absoluteSpeed, partOfTwoWayRoad and openlr stay undefined with the standard styles, which do
not request their tile tags; a custom style whose flow source asks for them populates them.
No flow tile carries free-flow speed, travel time, delay or confidence: for historical speed and
congestion over an area, use the Traffic Area Analytics service.
Defines the structure of a traffic flow feature, which extends the basic GeoJSON feature with properties extracted from the traffic flow vector tiles.