It's genuinely scary. I don't mean scary as in out pops a zombie and that's it... its scary throughout the movie.
I dont' want to say anything to give away the movie, but if you want, scroll down....
SEMI-SPOILER AHEAD
In the opening scene some kids come back from a road trip and run into a young girl walking alone on the road... they decide to pick her up and try to help her, so they drive to the nearest gas/rest station, but the girl tries to get them to turn around. When they decide not to, she shoots herself. The teens then stop by the station and reports the suicide. As the movie continues, the teens (and audience) soon find out why the hitchhiker acted the way she did.