Bibb v. Navajo Freight Line, 359 U.S. 520 (1959), helps clarify the circumstances in which these regulations will be sustained. There, Illinois required tractor trailers to be equipped with a particular design mud flap. The design commanded by state law was such that trucks legal in Illinois would be illegal in Arkansas which had a differing required design mudflap.
Take a look at the decision there, and then follow the commentary trail after it.
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