Professional football is a shockingly violent game.

Unless you have been on the sidelines of an NFL game, yards away from the action, you cannot really grasp of the severity of the collisions.

Which is why the NFL’s moves to limit blows to the head is an excellent idea. Otherwise we were destined to see a player killed on the field.

If you have watched even a high school football game up close, you know how startling the collisions are. Guys 16, 17 years old, just blowing up a ballcarrier as he goes out of bounds right in front of you.

And that’s just the kids.

By the time they move up to the college level, the violence has risen geometrically. And then the NFL? Insane, really.

The players are bigger and faster and more inured to violence as they climb the ladder to the professional ranks. They are encouraged to carry extra weight, and the idea of knocking out a key opposition player — literally, if possible — is still celebrated and encouraged, at least tacitly.

I covered the Los Angeles Rams for four years, three decades ago. Back when players were certainly smaller and perhaps a bit slower. But that era is pertinent because journalists were allowed to be on the field for the final five minutes of a game, albeit on either side of the 30-yard lines.

Still, that was close enough to be … well, horrified is the proper word.

Any big hit in the open field is like watching a head-on car crash. And within minutes you find yourself thinking, “How is it that no one gets killed?”

To be in the first row of the stands … you might get a notion of what goes on in front of you. To be in the upper reaches of a stadium, or in a luxury box or a press box … you really don’t have a clear idea of the violence below. You rarely hear the sound of an impact or the whoosh of air being smashed out of a diaphragm or the cries of pain from injured players.

To watch an NFL game on television is to have no sense of the danger at all. At that remove, the disconnect is total and the players are reduced almost to cartoon characters. Let’s see, Wile E. Coyote has an anvil dropped on his head; he is flattened; a moment later, he is building a new Acme-brand roadrunner-catching machine.

A receiver gets demolished on a crossing pattern … and TV viewers wonder “why doesn’t that guy get up? He’s wasting time …”

Players often don’t get up because they have concussions. Or broken ribs or collarbones or a separated shoulder. People just don’t bounce up from that in the way that Warner Bros. cartoons would suggest.

The NFL, late in its history, is acknowledging that concussions are a very serious medical event. It was common — probably still is — for a player to suffer a concussion, sit out a few plays, then return to a game. Minutes later.

We now know that is extraordinarily foolhardy. And extremely dangerous to the long-term health of the individual.

I had a serious concussion in the last tackle football game in which I ever played. At Lutheran High School in Los Angeles. I fielded an onside kickoff, and a teammate kicked me in the back of my helmet as he attempted to leap over me.

I was unconscious long enough that my father was able to see me motionless on the turf … come out of the stands on the other side of the field … walk around to my team’s bench … and be nearby when I finally came around, perhaps after something of a seizure. (No one ever quite told me.) And I regained consciousness only with the aid of smelling salts.

My first post-KO memory is of the lights (it was a night game) creating a halo around the head of our team’s coach, Jim Young, who cheerfully said, “Gosh, Obe, I thought you were dead!” It was meant to cheer me up. Back then, being knocked out was something of a badge of honor.

My badge, however, came with a night of “check on him every few hours to make sure he’s not, you know, in a coma.” It came with a month of headaches and at least two months of head-rushes so intense, whenever I got out of bed, that they were rather like blackouts — everything going dark before I regained equilibrium and the lights came back on.

I believe mine was the only concussion suffered by anyone on my high school team that season.

In the NFL, however, concussions are weekly events for nearly every team. Which is not sustainable. Too many guys are dying too young, and are found, via autopsy, to have serious brain damage.

And someday one was going to die on national TV.

Promising to fine and perhaps suspend players who launch themselves into a player’s helmet should help. These guys are professionals, after all, and $50,000 or $75,000 is serious money, even to the stars. It ought to curb their behavior. Think twice about headshots on defenseless receivers.

Let the Neanderthals squawk about how football isn’t the rough-tough game it could and should be. Times have changed. Players are bigger than ever, and at least as fast, and have learned to launch themselves into opponents like guided missiles. And they often play on artificial turf, which is famously unyielding.

The open-field headshots have to stop. And, perhaps, we can head off an NFL calamity. The one I still fear is out ahead of us.