Mudflaps?

I am having a hard time finding some information about MUDFLAPS! (crazy i know it!) It was brought to my attention that mudflaps are not required by certain states across the union ... hummm ... how much do they play into safe that can easily avoid the splash back from large tractor trailers - hummmm - i see how this can be a safety issues. Anyone have some insight??? Record of laws??? Let me know. Bsafe.