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

Well, there’s two separate things here.

1. How the change is marketed. It isn’t usually marketed to the public as “more vehicles” but “less congestion / less time.” It’s not clear that the public would support the level of investment needed for something that doesn’t actually reduce congestion.
2. The highway exits to side streets. If the side street exit is the congestion point, then making the highway wider just changes your congestion from:

=======================================

========================================>Exit

To:

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====================>Exit

That is, it just redistributes the backup to be “wider” instead of “longer.”