I don't find it necessary to do research prior to a road trip in order to find NPR stations. They are almost always found at the extreme left-hand end of the FM dial; I just hit my car radio's "Scan" button, and usually find more than one NPR alternative somewhere below 92.0 or so.

The New York City area is a very notable exception; the main NPR outlet (WNYC? Something like that, I don't remember) is up in the middle of the FM spectrum. There are several excellent non-commercial stations down at the bottom of the dial, but no NPR.

I first found some of my favorite NPR programs while out on the road, since our local affiliate has not always carried every network offering. For example, Fresh Air was not heard on WWNO in New Orleans until just this year; until recently, I was only able to hear it on the road or over the internet (and of course, I first learned of its existence by listening on the road.)

Warning ~ thread drift:

My very favorite non-commerical radio station, WWOZ in New Orleans, is not an NPR afiliate and does not broadcast any news or talk, just music, and very excellent music at that. ("More platter, less chatter.") It's about time I posted one of my periodic reminders that some of you will really enjoy listening in, at www.wwoz.org.