Stopping suddenly is dangerous. When you brake quickly, you could skid and lose control of your vehicle. You also make it harder for drivers behind you to stop without hitting you. Try to avoid panic stops by seeing events well in advance. By slowing down or changing lanes, you may not have to stop at all, and if you do, it can be a more gradual and safer stop.
Many highways have rest stops and service areas.
You can stop on the shoulder of the highway in an emergency. If you do, help other drivers by turning on your emergency flashers. It is also helpful to raise the hood, or tie a white cloth to the antenna. Many highways have controlled access. This means you can enter or leave the highway only where there are entrance or exit ramps.