1990: "Driving Cars"

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jello34543
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Re: 1990: "Driving Cars"

Postby jello34543 » Fri May 11, 2018 9:29 pm UTC

Leovan wrote:I'd also recommend motorcycles for driver's ed. There's no better inspiration to learning how to judge traffic and practice defensive driving than knowing you're half their size, have no protection, and they might not see you.


Agreed. I've occasionally pondered that it should be required to learn motorcycle riding before being allowed to drive a car (with limits on power:weight ratio and total power for new riders) It would never happen, people are way to attached to their cars, but part of me likes the idea a lot. Winter would be a bit of a problem though...

DanD wrote:It's also possible to take the vehicle into account. A tractor trailer is not safe going 120 on a highway. A low slung car, in good weather conditions, might be. A good automated system can be trusted to make that judgement call realistically, including adjusting for traffic, etc. Therefore automation allows the possibility of eliminating one size fits all speed limits.


Having vehicles on the same road going significantly different speeds is a really bad idea. Everything is about relative speeds. Your low slung car going 120, hitting a tractor-trailer going 60 is not meaningfully different from your low slung car hitting a brick wall at 60. The fact that slower drivers can't be counted on to keep right (US) makes things 1000x worse in your scenario.

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Re: 1990: "Driving Cars"

Postby Mikeski » Sat May 12, 2018 3:55 am UTC

wumpus wrote:For highway traffic, this should be trivial to compute by determining the spacing of cars possible by a human, and the spacing of cars possible by a human.

For US highway traffic, assuming 70mph traffic (sorry about imperial units, but metric units are only used to repair cars in the USA):
4 seconds (what I needed to write on my driving test in the 1980s): ~400 ft
2 seconds (what is currently recommended) ~200 ft
1 second (typical in Maryland driving) ~100ft
0.1 second (possible for a computer?) -10ft?

A factor of 10 assumes that either the drivers aren't tailgating (which likely only happens in places that *don't* need more roads) or the computer simply has *zero* spacing between cars. A factor of 3 to 5 seems possible, although how you would be certain that a human didn't slip into your convoy is beyond me. Don't forget the aerodynamic advantages to all that drafting as well.

You would be certain that human drivers didn't slip in simply because they can't. At 10 foot spacing, no one can change lanes without being part of the computer network, and asking for space between car NY-ABC123 and car CA-4DEF567. A 15-foot-long car won't fit in a 10-foot hole. :wink:

I don't want to drive ride at a distance where one dropped packet (I'm gonna stop: all cars in lockstep behind me, please hit the brakes now) means a giant pileup. Waiting for sensors to positively determine the car in front of you is braking implies nonzero reaction time and would require more space. Computer clock speeds might be in the GHz, but sampling rate on radar/sonar is in the KHz to (maybe) 10s-of-MHz range. And the brakes themselves are mechanical actuators with reaction times in the milliseconds, not micro- or nanoseconds.

Even with a theoretical zero reaction time, 10 feet isn't going to be reasonable. It is only safe if every car has exactly the same braking ability. But, even just looking at Ford (pulling numbers from Car & Driver online reviews), you have a range of 145ft to get from 70mph-to-0 for a GT, to 209ft for an F-450.

If the truck is following two GTs, each at 10 foot spacing, and they all brake (at max power with zero reaction time) because a deer jumped into the road, the truck drives right over the top of both GTs and hits the deer in front of them: 64 more feet to stop, 20 feet of space, just over 31 feet of GT. (Yeah, it would really just push all 3 vehicles into a single pileup. Driving over the top like a mini-monster-truck sounds funnier, though.)

Even the difference between a plain-old Focus (179 feet) and the rally-car-trim Focus RS (154 feet) is way too much for a 10-foot following distance. Or a baseline Taurus (180 feet) vs. a baseline Mustang (156 feet).

I suppose every car could broadcast its expected braking distance, knowing its own brake and tire wear, road conditions, etc, (dynamically, to adjust for expected brake fade, even) to keep the following distances minimal, but they aren't going to be "10 feet" except in the most laboratory-esque circumstances.

Safe spacing = reaction time + difference in braking ability. 200 feet for a human is usually reasonable (at 70mph, that gives you 1sec reaction time plus 100 feet of braking delta, and humans are a bit faster than that if they're paying attention, and 100 feet is a generous delta; that's a moderately-worn big SUV following a new sports coupe).

So, an all-human 215-foot pitch between vehicles (200' space + 15'-long car) versus a very-generous 75-foot pitch (60' space + car) gets close to a 3x improvement. Something under 2x is a more-reasonable estimate, given tailgating in high-traffic areas cutting the human distance down. (115' to 85' is 1.35x)

(Then we can add the triple-trailer semi trucks to the equation... or even just F-150s with big camper trailers with brakes, or small U-Haul trailers without brakes...)

wumpus
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Re: 1990: "Driving Cars"

Postby wumpus » Sat May 12, 2018 4:50 pm UTC

Mikeski wrote:
wumpus wrote:A factor of 10 assumes that either the drivers aren't tailgating (which likely only happens in places that *don't* need more roads) or the computer simply has *zero* spacing between cars. A factor of 3 to 5 seems possible, although how you would be certain that a human didn't slip into your convoy is beyond me. Don't forget the aerodynamic advantages to all that drafting as well.

You would be certain that human drivers didn't slip in simply because they can't. At 10 foot spacing, no one can change lanes without being part of the computer network, and asking for space between car NY-ABC123 and car CA-4DEF567. A 15-foot-long car won't fit in a 10-foot hole. :wink:


You really don't want to drive in Maryland (or DC/Northern VA). You'd be surprised at the number of people who will drive like they have one of these computers on board.

Braking distances are pretty irrelevant if you let the heaviest vehicle migrate to the front (assuming that even old-base-cars can out brake a large SUV or bigger truck). If the lead car takes 200+ feet to brake, all other cars simply outbrake him at <200 feet and don't have a problem. Granted, there are still going to be issues where you can outbrake the danger but the guys behind you can't, but conditions needing braking between 200 feet and 150 feet are rare*.

* Back in 1994 I was driving a rental car on a Chicago highway when I crested a hill and suddenly saw a traffic jam in front of me. I slammed on the brakes, the anti-locked kicked in, and I stopped less than a car's length from the guy in front of me. So maybe not. No idea how bad it would be without anti-lock brakes (they were a new thing back then, especially on the cheap rentals my job provided).

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Re: 1990: "Driving Cars"

Postby gmalivuk » Sat May 12, 2018 6:04 pm UTC

Let the heaviest vehicle migrate to the front of what, exactly? You seem to be imagining a little cluster of ten or so cars isolated from heavy traffic in both directions, which is typically not what roads look like when we're talking about traffic *problems*.
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Re: 1990: "Driving Cars"

Postby Sableagle » Sun May 13, 2018 12:12 pm UTC

Logical answer: sod the cars. Use trams and trains.

jello34543 wrote:Your low slung car going 120, hitting a tractor-trailer going 60 is not meaningfully different from your low slung car hitting a brick wall at 60.


Actually it is different, because the brick wall extends all the way to the floor.

Crunchy pics behind spoiler:
Spoiler:
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Trains don't disappear under trucks.
Oh, Willie McBride, it was all done in vain.

DanD
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Re: 1990: "Driving Cars"

Postby DanD » Mon May 14, 2018 2:28 pm UTC

jello34543 wrote:
Having vehicles on the same road going significantly different speeds is a really bad idea. Everything is about relative speeds. Your low slung car going 120, hitting a tractor-trailer going 60 is not meaningfully different from your low slung car hitting a brick wall at 60. The fact that slower drivers can't be counted on to keep right (US) makes things 1000x worse in your scenario.


If both vehicles are in communication with each other, or have sensors with enough range to see each other well within their stopping distance, the relative speed is less of an issue. Not to mention automated lane selection based on a range of expected top speeds.

Vehicles on limited access highways going significantly different speeds is not a bad idea. Vehicles impacting at significantly different speeds obviously is. But A does not imply B unless there is insufficient control built in.

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Re: 1990: "Driving Cars"

Postby Znirk » Wed May 16, 2018 6:53 am UTC

DavidSh wrote:Clearly a randomized double-blind trial of the value of driving practice/instruction is called for. The control group would have to have some kind of placebo practice/instruction.

And because it's double-blind, the instructors themselves mustn't know whether they're providing genuine or placebo instruction. Clearly we'll have to scrape the Dunning-Krüger barrel for instructor candidates.


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