1. go to Edit > Preferences...
2. on the left change the tab to "Add-ons"
3. top right click on "Install..."
4. now select traffiq_full.zip that you downloaded and wait for a short while before the data is copied to your Blender AppData folder.
5. now you can find cars & more assets to add to your scenes in the sidebar of the 3D view under "traffiq" tab!