Dean R. Koontz - Velocity.

Velocity is a novel by Dean R. Koontz and was published in 2005. A bartender named Billy finds a note on the windshield of his car after a long day at work.
It says: "If you don't take this note to the police; I will kill a lovely blonde schoolteacher somewhere in Napa County. If you do take this note to the police; I will instead kill an elderly woman. You have 6 hours to decide."
This is just the beginning of a long string of events that force Billy to choose between two evils and the speed of these trials keeps increasing making it more and more difficult for Billy to try to find the killer.

The suspense is quite good and only a couple of times do the protagonist act in another way than I would in the same situation.

I would give it a rating of 8 out of 10.