Induction loop sensors are still the go-to solution for many traffic lights. It's basically a circuit buried under the road, and when a big chunk of metal, like an automobile, drives over it, it induces eddy currents which are detected and trigger a light change.

I don't have a source for this, but I do remember reading about cases of motorcyclists being acquitted of running red lights that used induction loops. Their light bikes didn't have enough metal to trip the sensor, so, even if there was no traffic from the other direction for miles, the light would literally never have changed until a car came up behind them, leaving them no choice but to run it.