I play on my mouth piece while driving into work, but the key is to play the mouth piece into a towel. This gives a comfortable resistance (you control the resistance level by how hard the towel is held). What I have found by doing this is my stamina and control has increased tremendously. I got the idea by reading a piece on Coltrane that he would practice for hours with a towel stuck into his bell. Playing against such resistance made his playing with the towel removed relaxed and fluid. So now I take my mouth piece ONLY with me on business trips and use towels per courtesy of Hyatt, Marriot, Wyndham, Rock Resorts, etc. to keep my chops in prime condition when on the road. Give it a try and let me know if it works for you as well after a week or so of practice.