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Comment by mikestew | Hacker News Reader

Before jumping to conclusions...

Nah, I'm jumping straight to this conclusion: if you hit something in front of you that didn't leap out in front of you at the last second, you fucked up. The object that you hit was erratically slowing and speeding up? You should have left more room to allow for the unpredictability. It was raining, can't see, the roads are slick? Leave more room in front of you.

Yes, I way too often don't do that, either. But if there's something in the road ahead of me, and I hit it? Man, there are few scenarios where I can claim that there was nothing I could do. And in the case of a full-sized vehicle in front of me, I don't care how erratically it's driving, don't run into the back of it.

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I remember taking Driver's Ed class many years ago. When we got to the section about fault in relation to rear-end collisions, the instructor said, "if you rear-end someone, it is your fault, full stop." The class then spent five minutes asking hypotheticals, to which the answer was always, "nope, still your fault."

That is correct. However, it's also the case that in the real world if you drive legally but do things like brake erratically, you will cause accidents.

Not if the vehicles behind you are self driving cars.

He's wrong. What if that car ahead just got there and is moving far below your velocity? And there's the case of where the car ahead stopped in a fashion a car can't--ran into something massive or the like. Under standard driving conditions if the car in front of you is involved in a head-on at speed you're going to hit it. 2 second following distance assumes the car ahead is subject to the same physics you are.

If you cannot see a car ahead of you in time to stop then you are going too fast for conditions.

The only time it can be not your fault is if the slow moving car switches lanes in front of you before there is time to stop.

The one exception is if a car pulls into a lane in front of you when you are traveling faster.

His actual statement was, "if you rear-end someone that you are following, it is your fault..."

If someone abruptly pulls in front of you, you weren't following them.

I have a couple more scenarios for you. Overdriving your headlights is a great way to hit something you don't see in time. The safe speed in average conditions on low beams is around 25 to 30 mph, and on high beams, it is around 45 to 50 mph. If there's any glare on the roadway from security lights and oncoming drivers, your safe speed drops 10 to 20 miles an hour.

Related to this is glare from the sun or artificial sources. I lived in a small city with antique-style globe lamps on Main Street. The veiling glare made pedestrians invisible, and even if you knew about the glare and watched for pedestrians, you would still be surprised when they became visible halfway across the street in front of you.

If your system is objectively right, but also objectively causing accidents with humans. Well you won't fix the humans...

> If your system is objectively right, but also objectively causing accidents with humans.

It's not causing accidents in these cases. The humans are.

The causality version of "cause", not the blame version. Accidents that would not have happened without the system.

Right, but also imagine how traffic would be if everyone drove with the pre-requisite distance to do that. Can you imagine I-5 traffic if everyone had 10 car lengths between them?

So while your statement isn't wrong, it's also not always pragmatic in the real world.

I’m guessing an accident caused due to not leaving enough room to safely stop is going to cause a bit more traffic than the alternative

So you're saying we should all drive with a 10 car buffer, then?

If not, then you already recognize the probability is less than 100%, so that has to be baked into your statement.

So you're saying we should all drive with a 10 car buffer, then?

The only comments saying that are...yours. If your argument is so lacking that you need to argue in bad faith, perhaps it is best to not bother at all.

Or perhaps you are not aware of the proper following distance (and therefore, part of the problem about which you complain). Two car lengths (EDIT: seconds, not car lengths; oops) is the general advice.

I'm just trying to understand exactly what they are advocating, because so many people seem to be making a dichotomous safety choice. It's not a simple model and my point is there are tradeoffs.

>Two car lengths is the general advice.

No. It's speed, roadway, and car dependent. Two car lengths isn't even sufficient at 25mph let alone at 70mph.[1] Which all goes to show how poorly people tend to think about these things and quickly resort to overly simplified mental models.

[1] https://one.nhtsa.gov/nhtsa/Safety1nNum3ers/august2015/S1N_A...

No. It's speed, roadway, and car dependent.

Moreso that my post has a mistake: two second following distance, not car lengths. Brain fart on my part; apologies for causing you to have to find a URL. But as general rules go, that increases the distance as speed goes up. No, it doesn’t account for everything, but good enough for most circumstances.

Of course it's pragmatic. Free-flowing traffic at 50mph beats people zooming up to 70mph then braking, then zooming again. Freeflowing traffic at 50mph annihilates traffic from accidents.

Traffic is rarely “free flowing” at any speed on these kinds of roads. Often I see “moving roadblocks”: clumps of cars going around or just under the speed limit jockeying around each other, impeding other traffic from moving around them. So-called “defensive drivers” are often unpredictably overly cautious: I’d wager they are at least an indirect cause of accidents quite often, but are severely under-represented in the statistics (if/when they’re represented at all).

> Often I see “moving roadblocks”: clumps of cars going around or just under the speed limit jockeying around each other, impeding other traffic from moving around them.

Indeed - this is the 70mph is slower than 50mph thing I mention.

Not sure if we’re talking about the same thing. These slower drivers aren’t any safer: they’re weaving around each other and impeding faster traffic from passing. They’re arguably more dangerous because of that. And the flow of traffic is constrained to the speed they choose, which is on average slower than it otherwise would be.

While they are correlated, flow does not equate to capacity. You can have better flow while still having reduced capacity. That’s not pragmatic.

I don't know what I-5 traffic is like, and kind of weird that you would refer to a local road on a global forum, but I'll assume it's like the M25.

Roads like that are currently operating at bursting point. There are incidents and accidents every single day and constant police presence is required to unblock them. If you alleviate congestion, more people use the road. They just go back to bursting point. In other words, it's utterly insane.

Can you imagine if there were accidents on railways or in the air every day? Imagine the scandal if train operators were found to be unsafely squeezing more trains on to the line that it could handle. Roads are stressful, inefficient and shit. Enforcing a safe stopping distance and pricing journeys accordingly, like trains, is where we want to be.

I-5 is a notoriously congested interstate in California. I don't live in California anymore, but used it simply because there are a disproportionate number of Californians on HN.

I don't disagree about "pricing journeys accordingly," but there are many reasons why this is difficult in practice in the US. Going through those points is a bit of a digression from my main point. Namely, that there are pragmatic tradeoffs that have to be considered. I'm consistently taken aback by the amount of "simple" solutions people advocate on HN and sometimes I wonder if it's due to software developers constantly working with the abstract rather than the concrete.

"Can you imagine how bad traffic would be if everyone drove safely?" is a hell of a take.

So is "can you imagine how much infrastructure would cost to ensure everyone drove completely safely"

Like most real-world engineering, there is a cost-benefit balance. Could we design an interstate highway system that allows everyone a 10 car buffer? Sure. Would we like how much it costs, the effects on the environment, etc.? Probably not.

As an aside, your comment seems to go against HN guidelines by taking the least charitable interpretation of the comment.

Why?

You might have forgotten the ultimate rule of road safety: Everything is a tradeoff. Safer is only sometimes better. Otherwise the speed limit would be 10mph, because it's quite a safe speed.

Most traffic is not caused by sheer volume - this is well studied. It is often caused by inability to maneuver, merge, etc. As a result, your i-5 traffic would likely be much much better if everyone left 10 car lengths.

You do not have to raise the average speed of travel very much to make up the theoretical loss due to increased spacing

Can you link to those studies about traffic? I’m aware of some of the studies regarding merging (eg the benefits of zipper merging) but I don’t think that covers the bulk of congestion.

This is why cars do not scale: by the time traffic slows down there are 5-6 times more cars in the lane than it can safely handle. So by the time people are asking for "one more lane" they really mean 6 times as many lanes, a regular 4 lane highway needs 20 more lanes!

Moral: support public transit.

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