The time it takes for a car to pass through a single green light cycle at a typical suburban intersection is usually between 2 to 3 seconds per vehicle. This duration, known as the saturation headway, is a key metric in traffic engineering. However, the total time for a line of cars to clear an intersection depends heavily on the green phase duration of the traffic light and the traffic density.
For a standard 30-second green light, you can expect approximately 10 to 15 cars to get through if traffic starts moving smoothly. This number drops significantly if the lead driver is slow to accelerate, a phenomenon traffic engineers call start-up lost time. At a heavily congested urban intersection, the passage rate can slow to a crawl, with only 6-8 cars making it through a green cycle.
Here’s a quick look at how different conditions affect the number of cars that can pass:
| Condition | Approx. Green Light Duration | Average Cars Passing Through | Time per Car (approx.) |
|---|
| Free-flowing traffic, no left turns | 30 seconds | 12-15 cars | 2.0 - 2.5 seconds |
| Moderate traffic with turns | 45 seconds | 15-20 cars | 2.2 - 2.8 seconds |
| Heavy congestion, slow start | 60 seconds | 20-25 cars | 2.4 - 3.0 seconds |
| Peak rush hour gridlock | 60 seconds | 6-10 cars | 6.0 - 10.0 seconds |
| Straight highway on-ramp meter | 10 seconds | 1 car per green | 10.0 seconds (by design) |
The biggest factor is often driver behavior. A quick, attentive driver at the front of the line can help dozens of cars pass, while a distracted driver staring at their phone can waste precious seconds for everyone behind them. If you're trying to gauge when to go during a yellow light, it's safer to assume each car needs about 3 seconds to clear the intersection safely.
