I would nominate Jersey barriers as well, they’re easy to mass produce and there are machines that lay them down. Many roads would be undivided if it were not for jersey barriers.
I owe my life to Jersey barriers. Around 1999 or 2000 I was driving an old Plymouth Barracuda (Slant 6 grandma's car, not a hemi monster) at 70 miles per hour in the fast lane on a freeway in Nevada. The car drifted to the left naturally because it needed an alignment and as I went to correct to the right nothing happened. The joint that connects the steering column to the steering gearbox had become disconnected. I didn't step on the brakes because they were also somewhat if he and might have sent me in any random direction!
Had it not been for the graduating slope of the jersey barrier I would have either gone over into oncoming traffic which was also doing 70 miles per hour or more, or I would have bounced back into traffic on my own side of the freeway with random results. I would not most likely not have survived.
Instead my front left tire caught the steepest graduation at the bottom of the jersey barrier which launched front left corner of the car up the barrier. But then a funny thing happened. I didn't flip over and I didn't go over it. Because the top section of the barrier is vertical I just came back down. This cycle happened several times until the car slowed down enough just to be sliding against the bottom of the barrier. At that point I was only doing about 30 miles per hour and could hit the brakes without worrying about what direction the brakes were going to take me. The car came to a stop. The only damage was a broken ball joint, a very messed up tire and wheel, and a bent fender lip!
A wrecker came and got me and delivered the vehicle to my house. I horse traded that car the very next week for an old Land cruiser that tried to kill me when it lost brakes, but that's another story. I drive better vehicles these days but only barely
