Default saturation flow is an assumption, and it is usually wrong
Signal analysis frequently starts from a textbook saturation flow — 1,900 passenger cars per hour of green per lane, or a local equivalent — adjusted by factors. On a real approach with narrow lanes, a gradient, heavy turning conflicts, kerbside friction or an unusual vehicle mix, the true rate can sit well below that. Every downstream number, capacity, degree of saturation, delay and queue, inherits the error.
Measuring it takes a camera and a careful analyst. We record discharge headways for each vehicle in the queue after the start of green, discard the start-up positions, and derive the saturation headway from the steady-state part of the discharge — which is what the calculation actually requires.
Outputs
- Saturation flow rate per lane in PCU per hour of green
- Discharge headway series by queue position, with the steady-state range identified
- Start-up lost time and end-of-green extension
- Degree of saturation by movement, at existing and design volumes
- PCU equivalence applied to the observed vehicle mix
- Comparison against the default rate your model currently assumes
Degree of saturation, and what it is telling you
Degree of saturation is demand divided by capacity for a movement. Below roughly 0.85 an approach generally behaves; approaching 1.0 delay rises sharply and non-linearly, and small volume increases produce large delay increases. Because that relationship is non-linear, a saturation flow figure that is 8% optimistic can put a movement on the wrong side of the decision to change the timing plan.
Frequently asked
How many cycles are needed for a saturation flow study?
Around 15 to 30 fully saturated cycles per lane group — the queue must extend beyond the measured positions for the whole green, otherwise the discharge is demand-limited and not a saturation measurement at all.
What is the difference between saturation flow and capacity?
Saturation flow is the discharge rate while the signal is green and the queue is discharging. Capacity is that rate scaled by the proportion of the cycle the movement actually receives green.
Can this be done from the same video as a turning movement count?
Often yes, if the camera position shows the stop line and enough queue length. Commissioning both from one recording is the cheapest way to get both datasets.
Get a price for saturation flow & dos
Send the site details and the periods you need counted. We will come back with a fixed price and a delivery date, usually the same working day.