There are a lot of bike accessories that will let cars know you're there. But it can still be hard for them to tell whether you're speeding up or slowing down — which can make it tough for even a well-meaning driver to keep a safe distance. Mykle Hansen's speed vest is designed to reduce accidents that come from misjudging speed, plus irritation and aggression that come from cars assuming that bikes will slow them down.
To make the vest, you'll need to be comfortable with electronics and know how to program an Arduino microcontroller. But you won't need any tools more exotic than a soldering iron.