John,
Joe is correct: the M values are in the units of the impedance. For example, using Streetmap Premium California and using 2 stops at -13048589, 4036266 & -13049065, 4036270 with the default travel mode of "Driving Time" the impedance is in Minutes. The output route geometry polyline (ToJson) provides:
`[-13048589.117193095, 4036266.1880055116, 0]` => Start therefore 0 minutes
`[-13048820.616989916, 4036267.50599994, 0.32905645174046566]` => 0.33 min or approx 19 sec
Regards
Mike