Its probably an average of 10 mph, stuck in traffic trying to get to work or home.
The estimate is a crap figure
Lets say you commute, the national average in cities is about 25 minutes.. but lets make it an even thirty to make the math easier.
30 minute commute each way to work... 1 hour per day 5 days a week = 5 hours
Lets say you go to the gym, and that adds 15 minutes each way... another 1/2 hour and you do that 3 days a week.. you’re pretty dedicated... so 1.5 hours per week.
Lets say you do typical shopping or errands during the week.. maybe 30 minutes per trip 2-3 trips a week... another 1 to 1.5 hours per week
And lets say on weekends you are running around like crazy and like to hit the open road and go places... another say 4 hours per day for 2 days.. 8 hours
So all that tallied up adds up to 15.5 hours on the top side. and I really don’t think folks spend 4 hours a day on the weekends in their car.. but giving that as an outrageous example. you still don’t get to 21 hours per week.
10-15 would be my guess for the typical person in their car.