I would have loved to have taught someone like you when I was teaching DT - I only ever ended up with students who wanted to make something to store their xbox games which they could plaster their favourite football team's logo all over! Well done for having the maturity to try and start something that could actually make the world a little bit better.
Assumingly the project demands you to design something with an electronic aspect to it. The questions of power source up thread are very valid. Everybody who cycles has had that moment where they're about to leave work, school, uni, whatever to realise it's mid winter and dark and they've forgotten to charge their lights so have to wait around for 20 mins to get enough charge in to get home/nip to the shop and spend their last fiver on batteries... Get some extra marks for including some smart materials - reflectives, electronic thread, etc...
The safety question is interesting - Is this to replace the arm out signal, or to support it with extra visability? When I taught Bikeability, a large barrier for some picking up a bike again in their 40/50/60s having not used one since they were young, was the fact they didn't feel comfortable raising their hand away from the bike controls - brakes, steering, etc. Could it be a useful tool for teaching bike training? Taking students out on the road, the trainer's leading and backstopping (doing the functional indicating to traffic) a group of students could show they're indicating even if they're not comfortable to take their hand away from the handlebars - getting them out onto the road sooner to keep the learning experience interesting... Again, could be useful for someone with a disability which makes the arm out signalling motion difficult.
As will become clear, lots of people have tried to do similar things already, putting indicators into bikes, helmets, clothes, etc. And arms don't tend to fail. People here might not realise the limitations of the course you're studying but, if you're mature and keep coming back with progress and ask for advice you'll get loads of wisdom (and some bluntness) on here, which can be used as feedback throughout your project and will go a long way to getting you a mega grade, regardless as to how successful the end product is.