Not just 15 feet, that'd be insane!
Given this confusion, it does seem like it explains the root of the disagreement here, but that said, I don’t think it’s unreasonable for people reading your comment to assume that when you said “car length” you meant the length of a car.
A car length is, literally, the length of a car. It does not vary with speed.
Exception: https://en.wikipedia.org/wiki/Length_contraction (coming soon in a software update to a Tesla near you)
To me, a car length should simply mean the minimum safe _length_ at which to follow the car in front of you.
I don’t know you, but I believe a majority of people underestimate safe stopping distance. That’s why the webpage I posted, which is one of many similar ones, exist. That’s also in part why there are so many accidents.
People are also bad at estimating feet, so telling someone keep 80 feet is also not helpful.
But people are pretty good at estimating how many cars can fit between them and the next car.
So telling someone, hey, you are tailgating that guy and only one car could fit in front of you, but you need 10 car lengths to be safe, has a lot more communicative power.
Not if taught correctly, which I think it's important to note that we can't really do via textual communication.
In the driver's seat, during a driver's education course, you might learn that a car length is the length of an average car, but you also learn how to count the seconds of stopping distance you have in front of you. In addition, you learn that weather, traffic, and other factors can affect stopping power/distance, the point being that the whole system is incredibly dynamic and variable.
One can't reduce a dynamic traffic situation down to "X car lengths = safe." Similarly, saying "1 car length = a safe distance" doesn't really help either.
The idea is to teach people what "safety" actually looks like. How it's referred to is secondary. The proof is in how badly I was misunderstood when I said 1-2 car lengths. I _know_ I'm keeping enough distance, because I know how to judge a safe traffic situation.
I don't really care what a car length is or isn't, it's fairly irrelevant because safety is a much bigger picture than units of measurement.
I get it. You'd prefer to use the phrase "car length" to not literally mean "length of a car" in this context. Apologies if I seemed pedantic but, as evidenced by this thread, your use of car length is going against the grain.
Safe following distance is often expressed in multiples of car length. What car? Sure, good point. Pick one; it's close enough! (If you're actually thinking about following distance in any terms at all, even roughly estimated, you're ahead of the safety curve and you clearly are!)
Ideally, following distance could be accurately estimated in metres and adjusted continuously by velocity and driving conditions. AI can do that. People just kind of eye it and say, "it was raining and I followed about four car lengths behind him" or "we were inching along and I was half a car length behind him". Good enough for meat brains!
Safe driving!
The Subaru's adaptive cruise control system treats car lengths as variable units based on speed. It's much more liberal than I made it sound in the original comment. It sounded reasonable when I wrote it, but infinite nuance strikes again I suppose.
Personally, I find it somewhat confusing, but people should probably be more aware that it's a thing and not necessarily someone mixing up terminology.
Cars in your area are on average 40 feet long?
A car length is about the length of an average car.
Either way, with a 40 foot "car length", 1-2 car lengths is then ~80 feet. The above stopping distance chart puts a 40mph stopping distance with reaction time at almost 140 feet. <80 feet away is driving way too close at 40mph! Especially for a car you know is doing a maneuver which could result in them braking suddenly such as turning into another field of traffic.
I just used the term "car length" with entirely too much room for interpretation and am now getting torn apart for it, my mistake. I swear I'm not tailgating people LOL
If you're going 40mph and approaching nearly stopped cars at <80 feet, you're absolutely tailgating people and creating extremely risky situations.
Don't get me wrong, I've had my emergency braking trigger for similar circumstances of someone turning into a parking lot off a feeder of a highway. But I don't blame my equipment, I acknowledge I'm not a perfect driver. The car knew it could have been quite a challenge to stop in time and so it took action. The better action would have been for me to be going slower and approach that situation with less speed, not to just blame the car for being cautious and forcing realistic safety margins.
Yes they are, further up this comment chain is:
> for example, if they're turning and will be gone in a second anyway
That car turning is a nearly stopped car, doing a maneuver which could result in them stopping suddenly and unexpectedly. Going 40mph less than 80 feet away from that car turning is absolutely an extremely risky manuver. You don't know they're going to be gone in a second, you're assuming it, and if your assumption is wrong its a collision because you cannot possibly stop in time. Emergency braking triggering in this scenario is 100% justified.
Either way, if you are rapidly closing the distance to a car which is changing lanes, you don't know for certain they won't cancel the maneuver and go back to being in the lane. You're still assuming that, and assuming you'll be able to slow or stop fast enough to handle it.
If your AEB is constantly getting "false" alarms, maybe it is not the AEB that needs adjustment. Maybe driving in a fashion where the car's AEB doesn't constantly give "false" alarms would be a lot safer.
Changing lanes is a type of turn because you _turn_ the wheel to change lanes. You also might turn 45 degrees in a roundabout, or 90 degrees at a stoplight. They are all turns.
I'd agree that it's an assumption. Though your assumption that the distance is closed _rapidly_ is also just that - an assumption, and one made in bad faith despite my efforts to correct it.
The mere act of being in a car on the road involves dozens of assumptions made every single minute about the traffic conditions. Very little is known for certain, ALL maneuvers are to some extent based on assumptions.
This conversation is no longer productive, thanks for your participation anyway though.