If your cat is a nervous traveller it may be a good idea to ask your vet for a sedative before going on a long car journey. Your vet will want to examine your cat first and may prescribe a drug which you can administer yourself (although they may have quite unpredictable effects). If you are given a sedative it should be administered about half an hour before the journey and will last for up to eight hours.
Do not sedate your cat before a flight because if it is drowsy it will not be able to adjust its posture for sudden movements and can be thrown around the box on a bumpy flight. Also there is some evidence that sedatives can be dangerous for cats travelling in the cargo hold. Sedatives appear to work differently in cats at higher altitudes and in most aircraft holds the air pressure is the equivalent of travelling at about 8000 ft (2400 m).