Arizona Republic article, Nov 9, 2008: Folding bikes win new fans on region’s packed buses.
Something to watch out for: although most folding bikes have wheels larger than 16″, the most compact folders can be smaller.The most common wheel size for folders in the US is 20″, e.g. it appears all Bike Fridays have 20″ wheels.
A “device” with a wheel that is not “more than sixteen inches in diameter” (see §28-101(6) for the whole rundown, bikes must have two or three wheels, and only the largest wheel matters) is not a bicycle, legally speaking.
So, operating such a device is a legal grey area. Not likely to cause any problems, but in the event of a collision it could get ugly.