This is the obvious spoiler alert. This post will contain key plot points of the film, if you haven't seen it, I suggest you watch the film and then come back to read.
The first point of discussion is the character of Ivan. The only thing Trevor says in regards to who the character is, is "I know who you are". I think it is fairly certain that Ivan is some sort of physical representation of Trevor's conscience after killing the child. Weaving through red lights with no reactions from cars of pedestrians, nobody knowing who he is in the place he claims to work, it is obvious that only Trevor can see him. Whether he is a hallucination due to his lack of sleep and clearly deteriorating health, or whether Trevor is a schizophrenic are both topics left for viewer discussion, all we know is that the character is only important to Trevor.
A more interesting discussion arises when the question of whether Trevor is already dead during the whole film, and is in purgatory whilst he is denying his sins. Some of the main causes for this theory are the flashback scene after he hits Nicholas with his car. We see Trevor driving away, driving through the same tunnel we have seen him drive before, he is driving in his red car towards a white light (the light of the day outside the tunnel). Previously in the film, he is following Ivan in the red car, and almost crashes, then his truck stops working and he is forced to pull over to the side of the road. Was this near crash an actual crash when Trevor was alive? Hence why he cannot drive any further, is he bound to the area in which he committed his crime, being reminded constantly and prompted to confess to his sins? The car crash point is solidified when Trevor goes to the police in order to track down the address of the red car's numberplate and they tell him that he totalled the car in a wreck.
The final shots of Trevor being escorted to his police cell with his desire to sleep, something he was unable to do in the purgatory he was in, shows he has acted and chosen to take the Road to Salvation. The entirely white room seems almost a heaven, and his overwhelming desire to sleep due to the conscience being taken off his back shows that whatever stage he was just in is now over.
