I would have a line several feet back from the start line, once the riders assemble there put them under starters orders, as soon as they have approached the tapes and are looking to be stationary, I would release the tapes. If any of them has committed an offence like tape touching, I would allow the race to finish and then exclude them after the race finishes. Anyone gardening or pretending to adjust their clutch would also be excluded after the race, NO EXCEPTIONS!