My major objection to helmets, it that they make people think that cycling is a dangerous activity, which it isn't. You really don't need to wear one, no you don't, and I strongly object to the bullying attitude of those who keep insisting that we all should. As al78 has just pointed out "there is no evidence to show that helmets make a significant difference to cyclists safety". If you are not convinced try visiting Denmark or the Netherlands, far more cyclist that you see here, and hardly a helmet in sight, although Bell Sport would love to change all that. [rant mode] Cycle helmets are a con! The Emperor has no clothes on! Wake up and see that you are being fleeced!!
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As for Hi-Viz, it is a good idea in theory, but sadly in practice there are drivers out there who just don't look for it. If you really want to be seen at night (or even in the day) there really is no substitute for a really good set of lights. Of course this doesn't guarantee that you will be seen (some people pull out in front of big red trucks which are lit up like christmas trees, and it pees of the fireman driving it when they do), but it gives you a much better chance...