[maxerickson]

I looked around some before I posted that. This article seems respectable enough and at least supports the idea that bikes are intended to be able to trigger some of them:

http://bikeportland.org/2010/09/27/bike-science-making-sense...

[fredophile]

It looks like those are specifically designed to be triggered by bikes. I've been at intersections with a 50-100cc motorcycle that would not trigger the light.

[maxerickson]

I have the idea that they are broadly intended to work for bikes and the like but that they aren't always installed or tuned correctly.