That red light, green light and maybe yellow light for in between sake is a great system alright. Even the more dense of the population knows the meaning and can read the results. It can be manipulated in many ways, though.
Sorta like the little gadget that I tote in my pocket to open the cargo doors on my vehicle. Some co-perp can stand a pretty good distance away and make any of the lights come on.
Or the fellow running the demo can press the right button whilst it is in his pocket.