You can build a Buttboard in the morning and exceed Freeway speeds that afternoon.
A sack of potatoes could haul ass on these things.
But it won't corner or stop safely. Your goal isn't going fast; it's about having fun.
(And keeping blood on the inside of your body.)
Safety proponent, Bob Swartz, demonstrates ATGATT (All-The-Gear; All-The-Time)
There are three main ways you could get hurt while Buttboarding:
1) Grind away part of your body against the pavement - (Abrasion Hazard).
2) Run into something at speed - (Impact Hazard).
3) Get your guts squished out by a car - (Avoidable Hazard).
The first hazard is prevented though the use of gear; the second is avoided through skill; and the last hazard is avoided by judgment. Actually, only good judgment is required to avoid all three mishaps.
Every competitive Buttboarder you'll see is wearing a full set of leathers, gloves, and a full face helmet. They dress like motorcycle road racers, because when you come off at freeway speeds it doesn't matter what kind of vehicle you just came off.
If you participate in the sport thoroughly, you will come off your board and slide to a stop at some point. If you wear appropriate protection you will likely dust yourself off, grab your board, and continue on. If you come off at high speed wearing only jeans and a t-shirt, you may go into shock, vomit, cry uncontrollably, and face a multitude of painful skin graft operations. There's nothing cool or macho about that.
And let's be clear about skateboarding and injury. Coming off at 20 mph (faster than you can run) makes big, weepy, "Thrasher Magazine" strawberries. You can suck it up for a month and tell you friends how you stick to the sheets all night. You'll have scars to show off years later. Pretty stoic. We're not talking about that.
At 60 mph unprotected skin vaporizes and you're immediately grinding away bone that the doctors can't fix. An unprotected fist on the pavement means your shredded fingers detach and hit the guy behind you. If you "tuck and roll" without a helmet, the giant belt sander that is the road will strip off hair, scalp, and skull, grinding into your brain before you even complete the somersault. It happens that fast.
But if your wear appropriate protection, you'll likely brush it off and keep riding. It's amazing the difference proper gear can make.
Don't kid yourself that "We aren't riding motorcycles, so we don't need motorcycle helmets." Bicycle helmets are made for crashes at bicycle speeds — 20 mph and under. If you go motorcycle speeds, you need a motorcycle helmet. Skydiving and paragliding helmets look futuristic, but they are made in case you bump your head as you jump out the airplane door.
Even at bicycle speeds (which are quite fun) you probably want a full face helmet. Because your face is so low to the ground, everything the guy in front runs over tries to go in your mouth. Horse droppings are bad, but a least they sort of taste like grass. Don't even think about riding with an open face helmet on a path where people walk dogs.
Good News - Bad News - Good News.
The Good News is that because your body is mere inches off the road, if you start to fall off your board you are already on the ground. Unless you tumble. When stand-up skateboarders lose their balance they fall and tumble and add impact injuries to abrasion. Buttboarders almost never tumble. It's easy to spread out and just slide to a stop. Which is no problem if you're wearing appropriate abrasion protection.
The Bad News is, other than a helmet, that there's no appropriate protection for hitting a stationary object at speed. Running into a brick wall at only 35 mph is like falling off a 5 story building. Only about half the people who've fallen off 5 story buildings survive. But the dead ones probably hit their heads, and at least you'll be wearing a helmet. Right?
The Good News is that brick walls don't jump in front of you while you are riding. Buttboards are very controllable and there is no reason you should run into anything. Ever. Typically you will ride on wide, predictable, clear paths; those completely free of obstructions. This isn't "Out of Bounds" skiing where you're supposed to slalom between the trees. Yes, there may be poles and trees and boulders on the sides of the road, but there is no reason to hit them. When people do hit things on the side of the road it's almost always because they are racing, or training for racing.
Racing adds a major element of danger to Buttboarding. Instead of going reasonable speeds to just have fun, you'll take greater than normal risks to win. However, the major advantage to organized racing is there will be NO CARS.
And as dangerous as racing can be, you won't read about Buttboard racing fatalities. However, every year or so you'll read about some kids bombing through an intersection and one gets run over. It happens with sickening regularity.
These aren't kids with purpose built Buttboards who've read the Street Luge Survival Guide and are wearing leathers and a helmet. No one's told them about what's reasonably safe and what isn't. They just find a hill and roll down it using the same skateboard stuff they ride around on. The outcome isn't much different than if they were riding "Big-Wheels", or bicycles.
Typically, it will be a straight residential two way street, down a hill and through an intersection with a stop sign. One of the kids will be posted "to stop traffic" while the other flies through the intersection. Invariably, the one with crossing guard duty will be a young teen, or a pre-teen. It's never made clear exactly what goes wrong at the crucial moment. Maybe the driver isn't paying attention. Maybe the kid realizes the car won't stop and switches to warning the friend. Maybe the driver thinks it's a prank.
But know this: Never expect a youngster to stop an aggressive SUV.
Particularly if your life depends on it.