> My understanding is that it is pretty much guaranteed, especially in larger cities.
Induced demand can only happen when there is a non-trivial amount of people who have and use some other option. (NYC, for example, where there is a large subway network. A new freeway might convince some subway-users to switch, inducing demand)
For a city with no alternatives (like no meaningful public transit), it's impossible to induce traffic demand, because demand is already at 100%. Traffic still grows as population grows, but a person can only physically drive one car at a time, it's currently impossible to induce any higher demand than that.
> Do you happen to have any examples of where induced demand has not happened
"A year after the freeway opened, traffic volumes along parallel roads like 44th, 56th and 68th streets dropped 40–50%"
That's pretty typical for proper freeway construction/expansion. It's just that we stopped doing that 20-30 years ago, but population growth hasn't stopped in the meantime, so we're way behind on our urban transit infrastructure (all of it, freeways, rails, and public transit).
> For a city with no alternatives (like no meaningful public transit), it's impossible to induce traffic demand, because demand is already at 100%. Traffic still grows as population grows, but a person can only physically drive one car at a time, it's currently impossible to induce any higher demand than that.
This is not true. An example is Houston building a 26 lane highway only to lead to ~40 increased commute times [1]. Initially commute times decreased, but then the population felt they could move farther out into the suburbs, get a more affordable house, and still have the same commute time. Once enough people did this traffic times got worse.
It'd seem in that case it's not that demand increased but that people moved further way from their final destination so their commute time was increased.
It may very well be that the same people are still driving or wanting to drive, but simply by living further away the commute times have gone up.
If people travel further then demand has increased. It is strange how people think about congestion in terms of number of people rather than miles driven. A single car that hits several choke points on a 50 mile commute is causing far more congestion than a vehicle travelling 5 miles. Yet the users are treated exactly the same and never get any social pressure to reduce their demand.
There's no such thing as demand being at 100%, certainly not in this context. Demand is a curve. If the cost of driving decreases--i.e. less congestion, easer traveling--people will drive further and more often. It won't happen instantaneously, of course, but demand will adjust even if the population stays the same.
That doesn't mean the new road will fill up completely. There may be other constraining factors. But if a route was highly congested before expansion, you can bet demand will go up after expansion.
> "A year after the freeway opened, traffic volumes along parallel roads like 44th, 56th and 68th streets dropped 40–50%"
That doesn't tell you anything about the total volume of traffic. And in any event it's precisely what you'd expect to see. If a line at the checkout counter has 10 people in it and new counter opens up, expect about 5 people to immediately move over. But _also_ expect a few more people to join one or the other line now that they'll clear quicker.
Induced demand can only happen when there is a non-trivial amount of people who have and use some other option. (NYC, for example, where there is a large subway network. A new freeway might convince some subway-users to switch, inducing demand)
For a city with no alternatives (like no meaningful public transit), it's impossible to induce traffic demand, because demand is already at 100%. Traffic still grows as population grows, but a person can only physically drive one car at a time, it's currently impossible to induce any higher demand than that.
> Do you happen to have any examples of where induced demand has not happened
https://en.wikipedia.org/wiki/M-6_(Michigan_highway)
"A year after the freeway opened, traffic volumes along parallel roads like 44th, 56th and 68th streets dropped 40–50%"
That's pretty typical for proper freeway construction/expansion. It's just that we stopped doing that 20-30 years ago, but population growth hasn't stopped in the meantime, so we're way behind on our urban transit infrastructure (all of it, freeways, rails, and public transit).