It's probably fair to assume that from the time that the Federal Government was building the airport and the access road in the 1960s, and reserved the median of the access road for future mass transit use, that everyone had a fairly good idea about what travel times out there would be.

You're talking about a right-of-way wide enough for two tracks, for a system that has only operated two-track lines. And as noted, skip-stop service doesn't really save that much time, and is probably not worth the operational hassle.