There is a fancy term to describe the maximum number of vehicles that can travel through an intersection. It's called the "Saturation Flow Rate" or SFR. In 1985, traffic engineers believed that unimpeded queued vehicles would travel through an intersection about once every 2.0 seconds. Based upon that, and probably a few studies, the Saturation Flow Rate was set at 1800 vehicles per hour. (This is obtained by dividing 3600 second/hour by 2.0 seconds/vehicle). It was a nice round number.

There was a problem with this value, however. When we did traffic studies at very busy intersections, we found that traffic moved through the intersection faster than this value. Sometimes our calculations left us scratching our head. We would measure traffic through the intersection greater than the calculated capacity. What was going on?

The answer is that the 1800 Saturation Flow Rate was too low. Today it is set at 1900, but in 1985 the rate was 1800.

Now the 1985 Highway Capacity Manual method was correct. Given all the inputs into a capacity calculation (lost time, cycle length, saturation flow rate), it will fairly accurately calculate average vehicle delay. But the 1800 SFR flow rate was not correct. It was too low.

In the late 1980s, we performed research of saturation flow rates at major intersections in the Phoenix area. What we found was very surprising. At busy intersections, we found that vehicles traveled through the intersection at 1.8 seconds per vehicle. And the results at some intersections were astonishing.

One major roadway through the Phoenix area is Grand Avenue. Back in the 1980s this road created several six-legged intersections. Since each roadway needed its own phase, the cycle length at these intersections could be almost three minutes. So when vehicles received a green light, they went. And boy did they go!

We measured Saturation flow rates at Grand Avenue and found that they were consistenlty between 1.6 and 1.7 seconds per vehicle. This translated into a Saturation Flow Rate of 2200 Vehicles per hour. This was substantially greater than the 1800 Vehicles per hour that was imbedded in the 1985 Highway Capacity Manual.

Today, the 1900 might still be low. Most agencies don't have the resources to measure Saturation Flow Rate, but they should. It gives a more-accurate picture of the actual traffic situation.