Brady Clark and Mykle Hansen created a vest that lets bicycle riders show off how fast they're pedaling:
How does the vest work?
In a nutshell, it uses a sensor on your wheel to send a signal to a microcontroller that then turns the numbers on and off on the vest.
Why did you invent this?
Have you ever outpaced cars and been honked at? Had a car almost wipe you out while making a turn? The Speed Vest was invented because we were curious to know if putting more information in front of drivers might change their awareness of bicyclists. That, and we suspect a lot of people don’t realize that an average person can bike 10-15 mph (15-25 km/h).
Link (with a video clip of the vest in action)