“Induced demand” meets “One less car”

When peak-hour traffic congestion gets unbearable and new roads are built, there’s an initial reduction in congestion and everyone is happy.  The congestion comes back surprisingly fast — a phenomenon known as induced demand.  Before the new roads were built, people would have been avoiding them at peak times: they might have travelled at non-peak times, or car-pooled, or taken the bus, or gone somewhere closer instead, or just made fewer trips.  With the new space, these people can now drive. That’s good for them: they must benefit from being able to drive or they would still be doing whatever they were doing before.  It’s bad news for people who were already driving in peak traffic; their new lanes are being filled up and they’ve lost most of the benefit of the new road capacity. Car unenthusiasts such as Greater Auckland (and, um, me) love to tell you all about induced demand, but even car enthusiasts will often admit it’s a thing.

On the other hand, Auckland is going through an expansion in bus services, bike paths, and near-city housing.  As more people bus, walk, and cycle, pressure on congested urban streets will decrease, as will carbon emissions from transport.  Every mass transit or active transport user is One Less Car.  Studies of short-term disruptions such as transit strikes confirm that public transport, and probably bikes, really do reduce congestion.

There’s a bit of a contradiction here, though.

If extra space on the roads provided by new construction is quickly filled up by new demand, you’d expect extra space on the roads provided by One Less Car to be filled up in the same way.  Just as the short-term congestion effects of adding or subtracting new road lanes overestimate the long-term congestion effects, the short-term congestion harms of taking away buses for a day would overestimate the long-term congestion benefits they provide.  People adapt.

For example, Seattle, in the US, has made a big effort to increase public transport in recent years, with some success. The proportion of households with fewer than two cars is increasing (in contrast to similar cities).   On the other hand, congestion (as measured by TomTom) and  vehicle miles driven are both slightly up.  The policies have been successful — there are more non-car trips than before and the stable congestion and driving statistics are for an increasing population — but congestion hasn’t decreased.  In Auckland, more people now work in or near the city and many more people get to those jobs without driving. A lot of cars have, in some sense, been taken off the roads, but congestion hasn’t decreased and motorway traffic volumes are stable.

Now, there was a recent research paper from the University of Otago (press release) looking at new cycling and walking paths in New Plymouth and Hastings, which estimated a small but persistent decrease in car use (about 1%).  But these aren’t cities where car use is strongly limited by congestion, so you wouldn’t expect much induced car traffic demand.

Even with induced demand there are real, important, benefits when people use alternatives to cars. The people who switch to bike or bus will benefit (or they wouldn’t do it). The people who weren’t previously driving in peak traffic and who now get to supply the induced demand will benefit.  Some people who would otherwise have been forced out of peak driving will be able to continue, and they, too, will benefit. But people who are in peak-hour traffic anyway don’t really benefit.  To them, it’s not One Less Car. It’s One Different Car.