induced demand on highways.

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According to induced demand, the optimal highway would be one that is the smallest. Meaning 1 lane would be an optimal highway, but why is every country expanding their highways and by default they have more than 2-3 lanes.

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Induced demand is not the simple, linear phenomenon that you suggest. Yes, in some situations, building a new road might cause people to drive who otherwise wouldn’t, increasing traffic. Conversely, removing a road may cause people to stay home who otherwise would be driving, thereby reducing traffic. But there is some minimum number of people who are going to drive regardless of how many roads there are, and if the infrastructure is insufficient to service this number of people then you are going to increase traffic by removing roads/lanes.

Optimal by what metric? The highway that moves the most amount of goods is the one the city cares about because that’s what highways are designed to do. A 3 lane highway will have the highest capacity and induced demand means more of that capacity will be used.

In addition to u/Beepboopbob1’s great answer about induced demand, let me just add that a single lane is never optimal because vehicles cannot pass each other, and so you will always be limited to the speed of whichever is the slowest vehicle ahead of you. E.g. where I live, trucks are limited to 80 km/h while regular cars can go 100 km/h (during the day, or 130 km/h at night). So with a single lane, everyone would be forced to go 80 because there’s always a truck somewhere ahead of you. Or, if a vehicle breaks down and blocks the lane, then that causes a complete stop of all traffic behind it. So you always want at least two lanes.

The same amount of people are driving the same highways, and induced demand isn’t necessarily MORE cars, just more of the same fundamental issues that exist in 2/3 lane highways that adding more lanes does not solve. As others stated, one lanes are less efficient for intuitive reasons such as no passing and driving the lowest driver’s speed. But, I would argue it’s not the number of cars or lanes that’s the issue, it’s the number of driving styles on those increased number of lanes and lack of sensical engineering to interchanges that causes MORE traffic issues than just 2/3 lane highways. For example, middle lane drivers and semi’s ruin 3 lane highways, and that issue is exacerbated with more lanes because people seem to think ONLY the left most lane is for passing, and on 6 lane highways it’s like every other lane is slow/passing. Furthermore, increasing 5 miles of a highway to more lanes and not the other lanes isn’t adding more cars, just creating more space to speed to the next bottlenecks.

So, it is not the number of lanes that is inducing traffic/not solving the core issue of jams, it’s the failure to interconnect the larger highways to the entrances and exits, on top of the free for all lane driving.

When I drive Chicago to Milwaukee on the 4 lane highway I always stay in the RIGHT lane even though I drive 80 because no one ever seems to drive in it! And the biggest jams, without fail, occur right before larger interchanges where even when the on/off ramp is long, people merge onto the highway without reaching speed.

More lanes do not induce traffic, the poor interconnectivity and inability of folks to follow common driving rules are the problem.

All that being said, even more heavily populated areas with lots of drivers should focus on traffic engineering of three lanes before the band aid attempt at adding more lanes.