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

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.