this post was submitted on 09 May 2024
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I once heard of an experiment in economics that offers insight into this.
Say you have 100 people. You give each of them one of two choices:
A : you get $40 unconditionally B: you get $70 - n, where n is the number of people who choose B
You end up getting, on average across experiments, n = 30.
If you move the numbers around (i.e, the $40 and the $70), you keep getting, on average, a number of people choosing B so that B pays out the same as A.
I think the interpretation is that people can be categorized by the amount of risk they’re willing to take. If you make B less risky, you’ll get a new category of people. If you make it more risky, you’ll lose categories.
Applied to traffic, opening up a new lane brings in new categories of people who are willing to risk the traffic.
Or something. Sorry I don’t remember it better and am too lazy to look it up. Pretty pretty cool though.
I heard a city planner talk about why adding a new lane doesn't help, and the term they use is "induced demand."
Basically, people are going to take the route that they consider the most convenient, and that usually comes down to time and effort. Traffic hurts both by taking more time and being more stressful to deal with. When you add a new lane to a road, people think that the traffic will be easier there, so they take that route instead of their normal one. So you're just adding more cars to the traffic that match or exceed the throughput of your new lane, basically putting you back at square one but a few billion dollars more poor.
You've essentially added a single lane one-way road to help ease traffic across the entire city.
so for these people the new lane will create marginal improvement, right?
That's the thing: Technically yes. It temporarily improves traffic. But only temporarily. IDK about you but spending billions of dollars to only temporarily improve traffic and then it ending up the same or even worse than before doesn't sound like a good investment to me.
but is it?
I thought the temporal improvement would be for everyone who already used the high way (because they will get to their destination a little bit faster). And for the few extra people, who start to use the highway but didn't use it before, the improvment will stay.
That's the thing, the number of new cars using that road ends up being at least one additional lane's worth. So traffic moves at the same speed as it was before the extra lane, just now with one more lane's worth of cars on that road.
If anything, you might see marginally better traffic on other roads because of the cars that started using the new lane, but you'd be talking about a handful of cars per road. Probably not enough for any discernible change in travel time or congestion, and each new lane you add later will have diminishing returns because it will be a smaller fraction of the total number of lanes coming from any specific direction.
It's called "Induced Demand".
As a road widening project is completed, traffic is alleviated for a short amount of time. Then as time passes word spreads, or more people move to the city, or kids get older and get their driver's licences. More and more people know this widened road is the fastest route, so more people take it, thus undoing the improvement. Then the cycle starts again - either with the same road being widened again, or another one a block over, on and on until the world is covered in asphalt.
The solution is to make alternative transit more appealing than cars. Bikes and public transit already have significant financial benefits, but lack infrastructure to make it more viable in North America. Busses get stuck in traffic, bikes are forced to share lane space with cars or sidewalks with pedestrians.
How is alternative transit the solution? Cities that have public transportation still have traffic jams.
There was an English traffic engineer that predicted that avg speed in central London will always be like 9mph. No matter how many lanes or public transit options you add. If there is no traffic, people will take cars until traffic jams are unbearable to give up. Then the system finds equilibrium.
The key is public transit that doesn't suck. For the last 100 years the car and oil/gas industries have spent billions of dollars undermining public transit.
Dedicated transit lanes, subways, light rail, protected bike lanes all make cars less appealing to those that want to use them.
Yeah, one of the best examples of this is the Vienna public transit network. About 1000 vehicles (bus, tram, light rail, subway) in service at rush hour, a daily total distance of over 200000km traveled, more year-long ticket owners than car owners in the city, and about 2 million "travels" per day, which is about 30% of all traveling done over the city (including pedestrian and bike traffic)
If that traffic would be routed only by car, the city would be a giant parking space; to compare, one subway train carries about 900 people in rush hour, which replaces 790 cars (avg 1,14 persons per car here). the subway interval in the rush hour is about 4 minutes. i live at one of the subway final destinations, which is on one of the far ends of the city - and i can be at the other side of town in about 25 minutes.
And constructing and running a public transit network is a pretty nice boost to the local economy, creates a whole lot of jobs. sounds like something a lot of us cities could make use of.
Mixed traffic works here, it allows mobility for all social classes (yearlong tickets cost 365€, so about 400$ incl. taxes), nearly all stations are barrier free.
Public transit that share lines with cars are always going to be worst that cars, but if you add exclusive lanes for Public transit they go smooth as hell. This is why metros are usually the best option on cities with good metro infrastructure.
having lived in cities with amazing metro, ok metro, no metro and towns with shitty train access and great train access...
this is 150% true. Having an underground station near your house makes the entire city 30 min away, using buses or horribly interconnected trains makes things 1 to 2 hours away.
Even living outside the city, having a direct rail to the nearest metropolitan center take an hour increased quality of life for me by a fuckload.
The key is that both adding car lanes and adding alternatives like transit are subject to induced demand, but the consequences of it are different for transit than for cars. Not only is the limit of the added capacity much, much higher for a train than it is for a car lane, adding more traffic to the lane up to that limit makes the performance worse and worse (increasing congestion), while adding more transit ridership up to its limit makes the performance better and better (increasing train frequency and therefore reducing wait times).
Similarly, induced demand for walking and biking is a good thing because more people doing those things improves public health, doesn't pollute like cars do, and takes up much less space.
So it's not that induced demand is bad, it's that inducing demand for cars, specifically is bad.
The goal for alternative transit is not to remove traffic jam, it's to transport people.
9mph is slower than a bike, it fits with my experience. When living in a city (in Europe, it might be different in the US) my feeling was always that bike was the fastest, public transport a bit slower but more comfortable (mainly protected from the elements and car drivers) and lastly car was the slowest and more stressful.
There will always be traffic, but public transportation allows for a higher throughput for the same speed and total surface area of the roads.
Let's be generous and assume that every car has 2 people in it (the truth is that the vast majority of cars, especially in the US, only have 1 person in them). Now imagine 15 cars vs. 30 bicycles. If we figure that you can comfortably fit 3 bikes in the same space as 1 car, you're looking at 150% throughput for the bikes compared to the cars at the same speed. Give them their own dedicated, separate infrastructure, and they can probably go faster than traffic while also removing the danger of bikes and cars sharing the road. If you figure buses can fit 20 people in the space of 2 car lengths, you're looking at 10x the throughput.
And that's not even getting into transportation that doesn't use the roads. The Boston T is a perfect example of this. Despite its notoriety for constant failures due to poor maintenance, and only being half the size it was 100 years ago, the T is considered to be the 3rd best public transportation network in the US. Why? Because the average commute time is about half the national average at roughly half an hour, and a full 50% of Boston's commuters use the T every day. That's half as many cars in traffic every day than if the T didn't exist. Could you imagine if Boston, notorious for its bad roads and heavy traffic, suddenly had twice as many cars driving on its streets?
Can you explain what this part means? What do you mean by category here?
Yes. That wasn’t the best word choice; maybe “group” would have been better. I meant groups of people who are willing to take some level of risk. Imagine the categories are “low risk takers”, “medium risk takers”, and “high risk takers”.
Compared to A paying out $40, if you make B $50-n you’ll only get the high risk takers choosing B. If you make it $70-n you’ll get high and medium risk takers. If you make it $120-n you’ll get almost everybody.
If risk taking is a value between 0 and 1, the categories are groups of people inside certain intervals. For example, low could be [0, 1/3), medium could be [1/3, 2/3), and high could be [2/3, 1].