Yes. I even reached through the sliding back window to grad the drivers arm and harass him, but the driver (my cousin), was a motorhead (as were all my cousins)
who raced go-carts, then mini-sprints, then sprint-cars, and laughingly threw my hand off and continued on.
I was in a village in Mexico once, where the local duty police cruiser was a pickup truck. After school, the school bus dropped the kids off on the highway, about two miles from the village. The cop met them there in the pickup, loaded a dozen kids in the back, and drove them to the village every day. What’s the problem?
Standing up, Baby!
While holding onto two mattresses.
Once a big mirror.
As with water skiing, the driver is what makes it safe or not, not what the person in back is doing. 
See location.
Yep, and here in Texas they are more often than not called simply “Pickup” not "pickup TRUCK’
Here is red Arizona, there is no prohibition about passengers (adult OR minor) or animals riding in truck beds.
I think I’ve done it a time or two in my neighborhood recently.
But the lasting memory is in college I rode on the back of a truck from where the res halls were to IHOP and back at, like, 2 AM. It was winter too. Froze my damn ass off…
(50-59): Not that often, but yeah, including on an Interstate. The last time I rode in the back of a pickup, my father-in-law was back there with me! (He was in his seventies at the time).
Been a good few years, but the last time was in Toronto, bar-hopping. We knew it wasn’t legal, but there were too many to ride in the cab, so a couple of us lay flat in the back of some guy’s pick up where we couldn’t be seen. Interesting perspective on the city, driving around and looking up at the stars and buildings from that angle. Good times.
Many times, but I remember one specifically. We left camp in central MN with a ton of us teenaged boys and all our gear in the back of a pickup truck and drove into Wisconsin to canoe a river. Probably 60 miles or so with the bed mostly full of stuff and 7-8 of us sitting on top of it and 5 people stuffed in the cab. We did get pulled over and lectured by a cop, so our adult leaders decided they wouldn’t drive us home like that. We had to sit and wait while they drove back to the camp and picked up more vehicles.
I’ve of course ridden in the back of an open pickup, we never thought anything about it, but the Boy Scout part stirred memories.
We were camping in the New Mexico Pecos Wilderness and one of the planned hikes was from the base camp into Santa Fe. Quite a day hike, and a lot hotter than I think they had planned. Anyway, we hit the outskirts about dusk and got a burger at a bowling alley, had pickups to take us back to camp. But it had rained hard in the mountains while we were gone, washing out the road, so we had to sleep in the back of said pickups about a dozen scouts per. Fun times!
And in Australia they’re called "Utes"and in South Africa “Bakkes” (pronounced “buckies”)
Well, someone had to keep the dog company. Might as well be me.
Yes, many times. Often with the tailgate down (so as to accommodate the entire baseball team) and 4 guys sitting on the tailgate. Four pair of legs hanging off the back of said tailgate and steel cleats dragging the road making a spark show. The tailgate was where everyone wanted to sit.
A few times, but never in our truck. My dad said it was dangerous. He was ahead of his time. 
I was expecting more horror stories.
You have a very different idea of danger than I do, I think.
In the Army yes. I don’t think I ever did on the civilian side.
When I was a kid my suburban family never owned a pickup but various friends of my parents did. I rode in the back of one a few times back in the mid-1960s. Not since.
Around here we frequently see lawn / landscaping crews riding in the back of pickup trucks. A couple days ago I was on the freeway following a pickup with three probably Haitian dudes in the back. They were sorta piled on top of all the lawn machines. We were both going about 80.
I can’t recall the last time I saw children in the back of a pickup. It’s gotta be decades.
=========
Unrelated story about riding in pickups. …
When I was USAF a squadron-mate had a pickup truck. And had recently bought a purebred AKC Labrador male puppy intending to breed it as a hobby for profit. Paid serious $ for the dog. Who turned out to be A) sweet as could be; and B) dumb as a box of rocks.
At about 2 yo (= just coming up on prime breeding age) the dog was riding in the back of the truck. And jumped out going about 40mph. He landed on all fours, did the splits, and abraded his testicles right off his body. All that as left was some scrotum hanging by a thread. Otherwise he was a little scraped up but basically OK. The vet cleaned him up, closed where the scrotum had been with a few stitches and sent him home.
Oh well; and thus are the dreams of Man captive to the Fates.