Use tank to tank method to calculate. Or alternatively, reset average mileage to 0, and Trip A to 0. The drive for at least 100-150km spanned over x number of days. When trip A shows approx 150km, then use that average shown on meter.
Almost impossible to get 16km/L average in HEAVY traffic as you say, or you’re calculating incorrectly.. if you are able to maintain 60 speed without lots of braking then yes it is possible.