It's often difficult to gauge how fast a bicyclist is travelling from behind, especially at night. To correct this problem, Mykle Hansen created a light-up vest that shows his current speed.
Using a standard speedometer, the vest sends the speed through an Arduino and lights up electroluminescent wire digits on the back. The lights are bright enough to both catch a drivers attention and warn them of your current speed, which is especially handy if they're planning on taking a right-hand turn after passing you. It's a bit complicated to get everything working together, but the results should help keep you safe. You can find the whole how-to guide over on MAKE magazine.
Speed Vest [MAKE Magazine]