JT
Memory's scratching a tad and I would need to dig out a couple of original AFMs to double check on landing distances .. but I recall that the same applied in the UK under the BCARs.
When I started in the late 50s the UK authority (ARB) used a 'rational landing distance' based on a reference wet runway. Certification braking tests were preceded and followed by runs of a "Mu-meter" to measure actual runway friction so that measured distances could be corrected to the reference surface.
If I remember correctly the regulations also included allowances for higher approach speeds and touchdown point.
FAA flirted with a similar concept in the 60s or 70s (memory fades!) using a 'diagonally braked vehicle' to measure the friction, but IIRC they came to the conclusion that it would be too difficult to classify every runway in use, particularly since the friction varied with time (rubber pollution amongst other things) so they abandoned the idea.