Luis! Did you postprocess this or was the motion-blur original?[/b]!
No, I only cropped it. It was a lucky shot. I tried doing the same with three or four other cars (panning while shooting) but this was the only one that got the effect I was looking for. The camera was set for aperture priority, instead of shutter priority. It was all very fast and there was little time to think. I'll know better, next time.
By the way, can anyone tell what those structures on the background are?