Legal Question

Well this is the first time I have ever hears this. I have my CWP, a friend just took his class two days ago and his instructor told him that if he is on an interstate his weapon must be in the trunk and unloaded... as soon as he gets off the interstate he can arm himself. His instructor said that its the law! I looked over the statues for concealed carry and did not find this anywhere.
I have never heard of this before.......
can someone fill me in??