Moderator: American AAdvantage, Travel Safety/Security & Texas, FlyerTalk Evangelist
Join Date: Sep 2006
Location: AUS / GRK
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There's a thread here in the US forum regarding the PHX airport, as well as multiple threads in the West forum that have much more detail on this.
For your particular situation, I'd tell your friend to take the Skytrain to the light rail stop, 44th St/Washington. The airport Marriott is about 1/2 mile north of there (near McDowell), and you can just meet him there, then hop on the AZ Loop 202, take that to either the AZ-51 (head north to the AZ 101 West to I-17), or to the I-10 to the I-17.
You'd be going way out of your way to go back to the rental car facility, and it would probably add 15-30 min to your trip (plus your friend dealing with the rental car bus).