I agree with balang that the blurs just don't look right together. Seems like the tree (in-game) blur had the camera swinging to capture the car, whereas the the road blur looks like a chase car camera or mounted camera (on car) blur. However, mixing the two seemingly different blurs was a brilliant call.
To finally comment on Bram's quote up there, I think that the tree blur and lack of blur in the sky is all in-game. But my shots with blurred trees don't look like that...shoot.My new answer is two layers and a layer mask.
TW got my vote, because it's so different and the different blurs give a more extreme sense of speed.