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How to measure section speed and gap times and gather traffic event evidence
How to measure section speed and gap times and gather traffic event evidence

Section speed and gap time violation evidence gathering guide! Learn how to set it up in 3 simple steps.

DataFromSky avatar
Written by DataFromSky
Updated over a week ago

Evidence is a feature available from FLOW version 1.16 that allows you to export reports of different traffic events including traffic violations such as speeding, not maintaining sufficient time gaps between vehicles, u-turns, stop-signs, and red-light running, and more.

In this tutorial, we will demonstrate in 3 steps how you can set up evidence gathering for section speed and gap time measuring. First, we will go over how to set up the spatial and other necessary filters and then we will show you how to set up evidence report generation and also how to access the reports. NOTE: The Evidence module must be enabled by the administrator.

Step 1. - Creating spatial filters

Create 2 gates, one at the start and one at the end of the road section for which we want to gather section speed data. Make sure that the gates are parallel. Next we draw a movement between the two gates. When setting up the movement we also need to put in the distance between the two gates in meters. You can find the field for the movement Length next to the movement name field. Please note that the Length defines the length between the two gates not the actual visual length of the movement you see in the video analytics setup view.

Step 2. - Creating section speed data filtration chain

Next, we need to instance the gates and the movement in the canvas. To do that we drag them over to the canvas and connect them as shown in the image below.

As you can see we also put a category filter there to only count passenger vehicles aka the car class. You can learn more about our object classification scheme here. Once you have added the category filter, gates, and movement and connected them in the correct order we need to adjust the individual gate settings.

To gather snapshots for the section speed evidence we need to tick Collect Evidence under the Time mode and Evidence on each gate. You can access the gate settings by double-clicking it. On the second (exit gate) we also tick the Calculate gap under Gap data which will allow us to get the gap time and gap distance between the two gates. Last and the most important step is to add a table widget to the end of the filtration chain (on the movement in our case). This table will gather data on all the objects that meet the filtration conditions and will be used as input for Evidence generation.

Step 3. - New evidence task setup

Go to the Evidence section of the specific video analytics and click on the New task button. Under source of events select the table widget we have created. In the image evidence section click on the New image data and add the two gates we have selected for visual evidence gathering. In the protocol attributes part you can set the thresholds for gap time and speed. If the given object will go over them, they will be highlighted in the report. Now you are generating .xlsx section speed evidence reports! You can access and download them in the Storage section of the given Analytics.


You have learned how to create a simple section speed and gap time measuring with evidence gathering. We will give you the last tip and that is if you want to generate a report only for cars going over a certain speed you might also want to add a Speed motion filter at the bottom of the filtering chain (that will use the measured section speed data) and put the Table widget for evidence gathering on it. We hope you have found this guide useful.


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