I found what you must do (maybe). You must have "double jump on air". 1) Sit near the motorcycle. 2) quickly press for very short "left" (to get some speed, simple run) then 3) "left+shift" (speed RUN) without jumb (once). 4) Then, "left-shift-space" continuously. You must be jumping on top of streetlamps cause you gain huge speed. It is very hard. I havent succeeded with motorcycle. once, for only 1 second lost.
