Railway Express Agency Inc. v. New York 336 U.S. 106 (1949) New York city enacted a traffic law that said you couldn't drive around in a vehicle for the sole purpose of displaying ads. However, you could put your own logo on your vehicle, as long as it was engaged in legitimate business (like making deliveries). Railway Express was a delivery business, and was putting third-party ads on their delivery vehicles. They were convicted under the law and appealed. Railway Express argued that the traffic law was an unconstitutional violation of the 14th Amendment because it discriminated against their business. The US Supreme Court found the traffic law to be constitutional. The US Supreme Court found that that to withstand judicial scrutiny on equal protection grounds, a law must bear a rational relation to some legitimate end. That's the rational basis test. The Court found that preventing distractions to drivers was a legitimate end, so the law met the rational basis test. Railway Express argued that the law was underinclusive because it didn't ban all advertising (since you could still advertise your own business). However, the Court found that it is constitutional permissible to be underinclusive, as long as the law is at least a step in the right direction to the legitimate end. "The city's means is not to regulate all advertising, or all ads on vehicles, this is only meant to cease certain ads unrelated to businesses on those businesses' vehicles. There is no problem here; it's not arbitrary. The city could even go so far as to limit the size, shape and coloring if they wished, as long as the ends justified the means." Basically, this case said that if there is a legitimate purpose (in this case distracting drivers with ads), it is constitutionally permissible to write a law that addresses one small part of the problem (3rd party ads on commercial vehicles), while not addressing other similar parts of the problem (like billboards). That's true even if the line you draw is completely arbitrary.