Hacker News Reader

Comment by lotsofpulp | Hacker News Reader

If people drove such that they had enough stopping distance to come to a full stop on highway at 65+mph, traffic would get backed up very quickly.

Road capacity is a function of number of lanes, the velocity of the vehicles traveling in those lanes, and the density of the vehicles traveling in those lanes. If you reduce the latter, you reduce road capacity.

Theoretically, everyone would be safer if they drove far enough back from the front of the car to come to be able to come to a full stop. Practically, the political uproar of enforcing this would be untenable, but I would be entertained to see the forthcoming overpopulation vs pave over all the land with roads vs mass transit vs variable rate tolling conversations that ensue.

View on HN

I’m dubious that the equation is so simple.

> Road capacity is a function of number of lanes, the velocity of the vehicles traveling in those lanes, and the density of the vehicles traveling in those lanes. If you reduce the latter, you reduce road capacity.

And yet study after study shows that increasing the number of traffic lanes only worsens congestion.

https://en.wikipedia.org/wiki/Induced_demand

If you have 10 cars wanting to use a stretch of road capable of carrying 10 cars at 8AM at speed X with the safest distance gaps, and you add 10 more cars, then there will be some combination of congestion behind this stretch of road, the distance gaps getting smaller, or cars driving faster now that there are 20 cars.

If you add a lane so that 20 cars can travel at the safest driving distance gaps at 8AM at speed X, but this then causes 10 more cars to come on the road because they see the possibility of using a road that is maybe just under maximum capacity, then you now have 30 cars on a 20 capacity road, and once again you end up with previous situation as 20 cars on a 10 car capacity road.

Obviously these things are impossible to exactly predict and constantly in flux and so engineers and politicians and drivers are all crudely estimating the shifts of supply and demand curves, and the overestimating and underestimating will cause congestion, or complaints of excess road, etc.

You are correct but I'm afraid this is not an excuse to drive at speed 2cm from the rear bumper of the car in front of you.

I should add this my near daily experience driving to my office in a rural area at 5am when there is just me and a random car behind me. With 2 miles or more of empty road behind him.

> You are correct but I'm afraid this is not an excuse to drive at speed 2cm from the rear bumper of the car in front of you.

I anticipated this response, hence I tried to pre empt it by making my first qualifier that I am not talking about tailgating 2cm from the car in front.

> If people drove such that they had enough stopping distance to come to a full stop on highway at 65+mph,

This qualifying statement was supposed to convey that the theoretical safest distance to travel behind someone at 65mph, 75mph, or more is so big that I would not expect it to be followed anytime other than the dead of night.

You are correct, again, but please consider that in order to avoid killing or seriously injuring someone in a road accident you don't need to come to an absolute stop.

You can actually avoid a collision at moderate speed by simply giving your brain time to react to events happening in front of you. And, even if you do hit something, doing so at a moderately low speed may not turn out so bad?

For sure, I keep as much space in front of me as possible to avoid touching the car in front. I just do not expect everyone else to.

Multiple studies in The Netherlands have shown that decreasing the speed limit during rush hour increases the road throughput. So this isn't the simple equation you try to paint, there is a nonlinear interplay between the average velocity, the variance of the velocity, and the vehicle/lane density.

That will work up to a point, but I would bet there are many situations where the amount of cars that want to be on the same piece of road at the same time overwhelms those kinds of measures. And with the same result since congestion force everyone’s speed to be lower anyway.

I can think of quite a few chokepoints in the US (Manhattan/SF-San Jose/LA/Seattle/Austin) where no matter what you do, the crush of cars will be too much for the roads that exist.

In these cases though, I think public transit is the only real solution, but that is a different topic.

Reply on news.ycombinator.com