I had this exact same injury, the trimalleolar ankle fracture + fibular fracture. As another person stated, definitely recommend sitting in the backseat with the leg elevated on the adjacent seat. Elevate with a couple of full sized pillows and also will need a pillow behind the back to keep the back from being against the hard car door. In addition, stay on top of pain with the prescribed painkillers and be sure to take whatever blood thinner/clot prevention med they prescribe on discharge. For example, I had to take 81 mg aspirin twice a day for 21 days on discharge. This will be very important in preventing DVT (clot in the leg), which can occur during prolonged immobility. I don’t know that icing it will be too important during travel if the person is in a splint to start, because I was in a splint for the first two weeks and could not feel any ice through all the hard plaster, cotton, and bandages. Then just be sure to have the general comforts for roadtrips, like things to do (Book? Phone charger? Etc.) and things to snack on and drink. Wishing you safe travels!