Sweeper bike detectors project from Cliff Batson (N4CCB):
They are equipped with a Moteino transmitter running on 9V battery power that keeps them going all day for the event, while transmitting a beacon every 4s. They beacon their identity to the various checkpoints/rest points and in sag vehicles (cars with “bike detector” receiver units) to keep track of which bikes are near by.
Via LowPowerLab.
Check out the video after the break.