Tires can only get so much friction with the road and after that, brakes lock up. Purpose built skids wouldn’t be any better. We’re easily at that limit. I’m already seeing commercials for cars that hit the brakes for you, but I’m not sure how much time they shave off or what the unintended consequences will be.
If you want to decelerate at more than ~1G you need other ways of doing it. A maximally efficient 10x10 parachute would have to be ~16′ diameter to get 1G for a small car at 60mph. At 30mph you’re down to a quarter G.
Other methods I can think of would be forward facing rockets (~40lb worth of fuel) with their obvious downsides, an explosively fired anchor and winch system, or a rail on every street everywhere for the brake to grab (like the “driving” ride at disneyland). Nothing easy..
If the obstacle in front of you is a car limited to 1G, then to first order all you have to do is start braking at the same point he started braking. Computers can probably get really good at this. If your problem is that you’re driving into trees and poles, then stop driving your car into trees and poles.
[upvoted for being clear, concise, and informative]
To amplify slightly—Another way to shave off reaction-time is to predict when the car ahead is going to brake. In particular, there’s a common case where car A brakes, causing B to brake, causing C to brake. There’s a fair bit of ongoing research into so-called open road platooning, the premise of which is that a computer in car A should tell B,C, and succeeding cars “I am about to brake” so that C doesn’t need to wait for B to react before beginning to do the same.
It’s neat stuff. And given that self-parking and self-driving cars are starting to enter the market, I think we may see it deployed within 20 years. At a guess, commercial freight trucks will be early adopters.
This 1971 patent says the inventors tested a device that can stop a car from 50 km/h within 2 meters, which indicates deceleration of about 5g.
ETA: Apparently it has “anti-skid projections” on the bottom, which look like sharp protrusions that can dig into the pavement (presumably damaging it during the stopping process). Is there any reason why this might be physically implausible?
It’s certainly plausible to decelerate rapidly by grabbing onto the pavement. You can decelerate even faster by running into a concrete wall. I suspect that 5g is more than the car (or its inhabitants?) can sustain without injury. I’m sure that digging into the pavement won’t go over well with the local highway department.
Note also that 50km/hr isn’t all that fast -- 31ish MPH I think—and that kinetic energy goes as the square of speed. So at freeway speeds, you’ll do proportionately more pavement damage.
I was skeptical of the injury potential at only 5g, since fighter pilots and F1 drivers routinely experience 3g acceleration; but this study%20933-9000 “pdf warning”) indicates that 5g is just over the threshhold for whiplash damage. This may also be due to the unusually good physical conditioning of fighter pilots and F1 drivers; physical conditioning certainly has the possibility of reducing neck damage.
More importantly, a pilot’s seat is significantly different from a driver’s seat, and they’re designed to hold the pilot in a way that limits relative movement of different body parts.
I would guess that the design of the device can be adjusted to give a smaller deceleration if needed to prevent injury. To minimize pavement damage, it’s probably a good idea to place the device under the automated control of an emergency stopping system that can detect imminent collisions and apply either the brake or this device depending on the deceleration needed to prevent the collisions.
It’s not actually that easy.
Tires can only get so much friction with the road and after that, brakes lock up. Purpose built skids wouldn’t be any better. We’re easily at that limit. I’m already seeing commercials for cars that hit the brakes for you, but I’m not sure how much time they shave off or what the unintended consequences will be.
If you want to decelerate at more than ~1G you need other ways of doing it. A maximally efficient 10x10 parachute would have to be ~16′ diameter to get 1G for a small car at 60mph. At 30mph you’re down to a quarter G.
Other methods I can think of would be forward facing rockets (~40lb worth of fuel) with their obvious downsides, an explosively fired anchor and winch system, or a rail on every street everywhere for the brake to grab (like the “driving” ride at disneyland). Nothing easy..
If the obstacle in front of you is a car limited to 1G, then to first order all you have to do is start braking at the same point he started braking. Computers can probably get really good at this. If your problem is that you’re driving into trees and poles, then stop driving your car into trees and poles.
[upvoted for being clear, concise, and informative]
To amplify slightly—Another way to shave off reaction-time is to predict when the car ahead is going to brake. In particular, there’s a common case where car A brakes, causing B to brake, causing C to brake. There’s a fair bit of ongoing research into so-called open road platooning, the premise of which is that a computer in car A should tell B,C, and succeeding cars “I am about to brake” so that C doesn’t need to wait for B to react before beginning to do the same.
It’s neat stuff. And given that self-parking and self-driving cars are starting to enter the market, I think we may see it deployed within 20 years. At a guess, commercial freight trucks will be early adopters.
This 1971 patent says the inventors tested a device that can stop a car from 50 km/h within 2 meters, which indicates deceleration of about 5g.
ETA: Apparently it has “anti-skid projections” on the bottom, which look like sharp protrusions that can dig into the pavement (presumably damaging it during the stopping process). Is there any reason why this might be physically implausible?
It’s certainly plausible to decelerate rapidly by grabbing onto the pavement. You can decelerate even faster by running into a concrete wall. I suspect that 5g is more than the car (or its inhabitants?) can sustain without injury. I’m sure that digging into the pavement won’t go over well with the local highway department.
Note also that 50km/hr isn’t all that fast -- 31ish MPH I think—and that kinetic energy goes as the square of speed. So at freeway speeds, you’ll do proportionately more pavement damage.
I was skeptical of the injury potential at only 5g, since fighter pilots and F1 drivers routinely experience 3g acceleration; but this study%20933-9000 “pdf warning”) indicates that 5g is just over the threshhold for whiplash damage. This may also be due to the unusually good physical conditioning of fighter pilots and F1 drivers; physical conditioning certainly has the possibility of reducing neck damage.
More importantly, a pilot’s seat is significantly different from a driver’s seat, and they’re designed to hold the pilot in a way that limits relative movement of different body parts.
I would guess that the design of the device can be adjusted to give a smaller deceleration if needed to prevent injury. To minimize pavement damage, it’s probably a good idea to place the device under the automated control of an emergency stopping system that can detect imminent collisions and apply either the brake or this device depending on the deceleration needed to prevent the collisions.
Suhweet, thank you, that was just what I was looking for. :D