I do not see the constitutional authority for Federal Government directives . As I've pointed out already they use a system of back door funding mechanisms to get their will on issues like speed limits and drinking age etc. You can be assured that if they did they would make such a directive even though it would be a stupid law for much of the geographic territory of the nation.
Now I have done some digging into the specific case that prompted the NTSB to make it's recommendation . It involved a kid texting (already illegal in Missouri where the pileup occured ) and this little tid bit of detail they failed to emphasis .
Why the Proposed Car Cellphone Ban Is Wrong - Popular Mechanics