It is just asking for trouble, but you can talk and comfort the cat while on the road. If the cat is by car but less than eight hours, there is no need for a litter box. An old metal lid or cardboard box kept in the trunk will work for longer trips. You can also purchase disposable litter boxes. Water is always required, but food may be held back for short trips.
Harnesses are available and the litter box can be kept in the trunk.
Never let the cat loose in a strange place without a restraint. There is always a chance the cat might become frightened and escape. Before travel check to find out if cats are permitted in motels and hotels. If not, offer to pay for damages. Never leave any pet locked in a car.