Unless you're blasting through red lights and stop signs, you don't really need to care what speed you're riding a bike at. But maybe knowing that you're careening through traffic at a blistering eight miles per hour gives you some kind of thrill. In that case, check out Matt Richardson's Dynamic Bike Headlight hack.
Using a Raspberry Pi at its core, Matt turned a compact pico projector and a handful of other electronics into a bicycle headlight that does so much more. Well, not that much more. In addition to providing a little extra light on a dark road, the projector also displays the bike's current speed. Now if only cars had this feature, since it would probably shame drivers into sticking closer to the speed limit if they knew everyone could see how fast they were driving. [YouTube via Reddit via Damn Geeky]