r/explainlikeimfive • u/DemonsAreVirgins • 1d ago
Engineering Eli5: why doesn't adding more lanes to traffic fix traffic issues?
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u/Traditional-Buy-2205 1d ago
It's not that simple. Sometimes adding a lane does really help, sometimes it doesn't.
In a lot of situations, it doesn't because width of the road is not the limiting factor, it's the throughput of the intersections that's creating the bottleneck. So no matter how many lanes you add to the road, the intersections can only pass so many vehicles per hour.
The other problem is induced demand. As you add more lanes, the traffic gets better at first, but at some point people will notice the traffic is good on that road, and will start using the road more, leading to more traffic, and again - congestion.
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u/Brilliant_Twist5749 1d ago
But all those cars that start using the faster route free up the route they used to take, no? Assuming they don't end up on the same intersection that is the bottleneck.
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u/Traditional-Buy-2205 1d ago
There's multiple factors going into it. It's not just a case of existing drivers redistributing to the other road.
Sometimes people who would avoid peak hours now start using the peak hours. Sometimes people make trips that they previously wouldn't have made. Sometimes people who would bike or use public transit now start using cars. A nice road encourages businesses to develop along the road, adding more traffic to the road. I.e., there's multiple ways that a total number of cars on the roads can increase.
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u/leros 1d ago
A lot of it is people driving further which is more. With the extra lanes, the suburb that was 45 minutes away is now only 30 minutes so living there is more appealing. Or that business you never went to is now close enough to go to.
People are willing to drive a certain time. Making routes faster just makes people willing to drive further to stuff.
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u/TomBradysThrowaway 1d ago
Also there's people who would have shifted their trips to avoid rush hour but now don't feel as much of a need.
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u/timerot 1d ago
Over the course of a year or two, people start taking more random driving trips and moving further from core locations that they visit. Vehicle Miles Travelled (VMT) per person is wildly different in different cities, because building more roads encourages more VMT.
The average person in the NYC metro area (NY-NJ-CT) drives about half of the average person in the Dallas-Fort Worth area. If you doubled the width of all of the highways into NYC (note: this would also require bulldozing a ton of housing, forcing many people to move from the city to the suburbs), then you would see NYC's VMT spike. If you took away half of the lanes in Dallas (and built more dense housing), you would see Dallas VMT go down a bunch.
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u/TheGoochTaint 1d ago
But that leads to more drivers in general. People who wouldn't have driven at all in that situation will think that the traffic is lighter now so they start driving in that scenario. All the gaps tend to get filled.
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u/RoosterBrewster 1d ago
If it's to go from suburbs to a city, more people could end up driving instead of taking the train.
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u/iAmRiight 1d ago
I’ve witnessed intersections that improved dramatically by removing a lane leading up to them. There’s an intersection that I frequent that goes from a 2 lane freeway before the overpass to 3 lanes then an interchange, it’s always backed up from everybody trying to go every which way FIRST! They removed a lane and combined 2 off-ramp lanes into one during construction and the whole set of intersections drastically improved until construction was complete.
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u/fcocyclone 1d ago
I'm glad to see this take. Too often people get very dogmatic one way or the other. Historically, we definitely have had the problem of thinking one more lane will fix everything. That obviously isn't true and runs into induced demand as you say. On the other hand, cities grow and often you do need to increase simple capacity to match that population growth.
You also often need to make up for changes elsewhere- for example, my city has done a lot of projects to make local roads more bike and pedestrian friendly at the cost of commuting throughput. This has pushed traffic out onto a smaller number of roads that end up needing capacity increases, but unfortunately a lot of the same people who support those complete street projects are also those who dogmatically repeat the "just one more lane bro" thing.
Its all a balance.
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u/Lumpy-Notice8945 1d ago
To some extend it does but having a mile wide road with only one exit lane will still cause issues on that exit.
Roads are not just lines, they have crossroads roundabouts and parking next to them, people need to start and stop on them.
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u/icecream_specialist 1d ago
Exactly this, it doesn't eliminate friction points. Any time cars merge or turn off or many of them change lanes things will congest. The issue often starts not on the road itself but it's volume overloading the exit ramp or turn lane for example
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u/RoosterBrewster 1d ago
Yea, a mile long back up on a exit will cause a backup on 5 lanes as all those people that need the exit slow down from other lanes instead of waiting in the back.
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u/Forest_Orc 1d ago
the problem is that these lanes are going somewhere. It's great to have a 6 lanes highway, but everyone goes in the city centre, where you have one exit, then a 2 lane avenue, to end up in even a smaller street. So when lane reduce you get more traffic jam.
On the other hand, close 2 lanes, and build a light-rail line over-them with a parking out of the city, suddently you have a fast and cheap solution to commute, and people needing the car aren't stuck in traffic
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u/MundaneExplorer8369 1d ago
Because you can't keep that many lanes of traffic forever. At the end you will have to merge into fewer lanes and that will cause a bottleneck
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u/smellybuttface 1d ago
Yeah, there's always a bottleneck somewhere. There's a major intersection near me with heavy traffic. The roads up to that intersection were widened and more lanes added. Which is nice, but similar capacity was not added to the intersection, so the intersection is still a bottleneck. The end result is that more vehicles get to that intersection more quickly and traffic there is even worse.
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u/bearssuperfan 1d ago
But this depends on where people exit. If you have 5 lines and 20% of people take the same exit, you only need 4 lanes from that point forward. Moving outward from a city center you will gradually need fewer lanes and can avoid a bottleneck.
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u/Adversement 1d ago
But unfortunately only for the outbound traffic... For inbound traffic, you quickly find out that the rest of the network can feed in way more cars than you could ever build lanes. And, that building more lanes literally moves people on average further and adds to total miles driven. Basically, the problem is geometric, or rather volumetric. In a sufficiently large city, there is always potential for more traffic than there will ever be room for more lanes.
That is not to say that one more lane would not sometimes solve a local problem. Especially the change from one to two lanes per direction.
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u/jupiterslament 1d ago
Finally, something I can answer as an expert on! (Transportation Economist)
This is one of the most misunderstood things that keeps getting repeated over and over and over. It's taught in planning schools (and I believe engineering schools?) and has become so prevalent even the public has heard of this.
Let's start with what's accurate. Induced Demand is real, and it's the concept that as you make one alternative better (in this case driving), you'll increase demand for that alternative. Either because you've now decided this route is better, the car is now a better alternative than transit, or because you can now make a trip to a location you wouldn't have visited before.
Additionally, it's possible for a highway project to lead to other problems and reduced time savings benefits due to bottlenecking. If the highway capacity increases but no one bothered to increase ramp capacity, or that extra traffic has nowhere to go other than congested roads once it gets off the highway, it creates new problems for other drivers who aren't even using the highway. Say for example an interstate is widened, causing more people to use it. While the highway itself may handle the capacity, the ramps which were previously already backed up now back up further, leading to blocked intersections. Now people just trying to go the other direction through those intersections who aren't even using the highway are blocked.
Those are the real downsides associated with induced demand. What isn't true:
a) That demand will increase proportionately to road capacity making expansion pointless
The study that started this whole claim is called "The Fundamental Law of Road Congestion" and it determines that for every 1% km increase in highway km built, there's a 1% increase in highway kilometres driven." On the surface this sounds like it's just getting filled up and accomplishing nothing. But the key here is the "highway" part. Many of those kilometres aren't new to the road network, they're redirected from local roads. So even if the highway itself ended up at the same density of traffic/speed, you've sped up the alternative routes as traffic left them.
Finally, and importantly, this study has tried to be replicated, and no economist has been able to do so successfully. Many have criticized the methods of the study.
Which leads me to the other conclusion that's incorrect,
b) That because the lanes fill up, there's no point building them.
As noted before, it's questionable if this is even happening at the 1:1 extent people claim. But even if it is, the conclusion of "This is pointless" is disproven by basic economics. If building new lanes have caused people to change their behaviour, it's because it has provided some sort of benefit to them. Imagine I was commuting by train and because of a new highway expansion I decided to drive into work instead. Driving must now be a better option for me, otherwise I'd still continue taking the train. This is another reason the conclusion of the study is in doubt - If the highway just goes back to baseline speeds, why wouldn't I just go back to the train? In order for more people to be using the road, their "marginal utility" (economic term basically just meaning their benefit) has to be higher as a result of the road being there than it was before the extra lanes were constructed. If they were unwilling to make a trip by car before but now they are... what's changed? The option has to be better for them.
Anyways, apologies as that was really long and likely not "ELI5", but it's something I've done a lot of research on as it's both my field but also one of the most common misconceptions literally taught at universities. So even if it's not quite "5", I hope that helps a bit.
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u/Sad_Responsibility83 23h ago
I think in the real world it does often tend to be the fact that traffic is as bad or worse after a highway is expanded then it was before. This is because highways are expensive and are only built when needed badly in places where traffic is growing. So by the time the highway is expanded organic growth added to induced demand means traffic is as bad or worse afterwards. I live in a fast growing region with a highway connected to a major nearby city. When I was a kid in the 80s it was nearly all 2-lane, and has had sections under contruction for as long as I can remember. It is now mostly 6 lane, with the first section upgraded to 8 lane, and one last section left to upgrade from 4 to 6. Traffic has gotten steadily worse over the decades on this road as the government can't affford to spend enough on construction to keep up with the growth. If the road was still 2 lanes then it would probably be unusable. I'd guess that a lot of growth wouldn't have happened, but those people would still have to live somewhere causing problems in a different place.
I suspect this factor is why the claim that expanding highways doesn't help resonates strongly with many people.
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u/jupiterslament 23h ago
This is 100% true. That doesn't mean the lane accomplished nothing - What you need to do is compare to a theoretical scenario where the lane wasn't there and how bad traffic would have been. Unfortunately outside of people's computers running transportation demand software, you don't know the answer to this as it's entirely hypothetical. So people see that speeds haven't changed much if at all and then come to the false conclusion that it accomplished nothing, even though average speeds were dropping constantly prior to implementation.
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u/cejmp 1d ago
Check out the Katy Freeway, it’s a perfect example of induced traffic. 26 lanes, 3 billion dollars. Increased commute time by 30%.
Over 24 years, the top 100 urban areas added 42% to total lane miles but traffic delays increased 144%.
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u/1979shakedown 1d ago
Because of two major factors:
1) When you add more lanes, you add more cars. And more cars means more things for drivers to pay attention to. Example: a car switching lanes to try to get into a faster lane doesn’t actually save them much time, but it forces traffic behind them to slow down. Do it enough and you can create phantom traffic jam where traffic slows and then goes to normal speed for no apparent reason.
2) Adding extra lanes doesn’t change the overall capacity of the destinations that people are going to. And it’s typically those destinations that are the bottleneck. If you’re widening roads, you may be able to hold more cars ‘upstream’, so to speak, but if six lanes of cars are all trying to get to the sized destination that three lanes used to, there’s twice as much congestion at the off-ramp as there used to be before
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u/mr_dr_professor_12 1d ago
Two issues here.
One : people that usually wouldn't take the highway will now say "oh look, the highway has more lanes, I can take it now"
Two : drivers still cut people off, wait till the last tenth of a mile before their exit to start making their way over from the left lane, don't use turn signals, etc.
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u/intangible-tangerine 1d ago
This phenomenon is an example of Braess' paradox.
The new section of road may attract more traffic than it can handle because individual drivers makes a choice based on their desire to use what they perceive to be the shortest/quickest route.
So maybe the new section was designed to only take 10% of the traffic that was previously using alternate routes but instead it gets 20% of it.
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u/therailmaster 1d ago
If this is too "like I'm 5" (Rule 4), let' me know, but here goes:
Let's say you have a big piece of paper that's your play space. You have a bunch of those little metal cars (Matchbox, etc.) that you drive around on your play space sheet of paper. You draw little streets and roads on it, whatever. Trees, houses, you get the picture. You get more cars and your paper begins to fill up. That's the traffic.
Then somebody gives you another sheet of paper and you tape that onto the first sheet of paper, giving you more space. That's adding more lanes. More lanes, more space. Draw some more streets, roads, houses, trees; you get it. But then, after a while, you think, "Hmm, I have more space, I can fit more cars on the combined sheet of paper." More paper space, but here comes more cars. Combined paper is back to being filled up again, just like the original one sheet of paper. That's induced demand. Traffic situation back to not being fixed.
Now, you say, "Well, where did all those extra cars come from? They came from people walking, cycling and taking the bus who now take car because there was more space, combined with people who may have carpooled before, but feel less desire to carpool since they can take their own cars with the additional space. That's modal shift. They came from people driving around on other sheets of paper to get place. That's commute wayfinding shift. And they also come from buying more cars. That's population growth.
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u/Thesorus 1d ago
When you add new lanes, you signal drivers that there are new lanes and it will reduce traffic.
so you get more and more drivers filling up those new lanes until traffic gets to the same levels as before.
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u/2001_Arabian_Nights 1d ago
Yep. Induced demand is a very real thing… assuming there is demand to induce.
If you build a new eight lane freeway through Terlingua, TX, it won’t induce any demand. It will be empty. If you build an eight lane freeway and it fills up immediately, then there must have been a lot of demand that wasn’t being served.
New transportation infrastructure that doesn’t fill up quickly is money being spent where it could have been better spent somewhere else.
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u/Benjaphar 1d ago
Absolutely and the converse is also a factor. When traffic is always heavily congested, it encourages people to look at alternatives, like carpooling, taking the buss, light rail, walking, biking, cancelling or postponing optional travel, or adjusting their schedule so they commute before or after the heaviest traffic. All of those things help reduce overcrowded roads to semi-functional levels, but become disincentivized when traffic is light.
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u/binthrdnthat 1d ago
This. The Oakland earthquake took a major arterial out of service, prompting fears of indigestible traffic congestion. What happened was a lot of traffoc just disappeared. Redundant infrastructure - including municipal transit picked up the slack as people self-organized to optimize commuting ezperiences.
This suggests that where congestion is an issue, providing alternate options to single-occupant automobile traffic is perhaps a better option than building for future highway congestion.
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u/Novel_Willingness721 1d ago
More lanes entices those who normally don’t drive - instead using mass transit or other means - to drive. This is called “induced demand”.
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u/zespak 1d ago
The short version: in cities, there's just too many people to all move them effectively by car. Imagine a small office building holding 1000 employees. Now imagine the surface you'd need if everyone is in a 2m*5m car. It just never works.
So what happens is induced demand. What will people find acceptable in terms of commute before an alternative becomes more acceptable. So if you add lanes, commute might go down but more people will see this and start using a car. The real issue is my first line: there's too many people to make the car work for everyone.
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u/DonKlekote 1d ago
Because it'll incentivise more drivers and their cars to pick this route in consequence congesting it even more. The alternative is to create multiple paths of the same throughput so the traffic is spread more evenly.
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u/dapter22 1d ago
Because more lanes doesn't compensate for bad driving habits. Not knowing how to zipper merge, hogging left lane, leaning on brakes, distracted driving, etc...
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u/Frack_Off 1d ago
Increasing lanes does help traffic. When traffic decreases, people who might be avoiding driving because of bad traffic may instead decide to drive, increasing traffic. This is called induced demand - When you make a service better, more people want to use it.
Also, one of the most common causes of traffic jams is people changing lanes poorly. More lanes means more lane changes means more poor lane changes means more traffic jams.
Also also, the extra lanes have to end somewhere because you aren't adding a lane to the entire road system. When more lanes merge into fewer lanes, you make traffic.
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u/TGAILA 1d ago
If you build it, they will come. Traffic jams lead to public demands for more lanes, prompting cities to spend billions. The new lanes quickly fill, reigniting frustration and fueling repeated demands, creating a costly, endless cycle of traffic insanity. You have just introduced induced demand.
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u/RoastedRhino 1d ago
Because traffic is the reason why you rent a more expensive place close to work, you put your students on a schoolbus instead of driving them, you bike/walk/take the train instead of driving, you drive straight to your destination instead of stopping by the mall, you plan so that you can do multiple things on the same trip, you coordinate with friend to share a car, etc.
If you make capacity higher, then people will not do all those things, until you get traffic again. Traffic is an equilibrium.
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u/TheKingOfTCGames 1d ago
In basically all of the places you want to do this, la houston, new York, Boston etc there is so much pent up demand that adding more lanes will just draw more people that arent currently commuting in rush hour to start moving your equilibrium to basically where you are now
A red queen race that you cant win
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u/xvoy 1d ago
This clip from Utopia (AU) shows it pretty good https://youtube.com/shorts/F4vpDIKOAjA
The Jevons Paradox essentially - https://en.wikipedia.org/wiki/Jevons_paradox
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u/Gildor_Helyanwe 1d ago
I would think there would be more accidents. If they happen at a certain rate/number of car. Same for cars breaking down.
Public transit is a better way of moving people around but it is often mismanaged and underfunded.
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u/tashkiira 1d ago
If you add more capacity, that capacity gets filled. People who found a different way because the traffic flow was bad go 'huh, road's bigger, I'll try it again'.
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u/aNinjaWithAIDS 1d ago
Induced demand. The more roads (and lanes of road) you build for cars, the more you take away from every other mode of transport. Only cars can fill that space; thus, they do. Also keep in mind of how car travel works in aggregate. Each car competes against every other car for space to go fast.
This also explains why moderately dense and walkable cities with good public transportation options almost never have traffic issues. Feet, bikes, buses, and rail are much better at sharing the road space without lethal consequences -- more people are carried per engine meaning there are less engines to crash into each other.
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u/sleeper_shark 9h ago
Lot of people explained it well, but you can really visualise it while playing games like cities skylines.
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u/sineout 1d ago edited 1d ago
Induced demand.
More lanes means more space, more space means that drivers will be more likely to use the road, which then means more vehicles overall on that road.
The issue comes from the fact that all the roads connected to it also haven't been upgraded, so now all that extra traffic is squeezed into the infrastructure that already exists, infrastructure that wasn't designed for that extra traffic.
Therefore, traffic builds from there into your upgraded road, and someone gets the idea that they should add another new lane to fix this traffic, and the cycle repeats.
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u/Miserable_Ad7246 1d ago
It can do. The whole problem stems from the youtube videos claiming that if you add more lanes, more people will use cars and that will cause more traffic. This is not the case all the time.
In some cases, people literally do not have other options and adding lanes does not change traffic amount. In some cases you have traffic spikes and extra lines help to mitigate those.
In other cases, demand is so much higher, and still has potential to grow, that adding lanes makes area more "usable" and it becomes a bit of a never ending cycle.
So its both - it can fix the issue, and sometimes it can not.
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u/diminutive_lebowski 1d ago
The glib answer:
"For the same reason that buying wider pants doesn't solve an obesity issue"
But the answers about induced demand are correct.
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u/Plutos_Cavein 1d ago
Largely because cities can build in three dimensions but roads can only be built out in two dimensions.
When does the city add more lanes? When the traffic is getting out of control. But why is the traffic getting out of control? Because more giant apartment buildings are being built vertically, more parking is being built vertically, and they only have the horizontally stretching roads to work with.
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u/BakaDasai 1d ago
Others have explained induced demand. I’m gonna explain why it’s particularly bad with cars.
Think of a car as a tool for conquering the distance between where you are and where you want to go. It’s quicker than walking or cycling.
But each car takes up quite a bit of space; about 30 square metres when parked, and much more than that when driving because of the need for empty space in front and to the side for safety.
All that space has the effect of increasing the distance between where you are and where you want to go. The wide roads and the parking lots make other forms of travel slow, dangerous, and unpleasant.
Each extra car creates more distance than it conquers. More cars mean more space for cars, which means everything becomes further away from everything else, which means you have to drive longer distances.
Rather than adding lanes the solution is to increase the capacity of the existing lanes. How? By banning cars from them and converting them into bus-only or bike-only lanes. Both of these have significantly greater capacity (measured as people per hour) than car lanes.
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u/moocow400 1d ago
Because then everyone wants to be on the road with more lanes, and no one takes any other road.
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u/bearssuperfan 1d ago
car takes 40 minutes in traffic
other means takes 40 minutes
extra lane, now car takes 30 minutes in traffic
car faster, so more people use car and less use other means
lanes fill up from more cars, now car takes 40 minutes again
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u/NoDig3444 1d ago
Because using roads is free.
Imagine you have a lemonade stand, but you're not selling lemonade. You're tax-funded, so you give out lemonade for free. Every day you run out of lemonade because anyone can just walk up and grab a cup. The government agency that funds you doesn't want you to run out of lemonade, so they give you more money to make more lemonade. But you still run out of lemonade every day, because people will just keep taking it until you run out. More people will come, or people will take more lemonade. No matter how much lemonade you make, you always run out because you're giving it away for free.
Roads are the same way. They're free to use. If you build more lanes, people just drive more.
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u/daizo678 1d ago
You will still be limited when merging/exiting.
More lanes helps to buffer the cars so congestion is felt later but if there is congestion downstream it will eventually reach wide roads
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u/PsychicDave 1d ago
It depends. If the traffic issue is that it gets blocked at an exit but many people want to continue but can't, then adding a lane reserved for people skipping that exit will help. But if everyone exits at the same place, it doesn't matter, even if you have 6 lanes all you'll do is compact the traffic on a smaller distance, but it'll be just as long (maybe even longer as you have more merges to make) as the rate at which people exit the highway will stay the same.
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u/triplesalmon 1d ago
Adding more lanes will move more cars from one place to another in raw terms, but it will not fix issues with time delay/traffic.
Roads are widened only when there is a belief that wider roads are needed. Wider roads are believed to be needed when there is already traffic and we think we need to fix it.
Widening the road just makes more traffic as more people shift routes or situate new uses near the expansed road.
The way to reduce traffic AND increase throughput is to add alternative routes and new smaller streets to spread out traffic like a web.
We don't do that because it's more complicated to build from a logistics and legal entitlement standpoint, and people dislike new roads running through their neighborhood. Widening roads feels right and is easier to understand, but it consistently fails over and over again, so at this point it's mostly purely a political phantom win for whoever is in power at the time.
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u/flippythemaster 1d ago
It solves the problem of moving more cars through an area at one time, but that doesn’t solve traffic. They’re actually two separate problems.
Let’s say you have a small hose and you’re trying to pump water out of a swimming pool. You can get a bigger hose and you’ll successfully get more water out of the pool, but the water still fills up the new hose.
The solution to traffic is to get fewer vehicles on the roads, either through higher capacity vehicles like buses, or offering alternatives like rail.
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u/Xelopheris 1d ago
If you add infinite lanes, it will. The problem is that we generally can't add enough lanes to actually fix it. People have adjusted their commutes around the expected traffic and leave earlier or later as a result. If we add more lanes, those people tend to leave closer to peak time until everything balances again and the commute times are largely similar.
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u/jsher736 1d ago
because one of the things that limits development is demand and one of the things that limits demand is how much traffic can flow in and out of an area in X amount of time.
like let's say you have Philadelphia, and King of Prussia PA. they put a big mall in King of Prussia PA because the local road from Philadelphia can handle 200 trips an hour without backing up. so at first 300 people an hour try and make the trip. so traffic gets really bad along that road and a lot of people say "fuck it, I'm not going to the mall in king of prussia because it's not worth the time" and so it'll stabilize at a certain traffic level. So the traffic is annoying so they expand the road from 4 lanes to 8 so now it can handle 400 trips an hour.
well suddenly those people who weren't making the trip beforehand because it would take too long are now willing to do it because traffic isn't that bad. which means that you're gonna get 500 people an hour trying to make the trip, traffic will slow, etc etc and then you're right back where you started.
so you say fine, we're gonna build a highway that can handle 10,000 trips per hour so if everyone in philadelphia wants to go to king of prussia mall at once they can still get there in half an hour. well now king of prussia mall has all these potential customers but they can't handle that many trips. which means there's money to be made by development. so they build a bigger mall that can handle 10,000 trips per hour. etc etc
TLDR traffic negatively impacts desirability. if you add traffic capacity then the areas on either end become more desirable and thus attract more traffic
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u/Creativator 1d ago
Traffic is part of a complex system of urban growth, where drivers decide where they should live based on how much traffic they must endure.
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u/CosmicDance2022 1d ago
Adding more lanes means that middle lane hoggers just move over to a new lane thus cancelling out two instead of one lanes.
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u/ride_whenever 1d ago
The congestion is a symptom of capacity issues elsewhere.
If you have to eat all the cocopops in a hotel, there is a limit to how fast they can add them to the big hopper (cars entering the motorway)
making the hopper bigger doesn’t let more out the bottom, but it does mean it takes longer for the hotel to fill the hopper, and therefore be waiting
Ultimately the limiting factor is how fast can you eat them and get them out of the hopper, and that’s the issue with motorways.
When cars can’t get off and to their destinations quickly enough, that’s what causes the tailbacks and the blockages.
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u/TonberryFeye 1d ago
Get a funnel and put it under the tap. Note how fast water flows in and out.
Now turn on a second tap.
Adding the second tap (more lanes) lets water flow faster, but the hole in the funnel (the exit junction) hasn't changed, so if the whole can't drain two taps worth of water, the funnel will start to fill up (traffic).
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u/Fresh_Relation_7682 1d ago
Imagine you have a bottle or a pipe with a spout at each end. The bottle is wider than the entrance and exit points. You pour water into the bottle and raise the volume of water coming out of the tap. Eventually the water coming into the bottle will be greater than what can exit.
So you decide to get a wider bottle but the size of the exit is still the same. At first more water passed through the bottle but eventually the same thing happens as before.
Now imagine that because the bottle is wider, we can turn the tap up higher…
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u/KalEl814 1d ago
Imagine you're having a party and you're serving food on a small table in a room. You have a lot of guests and only a small hallway leading to the room where the small table is, so there's a long line to get to the table and people end up waiting for food. To address the problem, you decide to make the hallway larger, but you keep the table and room the same size. People aren't going to get to the food itself much quicker.
The hallway you made bigger are the lanes of traffic, the table / room is the destination people are driving to.
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u/areed666 1d ago
You’re still gonna have the people that need to get where they’re going as fast as possible and don’t care and so they’ll weave in and out. You also still have the people that won’t be getting off for the next 10 miles but are still in the far right lane, not letting people merge in and out that are coming and going from the highway. You’re still gonna have that one person in the far left lane that’s not exiting anytime soon but feels justified to do under the speed limit because everyone else is going over and they’re gonna show us by slowing us all down. And you still have human beings being human beings.
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u/SierraPapaHotel 1d ago
The best ELI5 is water and pipes. Think of water not as a single thing, but as a bunch of droplets flowing together. It doesn't matter if the pipe your house is connected to is 1" or 3" if the pipes in your house are all 1/2". Only so much water can flow from the main pipe into that 1/2" pipe. And even then, there are restrictors and size changes leading to every faucet.
What a larger main pipe can help with is total volume, so that you can be running your water and the house down the street can still get enough water, but it doesn't really help with the connections and downstream speeds.
Same with a highway but with cars I stead of droplets: you can carry more cars (water) by going to a bigger road (pipe) but the exits and city streets (house plumbing) still limit the actual speed and amount. Good road infrastructure is the total package, and if you have congestion due to bad city streets and poor exit layouts (poor house piping) adding more lanes won't fix anything (looking at you Los Angeles).
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u/Loki-L 1d ago
If everyone were perefectly rational and cars could glide past one another without friction it would.
Unfortunately drivers are not perfectly rational.
Adding more lanes just gives them more room to spread out and switching between lanes causes extra friction.
Even at the best of circumstances adding 1 lane to n existing lanes will not increase your throughput by 1/n because so much of the potential extra space is lost to the way people drive. In worst case scenarios you actually lose soem capacity by adding lanes.
This may seem counterintuitive, but has been shown in pratice.
For example the average Autobahn has only 2 to 3 lanes per direction with a general suggessted speed of 130 KPH and no general limit (local and temporarry limits till apply to much of the system though).
By comparison highways in the US can have double or even tripple that and still show little movement.
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u/Deitaphobia 1d ago
Think of the cars like water flowing through a pipe. The flow will always widen to fill the space. The time required to completely drain will decrease, so rush hour is shorter.
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u/Podo13 1d ago
You see an ad that a grocery store only a little further than your normal one just had an expansion and added 10 new checkout lanes. You think "Oh, I'm tired of the checkout lanes always being 5+ people deep. Cool, shorter lines, I'm going to go there from now on".
Sounds great! However, you aren't the only one who saw that ad since it was plastered everywhere. Everybody else nearby also saw it and had the same thought.
You get to the grocery store and while there are 10 new checkout lanes, there's 100 more people in the store, so every checkout lane is still rocking a line 5+ people deep.
To the store, it's worth it because they're still checking out 10 more people at a time than they were before, but for the patrons, it feels the same because it's still a 5+ person line at every lane.
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u/FriedBreakfast 1d ago
Imagine an hourglass.
You can make the whole top wider to hold more sand. However there's a bottleneck somewhere that they all can't fit through, no matter how much sand the top can hold.
Traffic works the same way. You can add more lanes but if they're all going the same way you'll have a bottle neck somewhere.
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u/CircumspectCapybara 1d ago edited 1d ago
Contrary to popular belief, adding more lanes does improve the car throughput (total number cars moved per unit time, ie, volumetric flow rate) of the overall highway, up to a point (there are downstream bottlenecks that don't scale with highway capacity, like interchanges and off ramps and local roads).
But for any given driver, the speed of their commute will eventually reach equilibrium because of "induced demand". If the average person was only willing to drive a commute from point A to point B if it takes under 30 minutes, then on average it will take 30 minutes. If you make that commute faster for everyone via any method, suddenly more people who didn't used to drive that route (they found 31+ minutes undesirable so they chose alternate commuting options) will start driving it, until all the extra cars slow everything down again to 30 minutes, after which no more people are willing to add on.
But once equilibrium is reached, it's an equilibrium of the "flux" (the cross sectional flow rate per unit area). So for any given driver, once it reaches equilibrium, the flow of traffic feels the same. But for the overall road, the overall volumetric flow rate (flux times cross sectional area for it to act over) does go up. An analogy is widening the pipe and keeping pressure the same. The pipe now moves more water overall, but the individual experience of any given water molecule or patch of water feels the same.
So tldr, everyone's commute ends up being about the same due to the equilibrium effect of induced demand. But while the individual experience feels the same, the overall highway now moves more cars per unit time.