Eli5: it’s said that creating larger highways doesn’t increase traffic flow because people who weren’t using it before will start. But isn’t that still a net gain?

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If people are being diverted from side streets to the highway because the highway is now wider, then that means side streets are cleared up. Not to mention the people who were taking side streets can now enjoy a quicker commute on the highway

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23 Answers

Anonymous 0 Comments

This is an economics question, not an engineering question.

Yes, the total capacity that can be handled by the road, increases. Thus, the total volume using the road also increases. This becomes good for businesses/organizations/etc . But because of that increased demand, some individual drivers may or may not actually get a less congested drive.

Anonymous 0 Comments

You’re talking about *induced demand*. The theory of induced demand is that *more people will drive*, not that more drivers from side roads will use the freeway instead.

Here’s the theory:

If the roads are small, that means they get congested quickly, making them less efficient. More people will choose to use the bus, bike, walk, take a subway, etc.

If the roads suddenly get big, driving becomes really convenient. That means *more people will drive*. This causes four problems:

1. When those people get off the major road, they will clog up the smaller roads and create more congestion.

2. To use those big roads, more people are buying cars. People who didn’t have a car buy one. Households that had one car might get a second car as well. All these cars need to be stored somewhere when they’re not in use, which kills cities and pushes more people out to the suburbs where they can have a driveway.

3. Fewer people use public transportation, so there’s less funding for it. This means public transportation gets *worse*, which encourages more people to drive.

4. Eventually, all the new drivers fill up the maximum capacity of the new giant roads, so you end up right where you started (except with even more drivers and even more congestion on side roads).

Anonymous 0 Comments

Better highways will absorb part of traffic from side streets. This would make commuting by car temporarily more convenient, this would convince more people to get a car and join traffic.

Net result is even more cars on roads.

Anonymous 0 Comments

It is a short term benefit. Eventually people may choose to live further from their work than they did before. Also, they may choose to shop further away from their homes causing smaller and more local businesses to struggle.

You don’t want to build zero roads. People and goods need to move around. But there are trade offs to making it super-easy for everyone to be able to drive all the way across the metro area for everything.

Anonymous 0 Comments

What they mean by “increase traffic flow” is often “lighten the amount of perceived traffic on a particular road”. So the point here is that the highway itself will still be crowded even if you expand it in an attempt to provide more space for the same amount of cars. The amount of cars in that space will just increase, not solving the original problem the expansion was trying to fix.

Anonymous 0 Comments

Well, there’s two separate things here.

1. How the change is marketed. It isn’t usually marketed to the public as “more vehicles” but “less congestion / less time.” It’s not clear that the public would support the level of investment needed for something that doesn’t actually reduce congestion.
2. The highway exits to side streets. If the side street exit is the congestion point, then making the highway wider just changes your congestion from:







That is, it just redistributes the backup to be “wider” instead of “longer.”

Anonymous 0 Comments

Induced demand is a well studied phenomena. I worked for a highway for a number of years and though I wasn’t a traffic engineer I had many conversations with them so my level of expertise on this is far higher than average but less than a civil engineer who works for a city. The connection between adding lanes and reducing service levels (defined by the total number of cars passing between two parts of a highway where there are no exits) is an unfortunate truth that no politician wants to hear about.

The simple explanation is that while the highway, in theory, can handle more cars per minute in perfect conditions – perfect conditions never exist. A couple of things collide to make this a reality. As more cars are added to the road, very predictably, the average speed drops in linear fashion. Car accidents become more common and have a larger impact on the overall service level. Popular exits backup even without accidents and impacts speed behind the exit in question. This is one of the reasons you see pictures of LA freeways that are like 12 lanes abreast and not one of them is moving very quickly.

The latest studies have shown that adding a lane does *reduce* commute times for an average of 5 years in the USA and after that it increases back to what it was, but now you are pissing off even more drivers. Considering the exorbitant cost of adding lanes and their proven ineffectiveness, it is surprising that local governments often insist upon them.

There is only one proven way to reduce traffic and commute times, reduce the total number of cars on the road. Thats it, right there, no other amount of gymnastics will ever match this very basic principle. If you are a metro area and you need to move a lot of people quickly in a wide variety of weather conditions, you need trains and dedicated (and preferably protected) bus lanes.

Anonymous 0 Comments

When people talk about this (it’s called “induced demand” btw) it’s not the total number of cars on the road that *individuals* care about.

It’s how long it takes for a specific person to get from point A to point B and when you add more lanes that time doesn’t go down, and can counter intuitively go down sometimes.

The most important concept that isn’t just going to be the same amount of cars there were before but the amount of cars on the road will go *up.*

Are there still more people getting from point A to point B? I mean on this specific highway road maybe sure. But if you had a bunch of people that were already getting from A to B on taking public transit and they just switch to cars and it takes about the same amount of time then no.

And if you had a bunch of people that were walking and biking that now switch to cars then well…now they are getting less exercise and polluting the environment more. Hard to consider that a “net gain” that was worth it. Especially because sometimes the traffic is so bad that biking is just as fast or at least comparable.

Anonymous 0 Comments

It’s a net gain in being able to transport people through an area. But the problem is that politicians always promise they’re going to fix traffic by expanding highways, requiring bulldozing houses and investing billions of dollars, and not actually fixing traffic.

It really doesn’t make anyone’s commute any faster. What you’ll typically see is less use of public transit as more people can get to work in their car in the same time.

One interesting way of looking at it is thinking about what argument you would make if public transit had induced demand. Double the number of busses and you’ll double the number of busses and you could double the number of people taking the bus. Nobody will complain that by adding more busses you’ve increased ridership. But the goal of a good transit system is high usage of the system. You don’t *want* more people driving on the highway because that is not going to make commute times better.

One other thing to think about is how congested the area where everyone is driving to is. If you have a city that is largely downtown-centric, then all those cars still need to get off the highway in a few exits and then drive down a few specific streets to get to their destination. Those streets didn’t get expanded, but now there’s more cars trying to drive on them. If the total capacity of cars moving through downtown is less than you can handle on the highway, then you’re still going to get backed up at those exits. It can actually be more dangerous as you have queues building up the off-ramps and into live lanes, and suddenly seeing stopped cars in front of you on a highway is never a good thing.

Anonymous 0 Comments

More lanes also adds exponentially more lane changes which by extension cause many more interruptions in the flow of traffic and increase the risk of accidents.