Noisebeam: Perhaps you are looking at the part of the road which is too close to the bike; this causes your brain to interpret a graeter sense of speed. Whenever I hit a steep downhill where I know very high rpm are approaching I look far down the road, relax and try NOT to think about the pedaling motions as this would disrupt my high cadence spin. Your brain already knows how to fire the correct muscles in the proper sequence so by consciously thinking about it you screw up the timing and add anxiety.
Gerry