Excellent shot - love it. Is it a composite??
sort of. The bikes were actually in that location, it just wasn't all in-camera. The light streak was a long exposure as the bikes drove by. I then re-positioned the bikes using the live view to approximately line them up with the streak and took between 15 and 20 exposures of each bike/rider with a handheld flash and small soft box. I masked and adjusted the layers to taste, added a slightly-transparent layer of each wheel with a radial motion blur, then burned the shadow on the ground (the bikes couldn't create their own shadows with their own headlights).