It's hard to believe, but if you're trying to use public transit to get to an airport, you might want to use Bing Maps instead of Google:
Instead of advising you to take one of the “Airporter” buses from San Francisco International Airport, Oakland International Airport, and San Jose International Airport to the north and south of the Bay Area, the app will propose a two- or three-step odyssey on Bay Area Rapid Transit rail and then local buses.
As Google describes things, putting those city-to-terminal routes into its mapping apps shouldn’t be that hard. A transit operator has to apply to be listed in Google Transit, publish its schedule in the standard General Transit Feed Specification (GTFS) format, and have Google run some quality tests on that feed before factoring it into directions.
Three Airporter services in the Bay Area, meanwhile, had not even gotten to that first step.
“Google has never reached out to us, but at the moment I don’t think we have our schedule in a compatible format,” David Hughes, charter manager at Marin Airporter, says via email. “We are currently working on a new website and we should have the formatting correct when it is published.”
By the way, Bing is not that bad in general. People use Google as their default (I do) but sometimes Bing has better results.
For Chicago, it's not as much of an issue, because the CTA Blue Line goes straight to O'Hare Terminals 1-2-3. But in this case, Google says I can take a bus to the Blue Line and be there in 58 minutes, while Bing doesn't seem to realize that my bus goes all the way to the Blue Line.
All of which suggests that you shouldn't rely entirely on one source of information.