Hills?
Did you know that two researchers, Paradis and Cooke (2006), experimented with hill sprints using 35 subjects and found that neither uphill nor downhill sprints were better than horizontal sprints alone at improving sprint performance? However, when uphill and downhill sprints were combined as a continuous sprint for training purposes, it resulted in a significant improvment to sprint performance.
P.S. they used slopes of 3 degrees.