The people who fall from the 18 foot bollards are generally not standing on top, but rather hanging on with their hands. Their feet are more like 12 feet above the ground - several feet lower if they are defeated by anti-climb plates. Often the “fall” is a semi-controlled slide down the bollards.
The result is that it is rare that Border Patrol has to recover a broken body or medevac a jumper by helicopter. The thirty foot ones are a real monster though.
Kids like to climb on the 18 footers for fun, up to where the plates start. Without the plates, a typical (but daring) teenage boy can climb the 18 footers (they basically stop women and children). 30 footers deter most boys/men from trying. With a good anti-climb obstacle on top, 30 footers will effectively stop men.
I’d like to see an area on the US side filled with pyramid shaped concrete blocks about 1 ft. apart and extending about 4 ft. from the base of the fence.
Even if the fence is only 18-20 ft., a fall would cause severe injuries... and food for the buzzards.