In my home town, and probably many others in North America, there are now sensors under the pavement at an increasing number of intersections. The sensors detect the presence of vehicles and affect the traffic light cycle at that intersection. You can often see where the sensors have been embedded, as the process leaves circular or rectangular patterns where the cuts are made. The sensors are commonly used where less-traveled roads intersect a major road. This effectively creates an on-demand system, where the light cycle shortens to give a green light to the less-used road when vehicles are waiting there. This makes a lot of sense: there’s no point interrupting flow on a major road when there are no cars waiting on the other road. The sensors are used in plenty of other situations. In left turn lanes, they trigger an advanced green. At busy intersections, they can help to optimize modify the light cycle throughout the day. They can be placed farther from the intersection to kick in when more than a certain number of cars are waiting. And so on.
Surprisingly, many drivers either aren’t aware of these sensors or they are just too dumb to cope with them properly. A few months ago I was at an intersection equipped with on-demand sensors. I was the second car waiting at a red. I noticed immediately that the first car was too far back to trigger the sensor, and sure enough, a complete light cycle went by and we never got a green light. The opposite side got an advance and a regular green, but we never did. I got out of the car, tapped on the driver’s window and explained to the driver that she needed to pull up. She did so and we got the next light. Of course, I’ll never know if she really believed me. She might have thought I was a nutcase and we got the green because it was (eventually) our due. Sigh.
If you’re observant enough to notice the sensors, you can even tweak them to your advantage, with a little experimentation. Near my home there are sensors in a left turn lane that I use regularly. I recently noticed that there are sensors at the front of the lane, and about three cars back. I discovered that the advance green doesn’t kick in unless there is a car sitting on the sensor that’s farther back, so I played around with it. I found that if I triggered the front sensor, then backed up and sat on the other sensor, it would trigger the advance. That’s not usually practical and it could be dangerous, so I don’t try that unless the road is fairly clear in my direction. But if I’m the second car in the turn lane, I don’t pull up behind the first car, I stay back and sit on the back sensor, triggering the advance. Fun, and useful!