> I wonder if the ergonomics or geometry of a bicycle could be modified to better protect a rider’s neck.
There has been a huge trend in this direction in recent years! For example in 2000 the head tube angle on a typical “trail bike” was something like 71 degrees, but today it’s more like 65 degrees, meaning the front wheel is a lot farther out in front of you. Bikes have also just gotten longer, especially in the front end which adds to this effect. This all means that (assuming you don’t ride more difficult terrain to compensate) it’s way harder to crash “over the bars” on a modern mountain bike than in the past.
There are sections of trail that I would almost always OTB (over the bars) on when attempted on my old 1998 hard tail. After finally upgrading in 2018, I almost never go OTB because of the longer and slacker geometry.
One ride on a double black rated tech trail nearby, the air shaft in my fork seized up so I lost all front compression. Even without front suspension, I was able to complete the ride and even keep up with my group, my arms were just toast after that. Modern bikes are just that capable based upon geometry alone.
That said, I now ride at significantly higher speeds and ride much harder terrain. But even still, my crashes are different and more to the side than directly over the bars.
The more downhill oriented a mountain bike is, the more it resembles a dirtbike — more suspension travel (up to around 220mm, where the suspension starts to work against you and makes the bike too mushy for something that light and human powered) , higher and wider handlebars and more space above the seat (nowadays the seats actually move out of the way and come back when needed, operated by a lever on the handlebars).
I'd say that most honest to god OTB accidents happen on slower, gnarly, downhill stuff, where it's easy for the (still relatively light) mountain bike to get stuck and unable to roll over an obstacle in time (before the rider arrives), or for the obstacle to slap your steering to the side, which again suddenly stops the bike.
Dirtbikes are much heavier compared to bikes and the weight of the rider, and you generally ride them a bit faster and not necessarily downhill.
There has been a huge trend in this direction in recent years! For example in 2000 the head tube angle on a typical “trail bike” was something like 71 degrees, but today it’s more like 65 degrees, meaning the front wheel is a lot farther out in front of you. Bikes have also just gotten longer, especially in the front end which adds to this effect. This all means that (assuming you don’t ride more difficult terrain to compensate) it’s way harder to crash “over the bars” on a modern mountain bike than in the past.