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How to Calculate Saturation Flows
How to Calculate Saturation Flows

Getting saturation flows automatically on every traffic scene

Written by Daniel Štofan
Updated over a week ago

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 layout, turning radii, visibility, as well as vehicle composition. Saturation flow report describes the number of passenger car units (PCU) in a dense flow of traffic for a specific intersection capacity and time of the traffic signals. Saturation flows are usually required on each individual lane when modelling signalised intersections or roundabouts.

# How does GoodVision calculate saturation flows

GoodVision Video Insights provides saturation flows calculation as a spreadsheet report for each traffic movement or lane.

GoodVision’s algorithm detects free-flow conditions automatically within the desired time interval and uses them for the calculation. The algorithm is based on precise measurement of time-gaps between vehicles as they discharge into the intersection or a corridor across the stop-line. Computation includes also the traffic volume and PCU parameters (passenger car units).

# Scene preparation

Before getting the saturation flow report, the traffic scene should be prepared accordingly in advance. If you’d like to gain saturation flow data for each lane on your intersection individually, place separate lines to each lane entry of the intersection where the vehicles stop in front of traffic lights/signs. Then place another exit line on the other side of the intersection where the vehicles are leaving. Afterwards create movements between the lines to specify the traffic flows which you want to get the saturation flow outputs for.

Example of a drone scene description for saturation flow calculation

It is advisable to use a drone video footage for saturation flow calculation as from the top-down view, separate lanes can be identified easily. But also on some traffic scenes from standard traffic cameras it is possible, although maybe not for all lanes separately due to the lower camera angle.

For example, on the following picture the entries from the direction north and west are defined separately whereas the east entry will be calculated for both lanes of the traffic stream

Example of a traffic camera scene description for saturation flow calculation

# 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.

1. Click on “Get traffic report” in the top menu to open the report builder

2. Select “Saturation Flow” in the traffic report builder

3. Select object classes (vehicle types) to include into report

4. You can adjust the PCU values for each vehicle class. By default, these values are set according to the United Kingdom guidelines

5. Select traffic movements - saturation flows will be calculated for each movement individually

6. 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

7. 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.

1. 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.

2. MIN DURATION OF INTERVAL - the minimum accepted duration of the reading. Readings shorter than this duration will be ignored. Default value is 13 seconds.

3. 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.

4. 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

5. 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

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