Cyclist Speed Vest via MAKE
In the interests of safety on the roads, a lightweight vest for cyclists to display their speed:
This lightweight night-cycling vest displays your current speed in glowing, 7-inch-tall numbers easily visible to cars. On the back, an Arduino microcontroller reads input from an off-the-shelf bike speedometer sensor, and then switches power to sewn-in numerals made from electroluminescent (EL) wire.
Instructions how to make your own can be found here