Why would you want to limit that? How many times have you seen Horses transported that way, I have never seen that. I travel by vehicle a lot and live right on the interstate. No reason for the change.
To amend title 49, United States Code, to prohibit the transportation of horses in interstate transportation in a motor vehicle containing 2 or more levels stacked on top of one another. More: beta.congress.gov.
This is a state issue especially considering that the federal highway "commission" is growing more and more unable to fund the interstate system (which caused the recent law passed that states may now, without apparent regulation, impose taxes and tolls to use the federal highways...).
Leave it to the states just as you've left raising money for federal highway maintenance and construction to the states.