This, of course, has a profound impact on interstate commerce, and is therefor within the purview of Congress.
New York City spans two different states (New York and Connecticut), and roads are the transportation network that bring things from one state to the next. Honestly, I can't think of a more cut-and-dried case of interstate commerce than this.
This has noting to do with regulating interstate commerce. Interstate commerce can take place just fine with the uppercase font signs that New Yorkers are quite used to by now. And NYC is not in Connecticut.