YOU WALK TO the elevator,
you hit the Up button, and
you wait. And you wait. And...
you wait.
There really is a good reason
the lift keeps rising away from
you. And it’s not karmic pay-
back for never returning your
neighbor’s pruning shears.
With every press of a but-
ton, we’ve given our elevators
a doozy of a computational
challenge. The elevator system
must decide which car to send
for you, and when. It must
choose whether to go up from
the fifth floor to collect those
people on the seventh before
coming down to the lobby
to answer your call. It must
consider who’s been waiting
longer, and which of the many
paths is the most efficient and
least painful for everybody.
Elevator traffic is an elaborate,
delicate dance, and once you
see the steps, you can’t help
but tip your hat to the engi-
neers who choreograph it all.
Elevator routing wasn’t
always so complex. The ear-
liest electric elevators were
controlled by human opera-tors. An attendant standing
inside would drive the lift up
and down with a throttle of
sorts, stopping wherever he
or a dispatcher saw a waiting
passenger. But humans proved
to be clumsy, expensive, and
prone to strikes. By the 1950s,
electrical switches took over.
For elevators to direct
themselves, engineers had to
spell out rules for when to go
where. The simplest method
was for the elevators to shut-
tle back and forth between
predefined floors at scheduled
intervals. It was like taking a
bus; you waited for the 3:10 car
up to the 10th f loor, and then
found your way from there.
This, of course, was grossly
inefficient. During busy times
of day, the elevator cabs would
waste everyone’s time sitting
at a floor until their scheduled
departure. During off-peak
hours they would make point-
less empty trips.
By 1965, lift engineers set-
tled on the model we all know
and love and sometimes hate:
Passengers push buttons to
call elevators, and the eleva-
tors respond to these requests.
But here it gets tricky. As
requests to different parts of a
building pile up, how does an
elevator decide where to go?The Pain Index
WHAT IS THE perfect ele-
vator system? Does it serve
the person who’s been wait-
ing the longest? Or always
go to the closest call? Where
does it make the compromise
between speedy service and
keeping energy usage down?
Elevator engineers grapple
with all these questions, and
none of them are as simple as
they seem. Clearly, an eleva-
tor should try to reduce travel
time. But how should it prior-
itize your time? If you wait a
minute instead of 20 seconds
for a car to come, is that three
times as bad, or perhaps six or
even nine times worse?
Even the most basic of these
goals isn’t a given. Sometimes,
it’s actually better to make a
passenger’s ride longer. Imag-
ine two scenarios, one in which
your elevator takes 10 seconds
to arrive and then one min-
ute to reach your destination
(Fig 1), and another in which
each portion takes 30 seconds
(Fig 2). Many people find wait-
ing so painful that they’d
prefer the first option, even
though they’d reach their
destination 10 seconds later.
Accordingly, some elevators
optimize not for time, but for66 September 2019