Common sense decides that, the same as cars driving dangerously. Swerving, failing to stop at junctions, general dangerous stuff. Im sure that when they were written they didnt have trials and BMXing in mind.. theyre for 'cycling' not 'freestyle mountabike riding', which shouldnt be done on the road anyway, so youve got nothing to worry about. 'Dangerous driving' isnt really vague, anyone with common sense can determine if something is could cause a hazard, and especially a police officer/judge who has the final say in it anyway.