Pretty much everyone knows that public transit is more efficient than private cars at moving
large numbers of people, but we don't talk enough about just how big the difference is.
This is the Don Valley Parkway, a six-lane highway in Toronto. If you sit in one place
at rush hour and watch the DVP, you could see 6 to maybe 8,000 people in those cars
pass by in the busier direction over the course of an hour. If you do the same thing on the
nearby Yonge Subway you could see 30,000 people per direction per hour, with ongoing signal
upgrades pushing that up to 40,000 people in the future. That’s an enormous advantage
for the subway even before you consider that it only takes up two lanes, mostly underground.
Subways don’t carry freight, but when it comes to carrying lots of people, mass transit
wins hands down and it’s not even close. We're going to come back to this capacity
difference later in the video.
If you follow debates about building or expanding roads and highways you’ve probably heard
someone bring up the concept of induced demand, often linking explainer videos from places
like Vox, (where they use lots of colourful balls to explain), to argue against building
the new road or highway. Induced demand, more accurately called induced traffic, is the
idea that building new road space doesn’t solve congestion, at least in the long term,
because it encourages more driving to happen. When you build new lanes or roads, people
who previously avoided driving at rush hour start becoming less careful about when and
how much they drive. On top of that, other people and businesses see the new road capacity
and move to the area to use it. As a result, congestion eventually ends up about as bad
as before, and this is used as an argument against building the new road capacity.
Induced traffic is a very real phenomenon. It’s not infinite, you could build a big
highway in the middle of nowhere and it’s not going to become congested in the near
future, but there’s a bunch of research finding that 50 to 100% of new road capacity
in urban areas gets filled up after a few years. However, the induced traffic argument
against new car infrastructure has always seemed, maybe not wrong, but certainly incomplete,
because why does it only mention cars? What happens when we build another subway line,
for example—won’t that just become packed in a couple of years too, and isn’t that
a problem? In this video we’re going to explain why induced traffic for walking, biking,
and public transit is different than for cars, and why increased demand or usage for a mode
doesn’t have to mean slower travel for everyone.
Let’s get back to transit in Toronto. In response to overcrowding on the Yonge subway,
including at the major transfer station with the Bloor-Danforth Line, transit authorities
are planning to build a new automated subway called the Ontario Line to add capacity to
the city’s east end. When you think about it, the exact same points about induced traffic
and highways could apply here, except that we’d probably call it induced ridership.
In other words, this new subway line will cause more people to take more trips, both
people who currently live there and new people moving to east end Toronto because of the
new subway. Ridership will continue to grow and eventually at some point in the future
the Ontario Line will probably end up congested.
But if you told a transit planner that they shouldn’t build the Ontario Line because
it will just induce more ridership and eventually become congested, they would probably look
at you funny. Induced ridership is the goal of the Ontario Line, which is meant to encourage
growth in new transit-oriented communities. We want people to use the Ontario Line, whether
replacing car trips or taking trips that they otherwise wouldn’t have been able to do.
We prefer that it’s not so much new ridership that the line becomes overcrowded, that’s
bad for safety, efficiency, and user experience, but if and when that happens, it wouldn’t
mean the Ontario Line was useless and shouldn’t have been built in the first place. It would
mean the Ontario Line was really useful and we should probably build another subway line.
If that’s not possible, it’s still better to have an overcrowded Ontario Line than no
Ontario Line at all. The Yonge line is overcrowded now and no one thinks that means it shouldn’t
have been built.
If the basic principle of induced demand is the same, why does car infrastructure inducing
more car traffic seem so threatening while public transit infrastructure inducing more
transit ridership feels benign or even good? One big reason is what we mentioned at the
beginning of this video, the simple fact that public transit can carry a lot more people,
so reaching capacity is less of an immediate concern. Imagine we’re choosing between
two transportation projects in Toronto’s east end: the Ontario Line with its planned
top capacity of 34,000 passengers per hour per direction, or a six-lane highway resembling
the Don Valley Parkway with its 6 to 8,000 people per hour per direction. Both projects
will induce more people to take more trips, both have the potential to hit capacity and
become overcrowded, but one will reach capacity much faster.
It’s more than that though. It’s not just that transit can handle high volumes, it’s
that transit usually gets better with more ridership. Most of the ways that a transit
agency adds capacity to a route make service better for each individual rider, whether
that’s higher frequency, using trains over buses, giving signal priority on roads, or
making the line tunnelled or elevated. Transit riders don’t want crowding but they do have
reason to want their routes well used. Driving works differently—you want enough other
drivers to justify your road or highway existing but aside from that, traffic is really only
a threat that can slow you down or aggravate you. No one wishes for more drivers on their
road to get more lanes, but you might wish for more ridership on your transit route so
that you can get better frequency. If transit service gets better with more ridership but
the driving experience gets worse with more traffic, it makes sense that induced car traffic
feels more threatening than induced transit ridership.
There’s one more important difference between transit and driving when it comes to induced
demand, which is pricing. One simple way to reduce congestion is to charge more money,
discouraging people from making less important trips. For transit this is straightforward,
you increase the price of a fare or a pass, ideally offering a low-income subsidy if necessary.
The driving equivalent to this would be a road toll or congestion charge, but at least
in Canada we’re not set up for that—toll roads are rare and very unpopular. Increasing
the gas tax might kind of work as a substitute but people can partially get around that by
buying a more fuel-efficient car or stocking up on gas in a different jurisdiction. Thus,
another reason that induced demand is less of a problem for public transit is that it
has a simple form of congestion pricing built in.
We’ve focused on transit so far but we fully expect induced demand to apply to walking
or cycling infrastructure too. Imagine that your city has a mixed use pathway that’s
overcrowded and the local cycling advocacy group wants to double the width of the path
so that pedestrians and cyclists each have their own dedicated space. This probably will
induce more people to walk and cycle, and like with transit that doesn’t seem so bad,
whether people are replacing car trips or they’re just being more mobile in general.
Partly this is because more physical activity is a win for public health, and partly this
is because trips taken by walking and cycling lack the negative externalities of cars. A
bunch of people walking and cycling through your neighbourhood just don’t have the same
noise, safety, and pollution side-effects as the same number of people driving through
your neighbourhood. Infrastructure for walking and cycling is also relatively cheap and space-efficient.
No neighbourhoods are being demolished to build bike lanes but lots of neighbourhoods
were erased from existence to build highways in mid century North America.
Based on these examples, the concept of induced demand on its own is not actually a slam-dunk
argument against expanding roads or building new highways. The problem with new car infrastructure
isn’t only that it’ll induce more trips, because the same can be said for a new subway,
bike route, or walking path, assuming they’re built to conveniently connect people to places
they need to go. Instead, the problem with new car infrastructure is related to the specific
downsides of cars, from the negative externalities related to noise, safety, and pollution to
the basic geometric fact that they’re an inefficient way to move a lot of people in
a city, which makes them especially prone to congestion, even when we dedicate a lot
of space to them.
Rather than thinking about induced demand as a special property of cars, we should think
of induced demand as the basic fact that transportation demand and usage patterns aren’t fixed.
People make short-term choices about how to get around and long-term choices about where
to live based on the infrastructure available to them. If you build wider roads, people
will drive more and they’ll plan their lives around driving. If you build fast and convenient
transit lines, people will take transit more and plan their lives around transit. If you
build a safe and comfortable bike network that connects people to places they need to
go, people will bike more and so on. The question isn’t really which types of transportation
infrastructure induce demand, because they all do; the question is which type of demand
you want to induce.
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