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?

Question

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

In: Engineering

Answer 1

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.)