New Scientist - USA (2019-12-21)

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STREET layouts have a major
influence on people’s decisions
to drive or travel by foot,
according to the first global
analysis of street connectivity.
The findings could be used by
urban planners to design cities
with lower climate impacts.
Christopher Barrington-Leigh
at McGill University in Canada
and Adam Millard-Ball at the
University of California, Santa
Cruz, assessed the connectivity
of street networks in different
cities. They tallied the numbers
of intersections, streets
radiating off each intersection,
dead ends and loops, and
measured the straightness
of the routes between each
intersection. This was done
for all 46 million kilometres
of the world’s mapped roads.
The results show that cities
with grid-like street patterns,
such as Buenos Aires, Osaka ,
Alexandria and Taipei, have the
best connectivity. Old European
cities like Paris and Vienna
also score well – despite their
intersections tending to be
irregularly spaced and three-
way instead of four-way –
because they still form highly
connected networks.
At the other end of the
spectrum, cities that favour
cul-de-sacs and crescents – like
Bangkok, Raleigh in North
Carolina and Manchester
in the UK – are the least
well-connected because their
curvy, dead-end streets create
disjointed suburban islands
(PLoS One, doi.org/dgv8).
Since well-connected streets
make it easier to walk, cycle and
access public transport, cities
with such streets tend to have
lower rates of car ownership
and higher proportions of
people walking to work.
This suggests that how we

design urban spaces now will
have long-lasting impacts
on the use of cars and their
greenhouse gas emissions,
because once streets are laid
down, they are essentially
“locked in”, says Barrington-
Leigh. “The choice about
street connectivity in new
developments is one of
the largest climate-relevant
investments that humankind
is making and yet it’s been
grossly overlooked,” he says.

In the US, prioritising
street connectivity in new
developments would reduce car
emissions in 2050 by 9 per cent,
says Barrington-Leigh. The
impacts are likely to be even
bigger in places like China and
India, where entire cities are
being constructed from scratch,
he says. Overall, the amount of
urban space in the world is set to

triple between 2000 and 2030.
Well-connected streets may
also cut carbon emissions
by making it easier for people
to share resources, says
Barrington-Leigh. “In contrast,
if you live in cul-de-sac hell in
the suburbs, it’s harder to get
anywhere, so you might have
a swimming pool in your
backyard instead of going to
the local public pool, or a home
theatre in your basement
instead of going to the cinema,
or a large freezer because you
can’t go shopping as often.”
Street grids were popularised
in Ancient Rome and China
because they enabled efficient
transport of people and goods.
Cul-de-sacs caught on in the
20th century in the US and UK
when cars enabled people to
spread out and planners tried
to create safe havens for kids
to “play street hockey or run
over to their neighbours’ ”, says
Barrington-Leigh. But research
shows that people are actually
more likely to be run over and
killed in cul-de-sacs than on grid-
like streets, possibly because
pedestrians are less cautious
and the curvy roads make them
less visible to drivers. ❚

GOOGLE has completed the biggest
quantum chemistry simulation
ever, modelling the behaviour of
a long chain of hydrogen atoms.
Quantum computers use
quantum bits, or qubits, that exploit
the properties of quantum physics
to perform calculations. “The atoms
are quantum, the computer is
quantum, we’re using quantum to
simulate quantum,” says Linghua
Zhu at Virginia Tech, who wasn’t
involved in the work.
Until now, the largest molecule
simulated with a quantum
computer was beryllium hydride,
which is one beryllium atom and
two hydrogen atoms, says Zhu.
At the Q2B conference in
California on 10 December, Google’s
head of quantum algorithms, Ryan
Babbush, announced his team had
simulated a much larger molecule:
a chain of 12 hydrogen atoms.

“That’s a result that we’re pretty
excited about, because this is more
than double the number of qubits
and the number of electrons as
any prior quantum chemistry
simulation, and it had the same
level of accuracy,” Babbush said.
The simulation of large molecules
is an expected near-future use
of quantum computers. Such
molecules are hard to understand
using classical computers – the
largest we can simulate accurately
is pentacene, which contains
22 carbon atoms and 14 hydrogen
atoms, says Jamie Garcia at IBM.
The team used the same
Sycamore quantum computer
recently used to achieve quantum
supremacy – a calculation not
possible on an ordinary computer –
but didn’t use all of the qubits for
the simulation, said Babbush. ❚

10 | New Scientist | 21/28 December 2019


“ The atoms are quantum,
the computer is quantum,
we’re using quantum to
simulate quantum”

Urban design

Alice Klein

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News


Paris scores highly for
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200%
The increase in urban space set to
happen between 2000 and 2030
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