Not only is it safer for the cats to travel in their crates or carriers, but cats usually prefer it. The confines make them feel safer--especially if you also include their favorite blanket or toys to make the surroundings more familiar. It helps to get your pets familiarized and friendly with their carriers before hand. We store Skeeter's toys in his carrier, and he'll lay in it and take naps occasionally. It feels like a "safe place" to him now. Cats will cry in the car, but that is because the movement/sounds of the car frighten them. So, I would also suggest taking your little ones for car-trips around the neighborhood, then perhaps around town for a while.
Most of the time, cats will settle down after a while of being in the car. Most likely, they'll sleep most of the way (like most of us!) if you get them familiar enough to the car. Keep the windows closed and keep the car cool inside--they don't like the sound of the wind, it scares them. Play soothing music. And I would suggest keeping food and water rationed to prevent car-sickness if they are proned to it. I hope this helps!