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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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.
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