You must use the _________ to drive at a speed where you can always safely stop your vehicle.

You must follow the four-second sight distance rule to stop safely. Pick out a stationary object as far ahead as you can clearly see (e.g., a sign or a telephone pole). Start counting “one-one-thousand, two-one-thousand, three-one-thousand, four-one-thousand”. If you reach the object before you finish "four-one-thousand", you need to slow down.