[autoexec]

Busy roads are busy because of existing demand, new demand can be created by new roads/lanes and cause the expanded roads to become busier, but that's a really good thing not a bad one. New roads should be planned/expanded with that in mind because the moment existing demand starts to exceed what the current roads can handle it becomes time to build new roads or expand existing ones again.

Allowing outdated and inadequate infrastructure to make people miserable and hinder a thriving local economy is a terrible idea. Improving and adapting our roadways to meet the needs of a growing community is exactly what we want cities to do. We shouldn't use induced demand as an excuse to neglect that responsibility, we just have to understand the effect so that we can plan for it.

[PaulDavisThe1st]

Let's combine two things:

1. the observation that, AFAIK, there are no instances anywhere in the world where adding roads or lanes to roads resulted in over-capacity for the roads

2. your claim that this is because of existing demand that was not previously being met

This would appear to imply that the existing demand - which I'd like to call "implicit demand", because it was not visible via behavior until the roads were expanded - is in fact so large that to meet it would require building a road system of so vast a scope that we clearly would not want it.

Put differently, if building roads merely exposes existing demands, and the 10-100x fold increase in roads over the last, say, century, has not yet met those demands, then we should conclude that it is either impossible to meet those existing demands or will require basically the total destruction of so much land that it will be untenable.

[firedaemon]

Mostly I think the idea of induced demand should encourage us to increase the availability of transportation in “novel” ways as opposed to just adding more road. You can add more road forever and just get more traffic. If instead you add rail, say, you satisfy demand and avoid a concrete highway hellscape.

[autoexec]

I 100% agree. Too often though it's used as an excuse to not update roadways in places where there are zero efforts being made to introduce new public transportation.

It's also worth noting that induced demand exists even for public transportation systems. A single rail line can solve the bandwidth problem for a longer time than a new lane on a highway, but if growth continues you'll still end up needing to add new trains and rail lines or add additional modes of mass transportation.

Induced demand is never a reason to leave the problem of bad infrastructure unaddressed, it's just something we have to take into account as we try to improve things.

[NeoTar]

There were suggestions (pre-Covid) that the Elizabeth line, which increased east-west capacity in London by some ridiculous figure (over 50% if I recall correctly) could be saturated by induced demand (or if you prefer the realisation of suppressed demand) within six months of opening.

[marcosdumay]

Those also have the same induced demand effect people are talking about. It's just that they can absorb a lot more usage than roads.

Adding a passenger rail is in principle not very different from adding a 10-lane corridor. Including the problems of what those people do to enter or leave it, and the fact that it too can become overloaded.

That said, yes, of course the rail is a much better option.