It is not really tenable to argue that Congress can't pass a law requiring FHWA to approve a certain typeface family for use on highway signs. However, it is perfectly reasonable to argue that in so doing, Congress breaches an institutional norm in favor of leaving technical decisions to engineers in the permanent administration, trusting that they will make their choices in the public interest and on the basis of careful study of the various options, using decision-making tools such as cost-benefit analysis.
This is admittedly an idealized view of how things are to work. Back in 1958, green was chosen as the background for guide signs not on the basis of controlled legibility testing, but rather through a glorified popularity contest. And in this particular case we are focusing on Clearview versus the FHWA series, a controversy which is dwarfed by the fact that FHWA's mixed-case requirement amounts to a loophole allowing agencies to use mixed-case Series B on freeway guide signs, without regard to its unit legibility.