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Comment by lotsofpulp | Hacker News Reader

Once a road hits a certain density of vehicles, the only way to fit more vehicles on the road is to close the gaps between the vehicles.

One cannot realistically expect urban regions to have drivers that leave large spaces of road empty, it wouldn’t back up the congestion even further onto other roads.

It’s especially exacerbated because all vehicles and drivers have different reaction times and acceleration, so it’s inevitable that gaps will increase and decrease. Taking advantage of the gaps actually increases the road’s throughput, and is a sufficient tradeoff to risk and energy efficiency for almost all drivers worldwide.

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Depends on the type of traffic. You're describing gridlock, but temporary jams are more common on freeways. In those situations, if everyone left extra following space, it'd actually improve the overall throughput since people take extra time to stop then start. In some places, the police or automatic signs will even tell people to slow down in advance when there's a jam ahead.

> Once a road hits a certain density of vehicles, the only way to fit more vehicles on the road is to close the gaps between the vehicles.

Even better is to get the cars off the road by having good flow and not jamming it up.

"Closing the gaps" means tailgating or switching lanes to get into those sweet, wide open spaces. These are the primary causes of phantom traffic jams on congested freeways. There's a pretty good deep dive covering this[1].

[1] https://www.autoblog.com/2022/11/03/phantom-traffic-jams-pre...

>Even better is to get the cars off the road by having good flow and not jamming it up.

This is not practical in most cases of congestion, which are caused by the number of vehicles exceeding the road's capacity. There is no other choice after a certain point, vehicles will begin to pile up and speed up and slow down creating the traffic waves.

That video is correct for only a minority of times where the road capacity has not been met, but people are accelerating and decelerating too fast causing the traffic waves.

However, during times of too many vehicles on the road, such as rush hour, there is nothing anyone can do, other than use as much of the road as possible (i.e. less empty space).

> However, during times of too many vehicles on the road, such as rush hour, there is nothing anyone can do, other than use as much of the road as possible (i.e. less empty space).

Your analysis misses safety and so while your facts are correct your conclusion is wrong. When more cars want to use a section of road than the road can handle you must not put more cars into the empty space without all the cars slowing down (which in turn means the road can handle less cars per time). I know that is not what everyone does, but they (we) are wrong.

I intended for “risk” to be short for “safety risk” in the below portion of my original comment:

> Taking advantage of the gaps actually increases the road’s throughput, and is a sufficient tradeoff to risk and energy efficiency for almost all drivers worldwide.

> the road can handle you must not put more cars into the empty space without all the cars slowing down (which in turn means the road can handle less cars per time)

I agree that there is a range where cars weaving in and out causing people to brake unnecessarily causes more congestion, but there is also a point where there is so much congestion that “filling in the gaps” becomes the optimal move, because the road’s capacity has been reached and maintaining large gaps would simply push congestion further back onto other roads.

The problem is that there is no filling in of gaps without lane changes that cause the congestion. Everyone is making what they perceive to be the optimal move for them: "I'll just change over to that lane, because there's open space there and this one's going slow," but that individually optimal strategy isn't optimal for the movement of traffic as a group.

You don't even need to maintain large gaps - you just need about two seconds of time between you and the car in front of you, and that varies based on the speed of traffic. Everyone going ~30mph with gaps is going to have a lot more throughput than everyone averaging 10mph with short gaps. Of course this is before you get into the increased risk of collisions with smaller gaps. One fender bender is enough to bring everything to a halt, and statistically with enough cars and short enough gaps those are basically guaranteed.

Yeah, try setting your nav from Los Angeles to Irvine or San Diego to Irvine on any given day. Guaranteed 2-3 wrecks, plus some jams that the nav might think are wrecks but are just phantom jams.

Highway safety people who work for the do expect that. If their is a problem that is their problem not mine. My problem is arriving safe.

Read the above as a rant for better transit.

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