The White Van
Saying you saw a white van leaving the scene of a crime is about as useful as noting that there was a bird in the sky at the time. Look around you right now . . . go to your window and look outside. If you don't see a white van somewhere in your line of sight within 60 seconds you are probably living a sheltered life. For those inclined to paranoia it's pretty damn easy to convince yourself that white vans are shadowing your every move.
Keep in mind the white van didn't appear until sniper shot #8. After that, the whole damn world was seeing speeding white vans. Hell, didn't one witness say he collided with the white van? "White van" is synonymous with bafflingly undistinguished omnipresent vehicle.
Now, if witnesses had said they saw a grey Aston Martin at three separate crime scenes instead of a white van that would be a lot more useful.
UPDATE: Prince George's County Police now believe the sniper may be motivated by an unreasoning hatred of cans.