Q: I see changes in the way traffic is flowing at some larger intersections where I live. It used to be the turn lanes from the opposite sides would go at the same time. That way the through traffic from each side could go as soon as the opposite turn lane was complete. Now in many intersections, both the turn lane and through traffic from one side goes while the opposite side has all the cars for through and turn lanes waiting and idling. This does not appear to be as efficient as before. Why the change?
A: There’s that scene in the 2003 movie The Italian Job when the thieves we’re supposed to be rooting for take back the stolen gold from the double-crossing thief we’re supposed to root against. (Besides the ethical quandary of choosing which bad guy to root for, how can you ever root against Edward Norton?) A key part of their strategy involves hacking into Los Angeles’ traffic control computer system and manipulating traffic lights to direct the truck full of gold to their desired location.
While the hacking bit in the movie doesn’t hold up in real life, there really are rooms with screens and computers where traffic operations engineers can monitor intersections and make adjustments to improve traffic flow. Transportation departments use cameras, sensors and software to gather data that they use to make decisions on how maximize both efficiency and safety with traffic signals.
In your question you described a change from what is called a split phase to a lead-lag signal. To understand why that change might happen I spoke with a traffic operations engineer. Managing traffic signals is a data-centric operation. The engineers working with that data continuously review it to optimize travel. When traffic volumes change, so might the programming of the traffic lights. As an example, when we experienced a drastic change in traffic volume when Covid prompted shut-downs, traffic engineers responded by adjusting signals for the change.
And it’s not just overall traffic volume; signals are optimized to function differently based on the time of the day. The best signal timing for rush hour in the morning might be different from the timing in the evening rush hour when most of the traffic is heading the opposite direction, and certainly different form the signal timing at midnight. An engineer also has to take into consideration other nearby intersections. It’s not just what’s most efficient at one intersection; the whole system has to work together.
Along with efficiency, traffic engineers consider safety. They may make changes to a traffic signal to reduce vehicle conflicts, allow safer pedestrian crossing or manage hazardous congestion. I know of a freeway exit ramp that gets backed up during peak travel times. The signal for the exit ramp where it meets the arterial road has to be green long enough and frequently enough to make sure that we don’t end up with stopped cars on the freeway. During those times the arterial can have extended delays, but that’s a tradeoff to avoiding rear-end collisions at freeway speed.
Most of the time, signals operate as programmed without constant adjustments. But during a collision or other unexpected event, the engineers can adjust signals on the fly to respond to a situation.
It might not always look like it from your view in the car, but unless there’s a glitch in the system, what you’re experiencing at an intersection is probably the best balance of efficiency and safety that the engineers can achieve.