When travelling on a two lane highway driving 50 to 55 mph, you need a 10-12 second gap in oncoming traffic to pass safety. At 55 mph, you will travel over 800 feet in 10-12 seconds so will an oncoming vehicle. That means you need over 1600 feet which is about 1/3 of a mile to pass safety. It is harder to see and judge the speed of oncoming vehicles that are traveling 1/3 of a mile or more away from you.
