I just drove along I-55 through Mississippi a couple of months ago
and noticed that a large fraction of the cars on the road had cracked
windshields. I was told that this had to do with loose rocks on the
road, esp. near construction sites, and the rocks get kicked up in the
air by cars and trucks.
I don't think aerodynamics are going to have any noticeable effect
on any but the smallest of rocks. A flying rock probably has rather
little horizontal speed relative to the ground, and a car comes zooming
into it at 65 MPH (if it's doing the speed limit on I-55). To deflect
the rock with air means blowing on it so hard it'll be out of the way
by the time the car gets to it, and I'm afraid that most rocks, being
dense, don't get blown around very easily.
The best thing to do is to try to make windshields that don't
crack when you hit them with a rock going at 65 MPH relative to the
windshield -- this is quite difficult. A lot of car glass has a layer
of plastic to hold the shards together in case of a shattering
collision, but won't protect against the cracks. You could imagine
making a windshield out of some other material like acrylic plastic or
kevlar, but there could be issues with transparency and scratches
blurring the driver's vision. Some kind of netting might help, but that
again would be hazardous if it interfered with the driver's vision.
(published on 10/22/2007)