http://www.lunchboxcollections.com/mud-flap/
What is the difference between contoured and straight mud flaps?
359 US 520
Bibb v. Navajo Freight Lines, Inc 359 US 520 - Original Supreme Court case where the issue was first raised
The state that tried to implement the contoured mud flaps alleged that they were somehow safer than the straight ones. The trucking company successfully argued that this is an undue burden on interstate commerce.
Well this is just a guess:
But I would assume the straight ones, are straight.
And the countoured ones have a curve.
But hey, its just a guess.
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