Yep, that's true.
For my calculations for 200 to 500 mile trips I use 50mph for highways
and 30 mph for backroads.
Those figures are rather accurate and include traffic jams, fuel stops
etc.

On most occasions I'm on time +/- 15 min. even after a 500 mile drive.
Shorter distances have to much variables to calculate a accurate
average.

-Kees