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Comment by Firaxus | Hacker News Reader

Absolutely. In the context of high way driving especially with its high speeds, the slight slowing down to maintain follow distance even if people were constantly moving in is negligible to the overall trip time, and it’s highway driving where that follow distance is the most critical to saving lives.

If people sat down with the numbers and realized how little that slight slowing down in the moment someone moves in front of them impacts their trip time, how little it matters to the original trip time that someone passed them, the roads would be a lot safer, less stressful, and faster overall.

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If everyone slowed down, there would be far more congestion backing up due to reducing the throughput of the road.

I would argue that the people driving at distances closer to the car in front of them are reducing congestion by increasing the throughput of the road. Obviously this is mitigated by decreased throughput due to increased probability of collisions.

But if EVERYONE drove such that they could fully stop if the car in front do them fully stopped, you would have a public uproar over how much backed up traffic there was.

There road is basically constantly in flux and politicians could enforce minimum driving distances very easily with cameras and drive down collisions, but it would be massively unpopular and they would get voted out immediately. I presume that is why we already do not have speed cameras in the US.

It would be trivial to fine everyone breaking the speed limits. Or even driving too close to each other. For a tolled road, you can get go back however many years you want and retroactively fine people for going past the speed limit based on toll time stamps. But I figure the lack of this happening means the public wants to make this risk vs convenience trade off.

As soon as someone crashes, that congestion is going to go way up. In very specific conditions where a road is congested already, slowing down does cause a cascading effect, BUT, if everyone is already maintaining a proper follow distance, it takes longer for the road to reach max congestion in the first place. Consider it like a bunch of springs all lined up spring wise; if you have the same number of springs, and you’re trying to insert a new spring (someone is merging), the springs which were less compressed will more easily accept a new spring, while the ones where are already compressed will experience more strain.

And imo, I’d rather be going slow because “we’re going the follow distance” than going slow because “someone went too fast in an unsafe condition and crashed (or worse)”. In the face of sheer car volume, we’re all going slow(er, though note that we’re usually still getting there faster than a horse would have) pretty much no matter what, might as well be safe while doing it.

> BUT, if everyone is already maintaining a proper follow distance, it takes longer for the road to reach max congestion in the first place.

I do not think this is a realistic model of the way roads work due to rush hours and sudden increases in demand. The ideal rules are great for when demand ebbs and flows gradually, but things change when everyone wants to be on the road between 8AM and 8:30AM. Then, you can have all the protocols you want, but the system has reached max capacity and there is no avoiding that other than dealing with the consequences (congestion -> stop and go traffic -> gridlock) and of course a collision or two.

I am not saying this justifies driving like a maniac and cutting people off. I simply expect people to act a certain way over the boundary conditions of a situation are approached, and I will not expect it to be the same as how they act when we are not pushing the limits.

There are several major traffic accidents in Atlanta every morning during rush hour on the most important arteries. Most of these are probably caused by tailgating or aggressive behavior during merging. They turn a 30 minute commute into a 90 minute commute if any lanes remain open.

I can’t imagine that the downstream effects of maintaining a safe following distance and permitting people to merge would cause anywhere near the inconvenience of these daily, destructive events.

There are varying levels of safe driving distance. You can drive close enough to the car in front of you such that you can slow down quickly enough if they slow down from 70mph to 50mph, or if they slow down 70mph to 10mph, but gradually.

You can also drive further back such that you can slow down if the car in front slams on their brake and comes to a complete stop. This is the distance I am referring to which would be politically unpalatable to enforce.

I agree that people weaving in and out and tailgating and cutting people off are doing more harm than justified, but it all starts getting murky once the road hits capacity limits and people start trying to figure out the best way to optimize their travel times. Eventually, with enough congestion, the situation will deteriorate from stop and go traffic to crawling traffic to downtown Manahattan to Delhi and so on.

> it all starts getting murky once the road hits capacity limits and people start trying to figure out the best way to optimize their travel times.

that actually happens sooner when everyone is crawling up each other's asses.

once you get that first compression wave due to tailgaters slamming on their brakes then you've started the traffic jam.

I think that's an extremely suspect hypothesis. Most congestion is caused by:

- Construction

- Accidents

- Directly counter to your point: people driving like assholes and cutting others off or forcing others to take abrupt maneuvers that then cascade through the tightly packed traffic

Slow speeds and ample space do not cause congestion, changes in the flow of traffic do, and usually those are caused by people following too close and making sudden maneuvers. If enough space is given and the flow of traffic is not disrupted you could go as fast as you want with as many cars as you want and not experience congestion.

It's the mentality you're espousing here that I find so fascinating about road travel. There appears to be such dissonance that one will justify speeding or tailing or road rage in all kinds of ways.

Too many cars on a certain section of the road at a certain point in time is congestion. This can be caused by many things, such as roadwork, collisions, but more often than not, it is simply a result of lots of people wanting to drive in the same direction on the same road at the same time. Aka rush hour.

I am not justifying road rage or tailgating. I am explaining that it is inevitable once the road gets close to carrying capacity, which can only be increased in the immediate term by driving faster and/or decreasing the distance of gaps between cars.

You say you're not justifying tailgating, but it sounds like your prescription for rush hour is exactly that.

I suppose there are different characteristics in different types of congestion. Stop and go congestion is usually caused by disruptions to the flow of traffic, while slower than usual congestion is indeed caused by over-capacity as you indicate. Just moving slower would be all such times would experience if not for people tailing and cutting others off, leading to the stopping part.

Even moving slower will not prevent stop and go traffic if the road has reached its capacity, due to the various different accelerations and decelerations of cars.

My overarching point is something like this video:

https://m.youtube.com/watch?v=Suugn-p5C1M

There simply is not a way to avoid the consequences of being near or above a road’s capacity. Of course, I recommend everyone to play it safe and stay far enough back to avoid any liability of being too close to the car in front. But the reality is we make lots of calculations and choices in driving and at various times, we choose farther distances and other times nearer distances (even though we know it is riskier) and these collective decisions will propagate down through the whole road.

> due to the various different accelerations and decelerations of cars.

Right, changes to the flow of traffic.

> There simply is not a way to avoid the consequences of being near or above a road’s capacity.

I agree, that is a valid bullet point. But that is certainly not the only cause of congestion, and driving aggressively in congestion and attempting to justify it by keeping the roads clearer doesn't hold a lot of water I don't think.

But I see you're saying you're not advocating unsafe driving so that's good, and I take your greater point, thanks for expounding on it.

You're doing static analysis on a dynamic situation. The biggest issue with overcapacity is the start of compression waves of traffic, which are generated much sooner by tailgaters slamming on their brakes in a panic with each following vehicle having to brake harder than the one in front, eventually coming to a complete stop (or just causing an accident).

> If everyone slowed down, there would be far more congestion backing up due to reducing the throughput of the road.

This isn't true. The greater the following distance, the more time a driver has to respond. More time to respond means fewer and gentler inputs. In turn, this reduces propagation to following vehicles, and as a result traffic moves faster. It's the difference between turbulent and laminar flow.

> I figure the lack of this happening means the public wants to make this risk vs convenience trade off.

Joe Public has never sat down to think this through, nor is he qualified to do so.

Joe Public decides this by demanding their politicians to not enforce speed limits. A campaign would be dead in the water the second it talks about having cops issue more speeding fines or install speed cameras or using toll times stamps to issue fines. The US public is not sufficiently scared of injury or death from collisions to make it a political priority.

> In turn, this reduces propagation to following vehicles, and as a result traffic moves faster. It's the difference between turbulent and laminar flow.

And in turn, this decreases the rate at which vehicles enter the highway, which backs up onto the streets that lead to the highway.

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