A Tiny Traffic Light Setting Can Cut Your Commute — Or Wreck It

If your commute feels like a lottery, a new study suggests the culprit may be how traffic lights are tuned, not just how many cars are on the road. A researcher ran 240 computer simulations of one city street network, pitting lights that sense waiting cars (called "actuated" signals) against old-fashioned lights that simply run on a fixed timer. The result: timing settings mattered more than the technology itself.

The biggest surprises came from one knob — the longest a green light will stay green when cars keep arriving. At the heaviest traffic tested, setting that maximum to twice the planned green made average trip time 57.4 percent worse than the city's original fixed plan. Cutting it to 1.25 times the planned green flipped the result completely: 70.7 percent better than that same plan. Same streets, same cars, wildly different outcomes.

A second improvement came from a smarter rule for ending a green phase. Instead of switching on a timer, the light watches how many cars have actually cleared the intersection. Against the best of three fixed-time plans, that rule cut average time per trip by roughly 15 to 18 percent across three traffic levels — a real dent in daily delays, fuel burned and emissions produced.

The catch: this was a simulation, not a real city. Traffic demand was synthetic, drivers were assumed to react instantly when lights changed, and cars stuck too long were simply deleted from the model. Both timing tests also performed best at the shortest setting tried, meaning the study never found a true best value — only that shorter greens helped in this range. So it is not a recipe for your local intersection. The takeaway is that cities should test timing settings before crediting one type of traffic light with being smarter.