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Comment by grepfru_it | Hacker News Reader

Isn’t this a sign that you are following too closely? A driver should typically be able stop a car length per 10mph. So if you are traveling at 40mph you would need about 4 car lengths to react and stop. Ergo if you are following closer than that you may not stop in time. Just because you are using cruise control doesn’t absolve you of this requirement. My car allows me to set a cruise control gap in seconds which translates to reaction time. 2 seconds is the lowest gap mode and usually leaves me with half the distance necessary for a safe stop (although the car has aggressive brakes and can quickly slowdown at the expense of passenger comfort). Due to this I leave the gap mode at 6 seconds which is larger than I would like but would (and has) allow me to slow down in time for a stopped object in the road.

I apologize for any flame wars this creates, since people find driving to be deeply personal and any findings which may criticize their driving habits is generally frowned upon.

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* ... should typically be able stop a car length per 10mph.*

This is not correct, stopping distance scales as the square of speed. The distance you'd need to stay from the vehicle in front to be able to stop if it magically came to an instantaneous stop is far, far larger than anyone will reasonably keep.

The stopping distance at 65mph, including reaction time, is at least 300ft. That's ~20 car lengths, not 6.5 that your formula would yield. If conditions are wet, your tires are not in great shape, or you're not paying attention, it's going to be much longer.

Cars in front don't usually hit walls while they're traveling. It takes a while for the car in front to stop too.

But it is possible, when the cosequence is someone dying, the likelihood do not need to be high to act carefully. Anyway the example here was about a car in front avoiding a stopped car.

The thread is about how closely to follow a car. If the car is stopped, the answer is "as closely as you like, it's not moving".

This thread is about how closely to follow a moving car. The worry is if the moving car suddenly switches to being stopped. Will you be able to also switch to being stopped before hitting it?

Yea, some people seem to miss that just because the car in front of you can jump over in another lane doesn't mean you can at all. If you have a car beside you, and the car you're tailgating does not, then you're screwed in an emergency situation like this.

The scenario under discussion is when the car in front doesn’t stop but changes lanes, revealing a stopped car in your lane right where a moving car had been. It’s like the car in front of you was suddenly stopped by magic.

The solution is typically to swerve with the other car, but sometimes that’s not possible.

Drag works the same way, which is why a pickup truck can stop from 90 mph relatively quickly

Drag is insignificant compared to brakes, unless you're going very very fast.

if that were the case, cars would just casually accelerate up to near Mach 1 without any issue.

If a car has its brakes on, it'll be extremely hard to accelerate. The brakes create more movement resistance than air resistance does.

If a car has its brakes on, it'll be extremely hard to accelerate

It should be impossible to accelerate. None but maybe the most high-output engines should have enough power to overcome the car’s brakes. Aerodynamic drag is not quite a rounding error, but I don’t imagine it counts for a whole lot in this scenario in comparison.

No, because the engine is also insignificant compared to the brakes. A SUV might have brakes (briefly) capable of 900 HP of power absorption

Correct. Now please tell the goddamn drivers in the Seattle area. They aren’t listening to me.

I'm convinced that if police ticketed people for tailgating like they do for speeding it would save thousands of lives.

Following too closely is just how people drive, unfortunately.

The main reason I almost always have adaptive cruise control on is so that it keeps me at a safe braking distance from cars in front of me but I've found that most people do not care.

Just yesterday there was a corolla that was following a school bus so closely i'm sure it could sniff the bus' ass. That driver was one hard brake away from being decapitated by the school bus bumper.

I've used a few adaptive cruise controls that will not maintain a great enough following distance to come to a complete stop in the following distance (eg, in the case that the car in front of you swerves out of the lane revealing an obstacle).

Did that around here and you’ll have people swerving around you.

The problem I have with adaptive cruise: if you set your gap too wide, usually the suggested gap, people keep swerving into it, so it makes for a very nauseating drive if you are somewhere with not enough traffic that you are moving slowly, but too much traffic that people don't have to change lanes.

I'm sorry, but can't you just set it to medium or close distance if you're having problems with people jumping into your gap?

Ya you can do that, but then it feels like dangerous. When it gets like that I usually just shut it off and control distance myself (and "cough" adjust it to dissuade swerving and then back off for safety).

ah. my car doesn't have adaptive cruise control but I've rented cars that do. it took a bit to get used to medium and feel comfortable and not dangerous, but then people aren't cutting in.

Sounds like we need adaptive horn control for when people do that.

In America? Get ready for more road rage shootings

Just yesterday I was driving in the left lane with cruise control set to the maximum allowed velocity, with the maximum gap behind a car driving in front of me. Of course someone overtook me on the right (illegal in Germany) just to swerve in front of me and then drive at the same speed, just 20m further ahead. He ended up being first at the red lights. I’ll never understand it…

Regardless if you’re going the limit or not, it’s illegal to cruise on the leftmost lane when you ‘re not overtaking.

They were rightly annoyed. It’s also a hefty fine if you’re caught doing this.

If you are traveling 80 in the left lane while the right lane is locked in at 60 (trucks who are limited to power speed limits), it is simply impossible to travel in the right lane. You are constantly overtaking vehicles, so it works out legally, but, yes, if someone wants to go 90, there is a conflict. Ideally, you wouldn’t cruise in the left lane because there would be room to cruise in the right lane, but higher traffic situations don’t really allow for it (well, not so high that you can’t go 80 in the left lane).

This usually happens when you are approaching a city and are going to get a new lane or two pretty soon anyways.

If there was room for someone to overtake on the right I don't think your scenario applies.

You shouldn't overtake on the right.

Exactly. It’s illegal here in Germany. I don’t know if it’s ok somewhere else.

It's not, and it's a bit worrying that it's not widespread knowledge.

Well, it sort of does. Imagine your doing 100 in the left lane, as does the car in front of you and the three cars in front of them. There’s a truck doing 80 in the middle lane some 500m away. All of you are going to pass it. Now a sixth car starts tailgating behind the last in line and because there’s left a big safety gap he’s overtaking on the right.

I was the last in line, going at the exact same speed as the cars in front of me, just leaving enough space. Why would I switch lanes?

Plus it was leading to an exit anyway…

This is often true, but it varies from state to state.

https://www.mit.edu/~jfc/right.html

I know that. But what am I supposed to do when the left lane leads to the exit I want to go to?

Also: there was someone in front of me going the same speed, I just left a big gap

People don’t intuitively notice speed, they see following distance as a proxy for speed: people with longer following distances are perceived as “driving slow” and they feel they are going faster if they tailgate. Both untrue, but almost universally assumed without a second thought

Wow. I’ve been driving for 20+ years and never realized this is how some people think.

This is not a problem, just chill out.

I have to imagine the level of discomfort depends on the car. I've never ridden in a vehicle with adaptive cruise control, but if it responds quickly to someone jumping in line, that could go beyond annoying to motion sickness if it keeps happening.

Having owned a car with this feature, it's very uncomfortable when it suddenly and unexpectedly brakes hard to allow a car in.

And it can be very confusing to the vehicle behind you.

Ah. I see. My 2015 Model S doesn't do that, it just gently increases the spacing until it is as it was.

From some motorcycling book I remember the right response to that is to slow down even more and maintain following distance with the new car in front of you. If we let ego go and can be home 2-5 minutes late on average all will be well.

I've found that the adaptive cruise has made me an almost "egoless driver". It does keep a conservative distance, and aggressive drivers do fill in the gap, but I don't care as much when I'm not the one on the pedal.

With the full map and destination time in front me, I don't feel the urge to game the system (or maybe I'm just getting old).

I live in a cramped east coast city. If everyone around here followed the DMV guidelines for distance between cars, morning rush hour would probably be from 4am to 11am. Sure, the fact that everyone's driving in such a tiny little space is the problem, but the infrastructure here doesn't leave us much choice.

That's questionable. Flow should improve from increased spacing; it might even offset the density decrease.

Even if the length of the cars end-to-end is often greater than the length of craggy narrow street between tightly packed but unevenly placed intersections with anywhere from 3 to 8 entry points? Even when all of the 5 highways people are trying to enter and exit– all within the same mile– are all completely backed up? I'd have to see that to believe it.

Those seems like situations where extra spacing will help accomodated unexpeded mergings and lane changes without slowing traffic down.

The majority of the roads that aren't highways do not have lanes. The road layout is largely from a hundred or two years before cars existed.

I fail to see the relevance of this comment?

>>> Those seems like situations where extra spacing will help accomodated unexpeded mergings and lane changes without slowing traffic down.

>> The majority of the roads that aren't highways do not have lanes.

> I fail to see the relevance of this comment?

... you don't see how roads lacking lanes applies to the efficiency of merges and lane changes?

So the point you were trying to make is that single land roads don't face traffic issues from merges and lane changes? That seems to trivially be false in my experience. Having driving all over the USA on roads with a single lane in each direction, merges and lane changes are by far the biggest cause of traffic slow downs.

Sure thing, bud.

Do you suppose average speeds would be higher or lower without the frequent accidents due to failure to brake in time?

The traffic is not frequently caused by accidents. It's a small city so it's pretty clear when that happens. We have some of the lowest accident stats in the country. It's probably because any time there are a significant number of cars on the road, the entire region slows to a crawl.

I'm pretty much out of the habit of using cruise control even with newer adaptive cruise control because it really encourages maintaining a constant speed even on roads that are really too busy to allow it.

In this city, even at 3:30 in the morning on Sunday, cruise control is just useless. It's a spaghetti pile of twisty roads and a billion intersections. Even on the major thoroughfares, you're not going to drive more than 20 seconds without hitting a intersection. To make matters worse, it's all way too disorganized to efficiently coordinate-- almost nothing is perpendicular-- so when traffic is light they just have very short light cycles and you're nearly guaranteed to hit a red light in a few blocks, max. Most residential streets are only wide enough to accommodate one car, but in many cases there's no logistical way to make them one way, so you just have to get good at constantly negotiating who should yield and who should go with oncoming traffic. That, specifically, causes shockingly few problems! But it makes for complex traffic patterns that require a lot of cognitive load to handle. If there was good public transit here, it would be great.

The situation is similar in the Chicago area. Some roads are almost perpetually bogged down. If there is an accident or stalled car, it gets worse. I see two things that affect flow.

1. As traffic builds and distance between cars decreases, the flow becomes susceptible to "shock waves". (Just search Youtube for "traffic shockwave" for an example. I think it happens when someone taps their brakes and the next car behind them has to brake a little harder ans so on down the line until traffic comes to a complete stop.

2. As a commuter and occasional weekend driver I recognized two traffic patterns. During normal commutes, the same drivers drive the same routes every day and most settle into a pattern of maintaining speed, remaining in lane, moving to merge and so on in harmony with other drivers. There is the occasional outlier, driving slower than everyone else or weaving in and out and trying to get ahead by weaving and passing in whatever lane momentarily has space. On weekends I see drivers who are unaccustomed to the driving situation and simply don't mesh well with other traffic. Even though there is generally less traffic, it does not flow as well. There is more lane changing and stuff like crossing three lanes to get to an exit because they forgot where they needed to exit.

Excess traffic is a problem and unfortunately we seem to be able to overfill the available capacity any time it is increased.

When I worked in downtown Chicago I was very happy to be able to take commuter rail to work rather than to have to drive, but those options seem to be more the exception than a regular option. That's a different discussion.

Yeah I've had my fair share of big city traffic, but this is very different. In many ways driving here is a lot easier than driving in a place like downtown Chicago or Manhattan because it's much much much lower-volume, so there's a lot less action you have to pay attention to, but all of the roads are tiny and twisty. It's like heart circulation vs pinkie tip circulation. And since it's so small, even being slowed down to a crawl from one end of the city to the other as part of a longer drive would affect your drive less than even moderately bad traffic in a metropolis.

I often try to follow the car-length-per-10mph rule. It's often times not possible. Cars with just duck in front of you and people behind you will pass you. One of many reasons why I hate daily driving.

> I apologize for any flame wars this creates, since people find driving to be deeply personal and any findings which may criticize their driving habits is generally frowned upon.

I appreciate your attitude here. Mine is opposite. Dangerous driving injures, maims, and murders both perpetrators and innocents alike. Criticise bad driving loudly and often. You might save lives. I had to tell my mother in law that her daughter, my wife, lost her life in a road traffic accident. There are few aspects to serious road traffic accidents that are not horrific, and speaking up may save lives. Every year, 1.19 million people die as a result of a road traffic accident. Between 20 to 50 million people suffer non-lethal injuries, many of those leading to disability.[1]

[1] https://www.who.int/news-room/fact-sheets/detail/road-traffi...

Deepest condolences on your loss, and thank you for your candor—I can only speak for myself, but I’m definitely turning up my driving conservatism after reading your post.

I’m very sorry for your loss. I agree with your stance on being vocal about poor driving.

People speeding through the 30 mph limit in my village boils my blood like nothing else and I’m one of those parents who shouts at traffic on the school run, even driven by people I know. This has likely cost me a few invitations to the pub but I don’t care.

Can you lobby your gov't rep to install automated camera? It seems reasonable. If you are brushed aside, email/write/call a local newspaper or TV station. They would probably like to report it.

I've seen studies that those flashing speed camera/displays are pretty effective.

In the U.K. some years ago the police changed the term in common use from "Road Traffic Accident" (RTA) to "Road Traffic Collision" (RTC) in order to underscore that these were not accidental but mostly due to poor driving of one sort or another.

"Road Traffic Collision" is encoded in the applicable laws.

Well, to play the example out...

Stopped car in unexpected place.

1 car in front of your vehicle, between you and the stopped car.

Assuming the car in front of you switches lanes around the stopped car with 1 car length of space between it and the stopped car.

So 2 car lengths + your personal gap for you to slow down.

So you could be going no faster than 20mph worst case (0 personal gap).

The issue is that the car in front of you doesn't need to stop, so can potentially "suck you in" at an unsafe speed, before you realize you'll need to come to a complete stop (assuming your lane change is blocked).

It's a fair point, and at least I think of myself as a fairly conservative driver.

I think it is the only time though that I've been faced with a stationary car suddenly appearing where I wouldn't expect one, and that's very different to the car in front needing to slam on the brakes with whom you can share some stopping distance.

fwiw 6 seconds at 45mph is 120 meters. That we can comfortably go that far in 6 seconds is a modern marvel, but it's also 20 car lengths.

I understand 6 seconds is a lot of space. My cruise control has a delay before reacting so 6 seconds allows for the car to react and if it’s not to my liking then leaves me with the necessary space to stop. On the highway I usually control my distance by switching between 4 and 6 seconds.

What car is that? Of all the cars I've driven I've never encountered one that allows for more than ~2 seconds on the maximum setting. Thus leaving me a bit nervous.

I'm almost hesitant to believe that any car on the market has good enough sensors to do 6 seconds. That is a very long distance at highway speeds.

I have a Hyundai Sonata that does at least ~4 seconds. It works very well, although it can be annoying when people jump in front of you too closely.

It has 4 settings, but those don't correspond to seconds https://owners.hyundaiusa.com/content/dam/hyundai/us/myhyund... (page 6-117).

Quote: "For example, if you drive at 56 mph (90 km/h), the distance is maintained as follows: Distance 4 - approximately 172 feet"

172 feet at 56 mph is 2.1 seconds.

Not saying that you are wrong (maybe a recent model has better cruise control), but I think most people vastly overestimate the adaptive cruise controls in their cars and ~4 seconds would, for reference, vastly outperform 2023 Tesla Y in that aspect.

Huh, yeah I have a 2021 and your math seems correct.

I always drive setting four and it keeps a very comfortable amount of distance. I think something is incorrect here because I definitely get at _least_ three seconds, or maybe I'm counting too fast.

Thanks for looking into this & being friendly!

It takes FAR more than 50ft to stop from 40mph. I doubt you can even react in 50ft.

40mph is 58 feet per second, so that’s less than a second to

Your “rule of thumb” is AT BEST highly misleading.

Typical humans take about 250ms to react, and that’s before you actually do anything. There’s 15ft (at least) gone before you even touch the pedal.

Only a fool breaks the two-second rule: https://en.m.wikipedia.org/wiki/Two-second_rule

That rule explicitly states that it's not for being able to stop, though, it's just to have time to react to the driver ahead stopping. Two seconds isn't nearly enough to stop in front of a stationary object. At highway speeds it's even barely enough to not rear-end the vehicle ahead if it has to panic stop.

Yep, that’s right. Its a great rule though, and as explained in the article it works at any speed, highway or not. Personally I keep 4 seconds.

>Isn’t this a sign that you are following too closely?

If you open enough space ahead of you for a car, someone will cut in. Driving too closely isn't something only you can do something about.

In fact, because of the aforementioned behaviour, it's safer to drive too close because it's far more dangerous for a car to cut in in front of you because he perceives "enough" space to do so.

No it is not safer to drive too close. If someone cuts in front of you, you just slows down a bit to keep enough space and that's it.

> Isn’t this a sign that you are following too closely?

But they said it's FSD who's following too closely.

I'd expect automated system should react even better, because it pays attention constantly. That's the whole point, especially when it's called "Full".

Unfortunately for anyone relying on so-called "full self driving", that it may be capable of doing much of the driving you remain the driver.

It's not even a question of "taking back control". You're expected to be in control of the vehicle.

There's a lot of technical measures that prevent people doing harmful actions. Even in spaceships, where pilots are highly trained, focused, selected professionals.

For example, most cars prevent fiddling too much with its entertainment system while driving, even though drivers are expected to not do that while driving.

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