhere near it,
Assess and maintain the quad bike in the same way as any other farm vehicle,
Undertake training, and refresher training, to make sure that I use the bike as designed,
Keep the quad bike as a work vehicle and only let suitably trained workers use it,
Fit a suitable Crush Protection Device.

But above all, the most important lesson is DON’T USE A QUAD BIKE, as suitable and safer alternatives are available.