First things first: I need to mention I received this book in Goodreads giveaways.
In the near future Detroit is revived. Carpooling is the law, the moment you get on the highway you need to have four people in the car, and otherwise a heavy fine is issued. The problem is: it is not always possible to find passengers, especially the last fourth person. Thus a new job is born - professional hitchhikers of "fourths" as they call themselves. For a small fee they fill the vacant spot in the cars during rush hours.
Andre LaCroix is a disillusioned cop who also makes some extra money being a fourth. Therefore he is the first one to notice connection in some seemingly accidental deaths: all dead people are fourths. Trying to save his colleagues, he encounters corrupted cops and politicians (what else is new?) and some quite unpleasant facts about his family.
My biggest grip with the book is that it took exactly half of it before things got interesting. For some reason I could not connect to Andre (in my opinion, the reason for his disappointment in police work should be uncovered earlier) and I did not care about all the details in the job of fourthing. I also did not like some of the choices Andre made during the course of the book.
Still, it is a solid book deserving between 3.5 and 4 stars.