A Hospital in Portland, Oregon
Even if the book didn't explicitly say that Mia was airlifted to a hospital in Portland, which it does, with all the references to hipsters, vegans, and vintage clothing stores, you'd know it was Portland. A girl's family is killed, she's in a coma, and dozens of vegan hipsters in vintage clothes hold vigil. It's like the saddest episode of Portlandia ever.
The book's main conflict comes from the fact that Mia is in a hospital. A horrible car accident like this can happen anywhere, at any time. Mia says, "The ICU is like [a casino]. You can't tell what time of day it is or how much time has passed" (8.2). Which is a lie, because each chapter heading tells the exact time of day. Mia knows precisely how much time has passed. But we'll make the metaphor work by saying that being in the ICU is a gamble, and the stakes are literally life and death.
Being in the hospital provides the only conflict in the present-time portion of the book: Adam cannot see Mia because of hospital policy. This leads to him come up with a diversion. He gets a punk singer to distract the hospital staff so that he can rush in and see Mia. This plot point makes being in a hospital feel less scary and more like something from a sitcom.
To think about the setting a little more—we're not sure what year it is. The book was published in 2009, but kids in the waiting room are playing a Game Boy, even though the Nintendo DS came out in 2004. And teen girls fantasize about Brad Pitt. We doubt that many teen girls in 2009 were flocking to see Inglorious Basterds.
Gayle Forman lived in Eugene, Oregon from 1991 to 1996 (prime Brad time), so we imagine the book is set somewhere in that time period. [Source]