Doesn't help with your microwave sensors, but some of the newest traffic signals around here use cameras, which have worked reliably for me on my bike.
For a radar reflector on the front of a bike, you need the apex cut off a cube (corner plus three sides) facing forward. This would be like 1/8 of the westmarine reflector which is intended to be omnidirectional.
Corner reflector - Wikipedia, the free encyclopedia The material needs to be conductive. Effectiveness depends on its size compared to the wavelength of the radar. For K and X bands, 5 cm along an edge should be adequate.