OK, it's not really Rainbow Road. It's a glitch found by AmetDj2. But if you close your eyes real tight and replay the fond memories of performing the famous shortcut on Mario Kart's rainbowy space track, you...won't see a damn thing in your game of Forza Horzion and probably crash into something. What'd you expect when you closed your eyes? Sheesh, who told you to do that?
Forza Horizon rainbow glitch [YouTube]