A loose cat in a car is definitely not a good idea. Cats won't sit still and stare out the window while you're driving; they'll dig their claws into your seats and possibly into your head. A cat can easily sneak under your feet and get lodged under the gas pedal -- not where you want her to be when you're driving 70 miles (112.65 kilometers) an hour down the highway.
You and your cat will both feel much more secure if you put her in a carrier. Soft-sided carriers are lighter to carry if you're bringing your cat on board an airplane, but hard plastic or wire mesh carriers are sturdier and less likely to get crushed in the cargo hold. A hard kennel is a requirement if your pet is going to be checked on the airplane. Put your cat's favorite blanket and toy inside the carrier so she'll feel more comfortable in unfamiliar surroundings.
The carrier should be big enough for your cat to stand up and turn around in. It should be made of a leak-proof material that won't let in moisture. To fly, many airlines require that carriers be ventilated on at least two or three sides so your pet has plenty of air during the trip. Label the crate "Live Animal" if your cat is going to be flying in the plane's cargo hold.
So your cat won't clutch at the sides of the carrier in distress when you put her in there, acclimate her to it before you leave. Place the carrier on the floor of your home with the door open for a few weeks before your trip. If she doesn't wander in on her own, put a few treats inside to entice her, or pick her up and put her in there. With the door closed, leave your pet in her carrier for about a couple of minutes at a time, gradually working your way up to 5 minutes, then 10, and finally 30 minutes or more. Take short trips to the vet or the pet store to get her used to the idea of being on the road.