Keep your distance – proximity alert for cars passing too close to cyclists

Hack Details: I’m sick of cars driving too close to me. In the rush hour traffic, it might be just that they’re not paying attention, so I want my bike/vest to flash a warning at cars that pass too close.
What you need to make it happen? I’ve got some proximity (ultrasound ping) sensors, some Arduinos and a Raspberry Pi. I can sew lights into the vest for a bigger display to warn the cars with. (I’ll be bringing my sewing machine and some e-textiles gear.) If I can get a loan of a bike nearby for the event, that will save me bringing mine up the country. People with ideas on how best to implement the sensor or design a good warning pattern would be great. I need to find out where to get (and power) some really bright LEDs too.
Name: Tríona
Twitter/website: twitter.com/triploidtree