When I rode a steel frame, I was always able to trigger lights.

It makes it a little harder if the road's been re-paved so the sensor cuts aren't visible, but I usually found a hand's breadth to the right of the oil slick would do it.

On a carbon frame, there are many lights in my area where I could be waiting for a good long time (5 minutes or longer) for a car to come by that isn't turning right. Legal or not, if there's that little traffic, it's (a) safe and (b) unlikely to have a cop within sight.

Modern sensors are optical. I'm LOVING the newer ones that have been installed in my area.