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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The convenience of singing is not the difference, it is the time.
Eg. If people are willing to drive 40 minutes to work and then you make a fast highway, they can then drive faster and live further away.
That’s the same for any journey. So it isn’t just more people on the roads, it is also the same people travelling further.
The wording of your question doesn’t seem quite right, the word “flow” is ambiguous as it could mean number of cars or it could mean decreased congestion.
If you build or expand highways you will get more cars using them until you end up as congested as you were before you built them. If you limit your metric of what a gain is to number of cars on the road than you would describe a 4 lane highway with bumper to bumper traffic as a gain over a 2 lane highway with bumper to bumper traffic; if, on the other hand if you consider the cost of the extra 2 lanes in money and impact to human beings, especially those not using the road but living adjacent to it you would consider it a loss.
The politics of the phenomena is that new roads will be advocated for with the argument that they will improve everyone’s driving experience, that where once you were stuck in bumper to bumper traffic with the new road you would be able to speed along. The criticism is that this is a fallacious argument because of the extremely short term nature of the improvement as the very improvement in the speed of traffic attracts new drivers which in turn quickly causes congestion.
A wider highway increases capacity but not traffic flow. Think of getting a wider hose (the highway), but using the same size nozzle (the exits).
Traffic flow is mainly dealt with by adding more alternative paths to destinations and alternative transit.
The issue is that, in addition to the induced demand problem, increasing a highway inside a city or metro area, just takes the money away from other areas, such as busses, trains, metros, and other forms of people movement that aren’t cars.
Not to mention getting people across a highway is exponentially harder and more expensive, the more lanes it has. Either you build a bridge over it, which becomes more expensive the longer the span is (you’re not going to drop a support mid freeway), or you build the freeway over a tunnel/on a bridge span itself, which suffers the same problem.
In an ideal world, people have the option to drive. Some people just need to, like tradesman, people who’s jobs require driving to homes that may be outside the general metro area. But the general commute, is done through public or personal transit.
If you’re interested, look up the 15 minute city design (FMC/15mC). It’s the principal that basically means you don’t *need* to drive your kids to school, you don’t *need* to drive to Walmart to do the weekly shop, and even if you have parking, it’s not on-street/above ground, so it’s not taking up useful space.
It also makes use of mixed use zoning, like an apartment block with commercial on the ground floor, maybe a gym on the 1st, as that’s not a necessary ground development, and parks/green space instead of parking lots, to allow the residents and those around them to use it, instead of just those interested in accessing that plot.
And the capacity doesn’t increase linear. A 4th lane doesn’t give you 33% more. Trough lane changes higher risk of accidents,… it is maybe 20%.
Yes, it makes sense that more lanes would mean less congestion and quicker commutes. However, in practice, the “quicker commute” part doesn’t last long. More lanes means people makes plans to use those lanes – for example, they build more houses in the suburbs serviced by those highways. They might build more services, or otherwise add capacity at the destinations you can reach on that highway.
To be clear, this doesn’t mean that building more lanes is always wrong. But the result won’t usually be to “help commute times” – rather, commute times will tend towards the same, while allowing more overall throughput for car traffic on that road. Ie. it’ll still take 45 minutes to get downtown, but there will be more people getting downtown each morning.
(Also, I’d be curious how much these historical trends have been bucked by changes due to Covid/work-from-home. Maybe the “rules” have changed to some extent? Not sure.)
Probably not what op had in mind, but the first thing I thought of was braess’s paradox: in specific scenarios adding (or expanding the capacity of) a shortcut can lead to longer total transit times. Conversely, removing network capacity can actually speed transit up.
It is a phenomenon of game theory.
Imagine the following routes from point a to point d:
A-b-d. Where a to b is big and fast, but b to d is slow.
A-c-d. Where a to c is slow but c to d is fast.
If you add a shortcut between b and c, opening up the fast route a-b-c-d people will (depending on the specifics) flock to that fast route, and now the whole network is congested, *including the original two paths* (because the new path includes parts of both original paths, so if the new path is congested, the whole network is congested).
Not only that *more* people would drive, but also that people would drive *more*.
For instance, you forgot to buy eggs? The closest grocery store is two miles from your suburban house, but there’s a supermarket a few miles farther that has cheaper eggs, and also you could buy other things. Or people would go to live in farther and farther suburbs, so the congestion that previously happened 10 miles from downtown, now starts 10 miles back.
It’s possible to say that those are good things, because you chose them based on an analysis of cost and benefits, but you’re not taking into account the cost of building additional highway lanes (because you didn’t pay for them directly), or the non-existence of alternatives such as public transit (because you’re not *paying* money).
It is.
Induced demand is a sort of crackpot theory, because travel is quite inflexible in demand. Not wholly so, but if you gotta drive to work every day, that doesn’t change because of one more or fewer lanes.
Most living places/job combinations do not have a reasonable transportation substitute.
I got to say at least in New Orleans they increased the lanes on I-10 by two lanes and traffic flows GREAT. The bottle neck happens when we go from 5 lanes to three.
We used to have three lanes throughout and traffic would start as soon as you got on I-10 in the city all the way out through Metairie and Kenner. Now you can roll through those areas at a good pace until you get out to St. Charles.
Obviously because that improved traffic flow here that does not mean it will work everywhere but more lanes helped out a lot here.
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