[aerohead]

Ford did a lot of work on these with the Probe-IV concept car.They had them in front of the front wheels (but not behind!) and before and after the rears.The Probe also had active suspension so they could get away with stuff we can't.In addition to directing the air,the structures also improved the "sectional density" of the car,which softened pressure spikes in the areas where the wheels "erupt" from the body.The things we see on the Insight and Prius are probably a compromise for real world cars that must negotiate curbs,driveways,roadkill,tire "alligators",etc.,with a fixed suspension.The best example to feast your eyes on is the M.G. EX-181.Check it out at GOOGLE Images.