Possible peregrine sighting - need comparative raptor ID

[dale]

Like I OUGHT to be able to ID a peregrine at close range, right?
Except I was driving fast and it was flying low the OPposite direction RIGHT over my car.
I saw it for a lot less than a second.
It was definitely peregrine-sized, all pale underneath, and a dark helmet-like head top (not dark underneath). But seriously, I saw this thing for a FLASH. Could it have been any other raptor? A peregrine would have been migrating - it's not a peregrine area (highway near prairie in WI). There are scads of hawks around there, and I expect it was some sort of hawk, but that thing sure did look like a helmet.
Just curious what those who are more versed in hawks might know. Thanks.
Also saw a perfect male downy woodpecker today.