Asleep at the intersection

Today, while waiting to cross the street, I saw something that should never happen.

Here in BC, at many intersections, there are left turn lanes. In some of them, the traffic light will show a green arrow that gives right of way to those turning left. The crosswalk I was at was one such intersection. And as you might expect, the left turn arrows came on in both directions. However, there were no vehicles in either left turn lane.

Is it not obvious that the above situation leads to unnecessary idling, and therefore wastes energy? I see no reason why it is impossible to put a sensor in a left turn lane that, that while the light is red, detects and counts the cars passing over. Perhaps it could detect cars by pushing down slightly when a set of wheels passes over. This will happen twice per car (once per axle). The cars’ weight could press the sensor down. While not perfect (a semi or other long vehicle with more than two axles would result in an incorrect number of vehicles) this will be a good enough approximation to form a sensor system. The number of cars passing into the left turn lane could then be used to decide whether to activate the arrow the next time the light turns green. This system would prevent unnecessary idling due to unneeded turning arrows, and ought to be relatively simple to implement (a computer could probably do it).