Well, I finally got around to finishing off my distance measuring device and putting it onto my bike. It's basically a ultrasonic ranging module connected up to a microprocessor which in turn drives a small LCD screen. The ultrasonic sensor is currently attached to my rear mudguard, facing right. The system takes several range samples per second and displays the closest sample taken in the last few seconds on the screen. There's some pics in a thread over in the helmet cams section, but I'll repost them here: The sensor: The cable winds back underneath the rack and winds around the seatpost: None of my project boxes would fit it, so until I get something better, the circuit board and battery are currently stuffed in with my Smart headlight lead acid battery: Finally, here's the display, held in place with an elastic band: The system isn't meant to be waterproof (you can't cover the ultrasound sensor or it won't work), but it didn't seem to mind the rain as I cycled in for its first test today. It worked quite well, but there's a bunch of modifications I need to make: Adjust for the width of the bike - the sensor is at the centreline of the bike so I was always having to mentally adjust the value on-screen. Currently it shows the distance in inches - I'd like to alter it to be feet and inches. I would like to make it so that it shows the last few closest passes on the second line of the LCD - at the moment there is a temptation to look down quickly after the car has passes to see how close it was as the number only stays up for a few seconds I really ought to put an on/off switch on it! I should probably run it off the same lead acid battery as my front lights run off Anyway - I'm quite pleased with it so far!