Sounds like a light beam sensor system to me. Industrial waterproof critters are off-the-shelf, but probably pushing $100. This is a well-defined situation where I think you could whip up something and glop it in RTV. High intensity red LEDs have narrow beam angles, essentially pre-focused over 1 m. 555 oscillator and a very basic frequency discriminator to weed out sunlight and reduce false hits. Two sets, a couple of logic gates (or a uC), and the traffic lights and you're there.
ak