Saturation flow is a key measurement of on-street performance. It is an expression of the maximum capacity of a link as predominantly determined by junction characteristics like geometry, layout, turning radii, visibility, as well as vehicle composition. Saturation flows are usually required on each individual lane when modelling signalised intersections or roundabouts.
How does GoodVision calculate saturation flows
GoodVision Video Insights provide saturation flows in the form of a spreadsheet report. All needed measurements are extracted as vehicles discharge across the stop-line in free-flow conditions and thus unaffected by downstream interference such as congestion or exit-blocking.
GoodVision’s algorithm detects free-flow conditions automatically within the desired time interval and uses them for the calculation. Computation of the saturation flow per hour is based on the time gap between vehicles, traffic volume and PCU parameters (passenger car units).
How to get saturation flows step by step
Saturation flow report can be created in the report builder accessible on the Analyze screen in GoodVision Video Insights.
- Select “Saturation Flow” in the traffic report builder
- Select object classes (vehicle types) to include into report
- Select traffic movements - saturation flows will be calculated for each movement individually
- Select the time range for the report - you may select the peak hour, but feel free to select the whole survey duration as the algorithm is capable of finding all saturated free-flow readings automatically
- Potentially adjust the default algorithm values to match your country traffic guidelines and click “Get Report”
Created saturation flow reports will appear on the Traffic Exports page of your camera in a few minutes. Individual saturation flow Excel sheets are created for each traffic movement. This report saves you a substantial amount of time and manual video review.
Saturation flow report generation dialog
The algorithm is configured to TfL guidelines for the United Kingdom, but you can easily adjust the following parameters.
- END OF SATURATED FLOW - a condition when to stop the reading. If the time-gap between two consecutive vehicles is greater than this value, the reading will end. Default value is 4 seconds.
- MIN DURATION OF INTERVAL - the minimum accepted duration of the reading. Readings shorter than this duration will be ignored. Default value is 13 seconds.
- NUMBER OF IGNORED VEHICLES - number of vehicles on the start of every reading that should be ignored (e.g. on start of green). Default value is 2 vehicles.
- MIN NUMBER OF FREEFLOW READINGS - the minimum number of readings needed to calculate saturation flow for the traffic movement. If the algorithm cannot find this number of readings, the report will contain the intermediary result and a warning. Default value is minimum 10 readings
- PCU VALUES - the “passenger car unit” coefficients for each vehicle type. The default values from TfL guidelines for the United Kingdom are:
CAR: 1 pcu
VAN: 1 pcu
TRUCK: 1.5 pcu
HEAVY TRUCK: 2.3 pcu
BUS: 2 pcu
MOTORCYCLE: 0.4 pcu
BICYCLE: 0.2 pcu
A practical example showing how GoodVision extracts saturation flows from video footage can be found in the short video below 👇
Saturation flow calculation in the real traffic example
Saturation flow report can be created in the report builder accessible on the Analyze screen in the Video Insights.