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

You’re talking about *induced demand*. The theory of induced demand is that *more people will drive*, not that more drivers from side roads will use the freeway instead.

Here’s the theory:

If the roads are small, that means they get congested quickly, making them less efficient. More people will choose to use the bus, bike, walk, take a subway, etc.

If the roads suddenly get big, driving becomes really convenient. That means *more people will drive*. This causes four problems:

1. When those people get off the major road, they will clog up the smaller roads and create more congestion.

2. To use those big roads, more people are buying cars. People who didn’t have a car buy one. Households that had one car might get a second car as well. All these cars need to be stored somewhere when they’re not in use, which kills cities and pushes more people out to the suburbs where they can have a driveway.

3. Fewer people use public transportation, so there’s less funding for it. This means public transportation gets *worse*, which encourages more people to drive.

4. Eventually, all the new drivers fill up the maximum capacity of the new giant roads, so you end up right where you started (except with even more drivers and even more congestion on side roads).