Zebra vs. zebra-patterned sofa is a kind of problem designed to be hard for image processing algorithms. From the practical point of view however, you just have to require that policemen / construction workers wear specific patterns on the uniforms or even have special IR-reflective buttons/threads. That would solve like 95% of cases, and by the time self-driving cars become the norm, we'll have figured out at least some specific algorithms for that very purpose that would work in general case.