[lunarhighway]

i saw a piucture a while ago of a police minivan, that had a "spoiler" at the upper edge of the windshield, it basically continued the angle of the windshield a bit beyond the roof. my guess is it was put there to guide the air over the police lights wich where somewhere at the b pillar on the roof.

perhaps you can it so that along with the windshield the pressure bubble in front of the sign would make a smooth virtual path for the air to follow? just guessing