The "technical" term for an Interstate, a freeway, or an expressway is "controlled-access highway." The elements behind the safety and speed of travel on these highways are common direction, common speed, and no intersections to choke traffic or create conflicts. Simply stated, everyone should be headed in the same direction at close to the same speeds.
This uniformity and the lack of intersections has created and enhanced both safety and efficient movement. You might already know that our interstate system was first proposed and implemented by President Eisenhower. He had seen first-hand the benefits of these modern highways when he led our troops into Germany in 1944 and 1945 and saw the German Autobahn system. He saw how these high-tech roads facilitated the movement of troops and supplies. When he proposed the American Interstate System, it was the military benefits that first came to his mind. The Interstate System is also known as the National Defense Highway System.
We accelerate on the on-ramp, we decelerate on the off-ramp, but at the points we enter or exit, we should be traveling at almost the speed of traffic, if not exactly. But what happens if I need to turn around? Crossing a median requires deceleration on the left side of the roadway where the faster traffic is. The only other choice is slowing on the right shoulder, then making a kamikaze-change-of-direction across the traffic lanes into the median in the face of 70 mph traffic. Once in the median, we must find a break in the left-lane traffic. Depending on how congested the roadway is at the time, we have to either merge and accelerate back to highway speed in the left traffic lane -- where traffic is running at 70+ mph -- or once again shoot across all the lanes to the right side before merging back into traffic in the reverse direction. Does any of this sound safe to you? Yet drivers take the chance and do it every day.
The only legal and SAFE way to "turn around" on the freeway is to continue on to the next exit, crossover at the interchange, and reenter the freeway via the onramp in the desired direction. But why, then, do many freeways have crossovers? They are only for the use of highway maintenance workers and law enforcement. It is illegal for anyone else to use them, and it's extremely unsafe, so don't do it!
Keep the shiny side up!