By Angie Schmitt, September 28, 2015
Even if you never set foot on a bus or a train, chances are transit
is saving you time and money. The most obvious reason is that transit
keeps cars off the road, but the full explanation is both less intuitive
and more profound: Transit shrinks distances between destinations,
putting everything within closer reach.
A new study published by the Transportation Research Board quantifies the spatial impact of transit in new ways [PDF]. Without transit, the researchers found, American cities would take up 37 percent more space.
The research team from New York, San Francisco, and Salt Lake City
modeled not just how many driving miles are directly averted by people
riding transit, but how the availability of transit affects the way we
By allowing urban areas to be built more compactly, the “land use
effect” of transit reduces driving much more than the substitution of
car trips with transit trips. Total miles driven in American cities
would be 8 percent higher without the land use effect of transit, the
researchers concluded, compared to 2 percent higher if you forced
everyone who rides transit to drive.
On average, the study found, the “land use effect” of transit is four
times greater than the “ridership effect,” or the substitution of car
trips with transit trips. But the land use effect of transit varies a
great deal across urban areas, the study found. In places like
Greenville, South Carolina, it’s responsible for reducing driving 3
percent. In San Francisco and New York City, it’s 18 and 19 percent,
The authors suggest their model can help assess the effect of transit
investments on travel behavior with greater sophistication. For
example, adding a rail station to a neighborhood without one increased
the density of jobs and residences by 9 percent within a one-mile
radius, the study found. That would reduce driving about 2 percent for
all the households across the area.
In addition to new infrastructure, increasing the frequency of
transit service reduces traffic too. The researchers estimate that a 1
percent increase in transit frequency across a region would be expected
to bring about a 0.045 percent decrease in miles driven. And a 1 percent
increase in route density — a measure of how many transit lines service
a given area — would be expected to produce a 0.047 percent reduction
The report includes a “Land Use Benefit Calculator” [XLS] to help determine the total environmental benefits of transit projects.