February 14, 2025 • 39 mins
Leah Washington and her new boyfriend Joe Pugh are on their first day out together. They're at Alton Towers theme park, where they've chosen to ride the "Smiler" rollercoaster: a terrifying tangle of track that loops and swoops through a world-record 14 inversions. Leah and Joe are seated right at the front of the train and, as they reach the highest point of the ride, they steel themselves for the drop. But then, quite suddenly, the ride stops.
Down on the ground, the computer system that controls the rollercoaster is warning that another carriage is out on the track, right in the path of Leah and Joe's train. The engineers are certain the computer is wrong...
Algorithms are often faster and cleverer than humans, and they can help us avoid accidents. But computers can make mistakes. When should we trust our own heads, instead of the machine?
For a full list of sources, see the show notes at timharford.com.
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Episode Transcript
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Speaker 1 (00:15):
Pushkin. You might have noticed that things are a little
different on Cautionary Tales this year. In twenty twenty four,
we brought you a new episode every fortnight, But this
year we are doubling our output. New stories of heart
(00:35):
thumping peril, mind blowing mistakes, and jaw dropping scandal will
now be delivered straight to your ears every week. Here's
on for you right now. Not so very long ago,
I took my son with me to an amusement park
to celebrate his twelfth birthday. He's obsessed with roller coasters,
(01:00):
although usually he just experiences them through the medium of YouTube.
It's one thing to see the footage someone took from
the front seat. To actually be there, it's a different thing.
Riding a roller coaster is a strange kind of fun.
The volunteering to be terrified for the sake of entertainment,
(01:21):
and the roller coaster we'd come to ride certainly leans
into that idea. It's called the smiler. The conceit behind
the smiler is that people who aren't smiling enough will
have their lack of a smile corrected by a strange
Awwellian institution called the Ministry of Joy. The smiler's logo
(01:48):
is a ghastly grin connecting two hypnotically spiraling eyes. And
so we went to Alton Towers in England to ride
the Smiler. The ride doesn't soar in the high curves
of a classic roller coaster. Instead, it's an imp penetrable
(02:09):
looking spaghetti tangle of black and yellow, with the knots
and curls of the roller coaster track intersecting with a
huge five legged structure, some kind of diabolical machine, decorated
with a wraparound screen displaying dystopian messages and unsettling graphics.
(02:30):
It's hard to figure out what goes. Where As we
waited in line underneath the belly of the thing, we
gazed up at the tortuous coils of the ride through
black netting that added to the vibe of a correctional facility,
but was really to protect us from wallets and phones
(02:51):
falling out of the pockets of the riders in the
trains above. And those trains looped and swooped around and
around above us, two together on different parts of the track,
diving and rolling around each other like mating birds, but
there was no bird song. The sound was deafening. A
(03:14):
night Mayish theme tune like a nursery rhyme from hell,
the steel roaring as the trains rushed over ahead, so
close that it seemed we could touch them. And of
course there were the screams of the riders. They screamed
and they screamed, and they screamed as the ride turned
(03:35):
them upside down, over and over and over again, a
world record fourteen in versions. And as we gazed upon
the sheer awfulness of the thing, my son turned to
me and said, Dad, I'm not sure I want to
go on this ride. And he told me something else, Dad,
(04:00):
He said, you should do a cautionary tale about the Smiler.
I'm Tim Harford and this is that cautionary tale. Seventeen
(04:38):
year old Leah Washington's first big day out with her
new boyfriend Joe Pugh was a trip to Alton Towers,
the theme park which is home to the Smiler. Leah
suggested going on the Sonic Spinball coaster. Joe wasn't convinced.
Why line up for hours when the ride isn't even
(04:59):
that good. It was June twenty fifteen. The Smiler itself
was only a couple of years old, but had quickly
become an iconic roller coaster ride. So Joe suggested the
Smiler instead. Now that is a roller coaster worth waiting for.
(05:19):
Lea was nervous. She'd never been on the Smiler and
it looks terrifying, but she agreed, and so they patiently
lined up, edging forward to enjoy their turn. It was
a windy day that they were sheltered from the worst
of the gusts as the roller coaster cars swooped and
(05:42):
screamed above them. The line edged forward, and the minutes
ticked past half past eleven noon half past twelve. By
one o'clock, Lea and Joe could see they were close
to getting onto the Smiler itself, with the diabolical nursery
tune playing and the lines surrounded by unsettling images of compliant,
(06:07):
grinning faces. So we queued for a good hour and
a half. Leah later recalled, and then we got to
the front and they put us on the front carriage.
Those words are from a television interview she gave just
(06:28):
a couple of months later. In the interview, Leah is
serious but calm. She's a remarkably self possessed young woman.
She's also missing her left leg Smiler trains are short
and wide, four rows, four seats in each one. Leah
(06:51):
and Joe were in the front row. Me and Joe
got excited being at the front. The front row is
much sought after. You get the best views, the most excitement,
the most direct exposure to everything the Smiler has to offer.
But then we sat down, put the safety bars down.
(07:14):
Then we were sat for five ten minutes, and then
we had to get back off because there were technical difficulties.
That was a bit frustrating. That was Leah warried. Now,
not really, because all rides break down at some point,
but you didn't expect anything bad to happen. So Leah
and Joe stood at the front of the line and
(07:36):
waited to get back on. The Alton Towers Theme Park
has dedicated teams of engineers. The park wants to keep
the rides running smoothly and safely, with a minimum of interruptions.
The need for safety is obvious enough, but so is
the need to minimize downtime. There can easily be two
(07:59):
thousand people in the line to ride the Smiler, and
Olden Towers doesn't want people saying I queued for two
hours for the Smiler and I never even got a ride.
The show must go on, which might explain why. On
the second of June twenty fifteen, the Smiler was operating
despite the windy weather. When the roller coaster registered a
(08:23):
fault and Lear and the other riders were taken off
the train and asked to wait. Two teams of engineers
hurried to the scene. The line was only getting longer,
and two trains were sitting out on the twisting roller
coaster track full of increasingly anxious passengers, wondering what the
problem was and whether it was anything to do with
(08:45):
the gusts of wind. The first team of engineers started
to diagnose the problem, which was nothing serious, and also
decided to add a fifth train to the track while
the ride was suspended. That meant that once the roller
coaster was operating again, it would accommodate a few more passengers.
(09:08):
Seven minutes after the fault appeared, the engineers were able
to bring a train of relieved guests back to the station,
where they got off and wandered away to enjoy the
rest of the theme park. A minute after that, another train,
the last occupied train, arrived and the passengers disembarked. Now
(09:30):
all four empty trains were safely inside the roller coaster
station and the fifth train was added. This took another
five minutes, while Lea and Joe and two thousand people
behind them waited and wondered what was happening. As the
fifth train was being added, the roller coaster's electronic system
(09:52):
flashed up seven more fault codes. Each of them was minor,
but each of them needed to be acknowledged, checked, and
then cleared. By now Lea and Joe and the others
had been waiting nearly twenty minutes to get back on
the ride, and the engineers sent an empty train around
the roller coaster just to check that everything was working properly.
(10:16):
It wasn't. The train went out, but it didn't come back.
Like many roller coasters, the Smiler operates on a combination
of gravity and momentum. The trains don't have engines in them. Instead,
each train is pulled up a long slope by a
(10:37):
chain lift, then released to run the course of humps
and loops until finally coasting back into the station. Roller
coaster the clues in the name, but because the Smiler
has that world record tangle of fourteen inversions, because it
stays fairly low level with the tree tops. One chain
(11:02):
lift hill won't do the job, and so halfway round
the ride comes one of the defining moments of the
Smiler experience. There's a second chain lift, and instead of
being pulled up a long slope, the chain runs vertically. Suddenly,
your seat tips back so far that your feet are
(11:24):
higher than your head, and you stared directly up at
the sky, being hoisted higher and higher up a vertical track.
But in June twenty fifteen, that test train didn't come
back because it didn't quite reach the second lift, the
vertical one. It coasted to a halt just short of
(11:45):
where the lift chain would engage. Why unclear. The fact
that it had no passengers meant that it would have
been a little lighter and carried less momentum. Then there
were those gusts of wind. Whatever the reason, it wasn't
quite close enough for the lift chain to finish the journey.
(12:08):
They were puzzling over this problem. The first team of
engineers were joined by the second team, a pair of
electrical engineers. They all huddled together for a brief conference.
But one thing that doesn't seem to have been mentioned
was that a fifth train had been added to the
track anyway, it wasn't hard to figure out what was wrong.
(12:30):
The ride had sensors which showed that a section of
track was occupied by a train, that train they had
just sent out as a test. The engineers could also
look at CCTV images and see that train stopped just
shy of the chain lift. Three of the engineers made
their way down to the track's halfway point at the
(12:50):
bottom of the vertical lift hill. They put their shoulders
to the heavy train and started to push until the
train clicked into the chainlift and up it went, straight
up the vertical rails before coasting around the remaining loops
and corkscrews and back to the station. Lear and Joe
(13:15):
and the rest of the sixteen passengers had been waiting
for half an hour since being put onto a train
and then taken off again, with no knowledge of what
the Smiler engineers had been up to. But at long
last they were nearly ready to get the passengers back
onto the ride. Ahead of the train, Lea and the
others were board, there was another empty train. The engineers
(13:38):
sent it off around the circuit, and Leah and Joe
stepped forward to be strapped into the front row of
the smiler, ready for the ride of a lifetime. Cautionary
tales will be back after the break. Roller Coasters are
(14:10):
safer than lots of things people do for fun. They're
certainly a lot safer than riding in a car, if
you believe the numbers from the International Association of Amusement
Parks and Attractions. And why wouldn't you believe them. My
son and I ran about the same risk in driving
one hundred miles to Alton Towers and one hundred miles
back again that we would have faced if we'd ridden
(14:33):
six hundred roller coasters. So sure, roller coasters are pretty safe,
but they don't feel safe. They're not meant to feel safe.
And if you pushed a nervous roller coaster rider to
think about what might actually be dangerous about a roller coaster,
they'd probably tell you two things. First, I might fall
(14:56):
out of my seat, and second, the entire train might
fall off the track. That makes intuitive sense. The tracks
look narrow and exposed, and the trains go fast, take
the perilous curves, and for goodness sake, they go upside down.
(15:18):
But that's not really the problem. Here are the words
of Stephen Flanagan, an expert on roller coaster safety. Although
the public perception of the hazards associated with roller coasters
may be focused in the danger of a train parting
company with the track, in reality, the bigger and more
(15:40):
difficult to resolve issue has always been the hazard of
trains colliding on the track. When you get on a
roller coaster like the Smiler, you really don't need to
worry that the train is going to fall off one
of those gravity defying loops. You really don't need to
worry about anything. But if you did feel like worrying
(16:01):
about something, I don't suggest worrying that your train might
smash into another train. As Leah and Joe's train moved off,
a diabolic voice boomed over the speakers join us. Then
the train plunged into a dark section of track, shocking
(16:24):
the passengers by flipping upside down and right side up again,
before emerging into daylight and slowly clanking up the first
chain lift higher and higher and higher. Ahead of them
on the Smiler tracks was an empty train, and none
of the passengers realized that empty train was slowing As
(16:48):
it approached the top of a loop, the wind was
still gusting the empty trains were still a little on
the light side. The train came to a stop, then
slowly rolled back to settle in a dip. Once Lea
(17:09):
and Joe's train was released from the lift hill, it
would take twenty six seconds to loop over to that dip.
They were less than half a minute from disaster, except
their train just stopped right at the top of the lift.
(17:32):
We got to the first lift hill and it got
stuck at the top. So obviously me and Joe were discussing, Oh,
this isn't right. Why had the train stopped? Simple. Although
the Smiler engineers hadn't realized there was a problem, the
smiler's monitoring system knew perfectly well that there was a
(17:53):
train stalled out on the track. Automatically, it held Lear
and Joe's train at the top of the slope, the
highest point of the Smiler, waiting for the blockage to
be cleared. Catastrophe had been averted by an automated system.
So now what. Paul Meal was an expert who became
(18:20):
fascinated by the mistakes experts make. That was partly an
intellectual interest of his, but it was also very personal.
In nineteen thirty five, when Meal was fifteen years old,
his mother had gone to her doctor with some symptoms
which the doctor attributed to problems with her inner ear.
(18:42):
In fact, the cause was a brain tumor. The doctor
could have diagnosed the tumor by asking some basic questions
and performing some basic checks, but didn't. A year later,
the tumor had grown and spread. It was finally diagnosed
by a different doctor, and Paul Meel's mother died soon after.
(19:09):
Yiel later wrote this episode of gross medical bungling permanently
immunized me from the childlike faith in physicians' omniscience that
one finds among most persons. Meil became an academic researcher
and a clinical psychologist, and he never lost his curiosity
(19:29):
about the fallibility of human experts. In his early thirties,
he published a book that was to become famous, Clinical
versus Statistical Prediction. The book asked a question, should you
trust the judgment of an expert doctor, or is it
better to take a short list of symptoms, feed them
(19:51):
into a simple flowchart, and do whatever the flowchart says.
Such a flowchart might have saved his mother. This was
nineteen fifty four, so we're not talking about pitting human
experts against sophisticated artificial intelligence, talking about pitting human experts
against a crude formulaic process, but Meil found that the
(20:15):
crude formulaic process often beat the humans. Three years later,
he published an academic article with a punchy follow up question,
when shall we use our heads instead of the formula?
It's a question that's much more pressing today than it
was when he asked it in the nineteen fifties. Computers
(20:38):
tell managers who to hire and who to fire. They
tell radiologists whether a shadow on a scan is cancer
or not. They advised judges on who should be released
on bail and who should be detained, and they helped
social services prioritize calls about children at risk. The computers
(21:00):
they're everywhere, So when should we trust them instead of
our own judgment? Paul Meill wrote, I find the two
extreme answers to this question, namely always and never equally unacceptable.
(21:20):
At Alton Towers, the engineers were about to override the computer.
That's not always the wrong choice. Sometimes the human is
right and the computer is wrong. Unfortunately, this wasn't one
of those times. Lear and Joe and the others had
(21:43):
waited an hour and a half to get onto the Smiler,
then another half an hour while the technical fault was resolved.
Now they were perched precariously at the highest point of
the ride. A long minute ticked by. Then another excitement
gave way to waves of anxiety, boredom and annoyance. They
(22:08):
said over the tannel, we're experiencing technical difficulties. Bear with us.
They really weren't supposed to be poised and ready to
roll for such a long time. What was going on?
The Smilers engineers were trying to figure that out too.
The Smiler has a simple computer system designed to prevent
(22:30):
trains from smashing into each other at certain points on
the track. There are proximity switches which register when a
train enters a block of track and when it leaves
it again. If the system thinks a train is on
a particular block of track, it will automatically prevent the
train behind it from being released into that block. That's
(22:51):
what was happening here. Of course, the empty train hadn't
left the block. In fact, it was still gently penduluming
backwards and forwards, settling in the valley ahead of Leah
and Joe's train. So the smiler's computer prevented the occupied
train behind it from moving forward, leaving Lear and the
other passengers waiting and waiting at the top of the
(23:13):
lift hill. Meanwhile, on the ride's control panel, the zone
stop alarm was activated, notifying the engineers that one train
was being held because the train ahead hadn't left its
block of track. Hmm, well, the occupied block was the
one including the vertical chain lift, where the previous train
(23:37):
had stalled. It was natural to assume that if a
second train had stalled, that would be where it was.
One of the engineers went out to have a look,
but there was no train at the bottom of the
vertical lift. As far as he was concerned, then the
smiler's collision prevention system was just malfunctioning. Probably it hadn't
(23:57):
reset itself from the train which stalled fifteen minutes before.
That might seem like a big assumption, but this was
one of the engineers who arrived after the fifth train
had been added. He didn't know about it, or if
someone had mentioned it, it had slipped his mind. So
he thought there were four trains, and he knew where
(24:19):
four trains were LEAs at the top of the lift hill.
And three in the station. If it turned to look
over his shoulder and looked carefully, he might have seen
the fifth train valid at the other end of the
ride through the tangle of roller coaster track. But he
didn't look, or if he did, he didn't see. At
(24:46):
the bottom of the vertical chain lift was a control
panel to reset the block sensors. The engineer performed that reset,
telling the computer to forget its erroneous belief that there
was a train stuck on the track. Unfortunately, it wasn't
the computer which had an erroneous belief, it was the humans.
(25:09):
Retails will return in a moment. Leah's train has now
(25:30):
been held at the top of the chain hill for
six long minutes. There's still a chance the smiler system
won't release the train without an explicit override from the humans.
So now the critical decision is with the engineers back
in the station. But they don't explicitly discuss whether there
(25:53):
are four trains running or five. They don't send anyone
out to visually confirm that the whole track is clear, nor,
it seems, do they look carefully at the CCTV screens.
The smiler does have lots of CCTV screens very on
brand for a dystopian roller coaster experience, but the ride
(26:15):
is such a tangle of track that it's hard to
see clearly. Right now, the valid train is partly obscured,
and the engineers don't look carefully enough. Maybe they don't
look at all. They're all just assuming the computer must
be wrong, so they decide to override it. The smiler's
(26:39):
system still won't release one train into a section where
it knows another train is stationary, but it doesn't know
about the stationary train anymore because its memory of the
obstacle has been deliberately reset. Leah recalled it just set
back off without any warning. At last, Leah's train starts
(27:01):
to roll down from the lift hill and accelerate into
loop after loop after loop. Twenty six seconds away is
a stationary train. They were fine, We were going around
loops and everything. We just came round this corner and
I saw this car and I'm like, oh my god,
this isn't good. Paul Neil, the clinical psychologists who pose
(27:29):
the question about when we should follow the formula and
when we should use our heads, didn't have an easy answer,
but he did suggest a rule of thumb. If you,
as the human, know something that the algorithm doesn't know
that the algorithm can't know, that's a reason to think
about overruling the computer. But when investigators later looked into
(27:52):
the causes of the Smiler accident, they noticed that the
engineers had a patchy knowledge about how the ride's false
alarms actually worked. This meant they were often puzzled when
the alarms went off. They'd do some safety checks and
reset the system, but exactly why the alarm had sounded
was sometimes a mystery, so the engineers regarded the alarm
(28:16):
system as capricious and unreliable. If the engineers had been
better trained, they'd have understood the logic of the alarm
system and would have been able to think clearly about
when to override it and when not to. Paul Meel
warned us, if you're going to overrule the computer, you'd
better have a reason to think you know something that
(28:39):
it doesn't. But to do that, you need to have
a firm understanding of how the computer works. The Smilers
engineers didn't. As Leah and Joe and the others looped
upside down and saw the empty train ahead of them,
(29:02):
they had only a moment to brace for the shattering impact.
The train wasn't going especially far. You'd probably walk away
from a car crash at that speed, but unlike cars,
roller coasters don't have crumple zones. Leah was right at
the front with no protection, and as metal smashed into metal,
(29:26):
the two trains locked together and rocked backwards and forwards
twelve times, with the legs of the front row passengers
crushed between them. The next thing I remember was the
screaming and the blood. There was so much of it.
The metal of the safety bars had folded into our bodies,
(29:46):
said Leah later. All I wanted was to hold Joe's hand,
but when I looked down at it, I could see
there was no way I could. His little finger was
hanging off his left hand, and the middle finger on
the other was broken. All the front row passengers had
horrendous leg wounds. Both Joe's kneecaps were shattered. Leah's injuries
(30:10):
were even worse. I started getting pain. I couldn't feel
my toes, Leah remembered, and I started to look at
my legs. My left leg was all pushed up. They
were snarled up in a tangle of metal at a
forty five degree angle twenty feet above the ground, and
(30:30):
the Smile operators were apparently still confused because it took
seventeen minutes before anybody called for the emergency services. Up
on the coaster, Leah nearly died from blood loss, but
an air ambulance flew in blood for an emergency transfusion
(30:52):
twenty feet above ground. Later, she stopped breathing, but the
medics reaching her from precarious ladders and hoists managed to
resuscitate her. Once she was released from the train. Hours later,
Leah was flown to hospital, slipping in and out of consciousness.
(31:15):
She woke up twenty four hours later. I realized two
things were missing, my boyfriend and my entire left leg.
The doctors told her sorrowfully that they had faced a
choice her leg or her life, so they took the leg.
(31:37):
The woman sitting next to her on the roller coaster,
Vicki Balch, also had her leg amputated. Joe Pew was
elsewhere in the hospital. He and several other passengers also
suffered severe injuries, all because the Smilers engineers hadn't double
checked when the computer told them there was a train
(32:00):
out on the track. Given what she's been through, Leah
Washington is doing incredibly well. She's something of an Instagram influencer,
posting videos of herself learning to walk, learning to run,
(32:20):
learning to ski, or selfies of her looking fabulous in
a bikini. They look like a lot of Instagram self is,
in fact, except that she's looking fabulous in a bikini
with an artificial leg. Joe never regained the use of
one finger, but his knees were covered eventually, and the
pair are still together. Leah is now missus Washington Pew
(32:46):
after she and Joe tied the knot in twenty twenty four.
But that happy ever after ending so nearly didn't happen.
At that moment when Leah was unconscious and trapped in
crumpled metal twenty feet above the ground, when a team
of paramedics on hoists were trying to get her breathing
(33:08):
started again, it was touch and go. The medics later
told her if the weather had been any colder, she
wouldn't have made it. In a world where we're surrounded
by automatic systems, statistical formulas, algorithms, and computers, we're going
(33:32):
to have to get better at deciding when to trust
them and when to overrule them. There will never be
a hard and fast rule for when to do that,
But if you're thinking of ignoring the computer, it's wise
to have a logical reason to think you know better.
The Smiler engineers didn't. When investigators picked over the disaster,
(33:57):
they concluded that there was nothing wrong with the roller
coaster itself, nor with the automatic alarm system. It was modern,
well designed, and functioned exactly as intended. The problems emerged
from the working practices of the engineering team. It was
partly the fact that the engineers didn't fully understand their
(34:20):
own safety alarms, and it was also that nobody in
particular was in charge of the process of resetting the
Smiler after the very first fault alarm. There were the
ride operators, the first engineering team, and the second engineering team,
all trying to make decisions. There were loose, informal conversations
(34:41):
in which nobody had a full overview of what was
going on. There was no written process for working systematically
through a checklist before resetting the ride, and there was
no formal assignment of responsibility as to who had the
authority to override the automatic system and under what circumstances.
(35:06):
This lack of a formal process was critical. Without it,
the engineering team could just assume that the track was clear.
With a formal process, someone would have been clearly responsible
for checking. Fifteen months after the accident, a judge called
(35:26):
it an obvious shambles involving lack of communication and double checking,
which could and should easily have been avoided by a
written system of working, including a single overall supervisor. Automatic
(35:47):
systems can help us avoid accidents and make better decisions,
but Paul Meele's question will never go away. When should
we use our heads instead? Maybe the trick is in
the question. It's okay to use your head if you
really are thinking. Very often we aren't. As I waited
(36:14):
in line for the smiler with my twelve year old
son snaking through the maze of concrete under the black netting,
it was hard not to think about Leah Washington and
her accident. Maybe all this was going through my son's
mind too. As the que edged forward and his knees buckled.
(36:38):
I'm not sure I want to do this. Come on,
I said, he came all this way. It'll be sad
if you don't do it. So we did it, and
it was amazing. We had a great time, and he
was so proud of himself. But there was one moment
(37:06):
when we got to the front of the line, when
we were offered a place on the front row. We
looked at each other and shook our heads. No, not
the front row, thanks, We'll sit a little bit further back.
(37:43):
Cautionary Tales is written by me Tim Harford, with Andrew Wright,
Alice Fines and Ryan Dilly. It's produced by Alice Fines
and Marilyn Rust. The sound design and original music are
the work of Pascal Wise. Additional sound design is by
Carlos San Juan at Brain Audio Bend and daff Haffrey
edited the scripts. The show features the voice talents of
(38:06):
Melanie Guttridge, Stella Harford, a Hemborough, Sarah jupp As, Sam
and Roe, Jamal Westman and Rufus Wright. The show also
wouldn't have been possible without the work of Jacob Weisberg,
Greta Cohene, Sarah Nix, Eric Sandler, Carrie Brody, Christina Sullivan,
Kira Posey and Owen Miller. Cautionary Tales is a production
(38:28):
of Pushkin Industries. It's recorded at Wardoor Studios in London
by tom Berry. If you like the show, please remember
to share, rate, and review. It really makes a difference
to us and if you want to hear the show,
add free sign up to Pushkin Plus on the show
page on Apple Podcasts or at pushkin dot fm Slash
(38:48):
plus eight
Pushkin. You might have noticed that things are a little
different on Cautionary Tales this year. In twenty twenty four,
we brought you a new episode every fortnight, But this
year we are doubling our output. New stories of heart
(00:35):
thumping peril, mind blowing mistakes, and jaw dropping scandal will
now be delivered straight to your ears every week. Here's
on for you right now. Not so very long ago,
I took my son with me to an amusement park
to celebrate his twelfth birthday. He's obsessed with roller coasters,
(01:00):
although usually he just experiences them through the medium of YouTube.
It's one thing to see the footage someone took from
the front seat. To actually be there, it's a different thing.
Riding a roller coaster is a strange kind of fun.
The volunteering to be terrified for the sake of entertainment,
(01:21):
and the roller coaster we'd come to ride certainly leans
into that idea. It's called the smiler. The conceit behind
the smiler is that people who aren't smiling enough will
have their lack of a smile corrected by a strange
Awwellian institution called the Ministry of Joy. The smiler's logo
(01:48):
is a ghastly grin connecting two hypnotically spiraling eyes. And
so we went to Alton Towers in England to ride
the Smiler. The ride doesn't soar in the high curves
of a classic roller coaster. Instead, it's an imp penetrable
(02:09):
looking spaghetti tangle of black and yellow, with the knots
and curls of the roller coaster track intersecting with a
huge five legged structure, some kind of diabolical machine, decorated
with a wraparound screen displaying dystopian messages and unsettling graphics.
(02:30):
It's hard to figure out what goes. Where As we
waited in line underneath the belly of the thing, we
gazed up at the tortuous coils of the ride through
black netting that added to the vibe of a correctional facility,
but was really to protect us from wallets and phones
(02:51):
falling out of the pockets of the riders in the
trains above. And those trains looped and swooped around and
around above us, two together on different parts of the track,
diving and rolling around each other like mating birds, but
there was no bird song. The sound was deafening. A
(03:14):
night Mayish theme tune like a nursery rhyme from hell,
the steel roaring as the trains rushed over ahead, so
close that it seemed we could touch them. And of
course there were the screams of the riders. They screamed
and they screamed, and they screamed as the ride turned
(03:35):
them upside down, over and over and over again, a
world record fourteen in versions. And as we gazed upon
the sheer awfulness of the thing, my son turned to
me and said, Dad, I'm not sure I want to
go on this ride. And he told me something else, Dad,
(04:00):
He said, you should do a cautionary tale about the Smiler.
I'm Tim Harford and this is that cautionary tale. Seventeen
(04:38):
year old Leah Washington's first big day out with her
new boyfriend Joe Pugh was a trip to Alton Towers,
the theme park which is home to the Smiler. Leah
suggested going on the Sonic Spinball coaster. Joe wasn't convinced.
Why line up for hours when the ride isn't even
(04:59):
that good. It was June twenty fifteen. The Smiler itself
was only a couple of years old, but had quickly
become an iconic roller coaster ride. So Joe suggested the
Smiler instead. Now that is a roller coaster worth waiting for.
(05:19):
Lea was nervous. She'd never been on the Smiler and
it looks terrifying, but she agreed, and so they patiently
lined up, edging forward to enjoy their turn. It was
a windy day that they were sheltered from the worst
of the gusts as the roller coaster cars swooped and
(05:42):
screamed above them. The line edged forward, and the minutes
ticked past half past eleven noon half past twelve. By
one o'clock, Lea and Joe could see they were close
to getting onto the Smiler itself, with the diabolical nursery
tune playing and the lines surrounded by unsettling images of compliant,
(06:07):
grinning faces. So we queued for a good hour and
a half. Leah later recalled, and then we got to
the front and they put us on the front carriage.
Those words are from a television interview she gave just
(06:28):
a couple of months later. In the interview, Leah is
serious but calm. She's a remarkably self possessed young woman.
She's also missing her left leg Smiler trains are short
and wide, four rows, four seats in each one. Leah
(06:51):
and Joe were in the front row. Me and Joe
got excited being at the front. The front row is
much sought after. You get the best views, the most excitement,
the most direct exposure to everything the Smiler has to offer.
But then we sat down, put the safety bars down.
(07:14):
Then we were sat for five ten minutes, and then
we had to get back off because there were technical difficulties.
That was a bit frustrating. That was Leah warried. Now,
not really, because all rides break down at some point,
but you didn't expect anything bad to happen. So Leah
and Joe stood at the front of the line and
(07:36):
waited to get back on. The Alton Towers Theme Park
has dedicated teams of engineers. The park wants to keep
the rides running smoothly and safely, with a minimum of interruptions.
The need for safety is obvious enough, but so is
the need to minimize downtime. There can easily be two
(07:59):
thousand people in the line to ride the Smiler, and
Olden Towers doesn't want people saying I queued for two
hours for the Smiler and I never even got a ride.
The show must go on, which might explain why. On
the second of June twenty fifteen, the Smiler was operating
despite the windy weather. When the roller coaster registered a
(08:23):
fault and Lear and the other riders were taken off
the train and asked to wait. Two teams of engineers
hurried to the scene. The line was only getting longer,
and two trains were sitting out on the twisting roller
coaster track full of increasingly anxious passengers, wondering what the
problem was and whether it was anything to do with
(08:45):
the gusts of wind. The first team of engineers started
to diagnose the problem, which was nothing serious, and also
decided to add a fifth train to the track while
the ride was suspended. That meant that once the roller
coaster was operating again, it would accommodate a few more passengers.
(09:08):
Seven minutes after the fault appeared, the engineers were able
to bring a train of relieved guests back to the station,
where they got off and wandered away to enjoy the
rest of the theme park. A minute after that, another train,
the last occupied train, arrived and the passengers disembarked. Now
(09:30):
all four empty trains were safely inside the roller coaster
station and the fifth train was added. This took another
five minutes, while Lea and Joe and two thousand people
behind them waited and wondered what was happening. As the
fifth train was being added, the roller coaster's electronic system
(09:52):
flashed up seven more fault codes. Each of them was minor,
but each of them needed to be acknowledged, checked, and
then cleared. By now Lea and Joe and the others
had been waiting nearly twenty minutes to get back on
the ride, and the engineers sent an empty train around
the roller coaster just to check that everything was working properly.
(10:16):
It wasn't. The train went out, but it didn't come back.
Like many roller coasters, the Smiler operates on a combination
of gravity and momentum. The trains don't have engines in them. Instead,
each train is pulled up a long slope by a
(10:37):
chain lift, then released to run the course of humps
and loops until finally coasting back into the station. Roller
coaster the clues in the name, but because the Smiler
has that world record tangle of fourteen inversions, because it
stays fairly low level with the tree tops. One chain
(11:02):
lift hill won't do the job, and so halfway round
the ride comes one of the defining moments of the
Smiler experience. There's a second chain lift, and instead of
being pulled up a long slope, the chain runs vertically. Suddenly,
your seat tips back so far that your feet are
(11:24):
higher than your head, and you stared directly up at
the sky, being hoisted higher and higher up a vertical track.
But in June twenty fifteen, that test train didn't come
back because it didn't quite reach the second lift, the
vertical one. It coasted to a halt just short of
(11:45):
where the lift chain would engage. Why unclear. The fact
that it had no passengers meant that it would have
been a little lighter and carried less momentum. Then there
were those gusts of wind. Whatever the reason, it wasn't
quite close enough for the lift chain to finish the journey.
(12:08):
They were puzzling over this problem. The first team of
engineers were joined by the second team, a pair of
electrical engineers. They all huddled together for a brief conference.
But one thing that doesn't seem to have been mentioned
was that a fifth train had been added to the
track anyway, it wasn't hard to figure out what was wrong.
(12:30):
The ride had sensors which showed that a section of
track was occupied by a train, that train they had
just sent out as a test. The engineers could also
look at CCTV images and see that train stopped just
shy of the chain lift. Three of the engineers made
their way down to the track's halfway point at the
(12:50):
bottom of the vertical lift hill. They put their shoulders
to the heavy train and started to push until the
train clicked into the chainlift and up it went, straight
up the vertical rails before coasting around the remaining loops
and corkscrews and back to the station. Lear and Joe
(13:15):
and the rest of the sixteen passengers had been waiting
for half an hour since being put onto a train
and then taken off again, with no knowledge of what
the Smiler engineers had been up to. But at long
last they were nearly ready to get the passengers back
onto the ride. Ahead of the train, Lea and the
others were board, there was another empty train. The engineers
(13:38):
sent it off around the circuit, and Leah and Joe
stepped forward to be strapped into the front row of
the smiler, ready for the ride of a lifetime. Cautionary
tales will be back after the break. Roller Coasters are
(14:10):
safer than lots of things people do for fun. They're
certainly a lot safer than riding in a car, if
you believe the numbers from the International Association of Amusement
Parks and Attractions. And why wouldn't you believe them. My
son and I ran about the same risk in driving
one hundred miles to Alton Towers and one hundred miles
back again that we would have faced if we'd ridden
(14:33):
six hundred roller coasters. So sure, roller coasters are pretty safe,
but they don't feel safe. They're not meant to feel safe.
And if you pushed a nervous roller coaster rider to
think about what might actually be dangerous about a roller coaster,
they'd probably tell you two things. First, I might fall
(14:56):
out of my seat, and second, the entire train might
fall off the track. That makes intuitive sense. The tracks
look narrow and exposed, and the trains go fast, take
the perilous curves, and for goodness sake, they go upside down.
(15:18):
But that's not really the problem. Here are the words
of Stephen Flanagan, an expert on roller coaster safety. Although
the public perception of the hazards associated with roller coasters
may be focused in the danger of a train parting
company with the track, in reality, the bigger and more
(15:40):
difficult to resolve issue has always been the hazard of
trains colliding on the track. When you get on a
roller coaster like the Smiler, you really don't need to
worry that the train is going to fall off one
of those gravity defying loops. You really don't need to
worry about anything. But if you did feel like worrying
(16:01):
about something, I don't suggest worrying that your train might
smash into another train. As Leah and Joe's train moved off,
a diabolic voice boomed over the speakers join us. Then
the train plunged into a dark section of track, shocking
(16:24):
the passengers by flipping upside down and right side up again,
before emerging into daylight and slowly clanking up the first
chain lift higher and higher and higher. Ahead of them
on the Smiler tracks was an empty train, and none
of the passengers realized that empty train was slowing As
(16:48):
it approached the top of a loop, the wind was
still gusting the empty trains were still a little on
the light side. The train came to a stop, then
slowly rolled back to settle in a dip. Once Lea
(17:09):
and Joe's train was released from the lift hill, it
would take twenty six seconds to loop over to that dip.
They were less than half a minute from disaster, except
their train just stopped right at the top of the lift.
(17:32):
We got to the first lift hill and it got
stuck at the top. So obviously me and Joe were discussing, Oh,
this isn't right. Why had the train stopped? Simple. Although
the Smiler engineers hadn't realized there was a problem, the
smiler's monitoring system knew perfectly well that there was a
(17:53):
train stalled out on the track. Automatically, it held Lear
and Joe's train at the top of the slope, the
highest point of the Smiler, waiting for the blockage to
be cleared. Catastrophe had been averted by an automated system.
So now what. Paul Meal was an expert who became
(18:20):
fascinated by the mistakes experts make. That was partly an
intellectual interest of his, but it was also very personal.
In nineteen thirty five, when Meal was fifteen years old,
his mother had gone to her doctor with some symptoms
which the doctor attributed to problems with her inner ear.
(18:42):
In fact, the cause was a brain tumor. The doctor
could have diagnosed the tumor by asking some basic questions
and performing some basic checks, but didn't. A year later,
the tumor had grown and spread. It was finally diagnosed
by a different doctor, and Paul Meel's mother died soon after.
(19:09):
Yiel later wrote this episode of gross medical bungling permanently
immunized me from the childlike faith in physicians' omniscience that
one finds among most persons. Meil became an academic researcher
and a clinical psychologist, and he never lost his curiosity
(19:29):
about the fallibility of human experts. In his early thirties,
he published a book that was to become famous, Clinical
versus Statistical Prediction. The book asked a question, should you
trust the judgment of an expert doctor, or is it
better to take a short list of symptoms, feed them
(19:51):
into a simple flowchart, and do whatever the flowchart says.
Such a flowchart might have saved his mother. This was
nineteen fifty four, so we're not talking about pitting human
experts against sophisticated artificial intelligence, talking about pitting human experts
against a crude formulaic process, but Meil found that the
(20:15):
crude formulaic process often beat the humans. Three years later,
he published an academic article with a punchy follow up question,
when shall we use our heads instead of the formula?
It's a question that's much more pressing today than it
was when he asked it in the nineteen fifties. Computers
(20:38):
tell managers who to hire and who to fire. They
tell radiologists whether a shadow on a scan is cancer
or not. They advised judges on who should be released
on bail and who should be detained, and they helped
social services prioritize calls about children at risk. The computers
(21:00):
they're everywhere, So when should we trust them instead of
our own judgment? Paul Meill wrote, I find the two
extreme answers to this question, namely always and never equally unacceptable.
(21:20):
At Alton Towers, the engineers were about to override the computer.
That's not always the wrong choice. Sometimes the human is
right and the computer is wrong. Unfortunately, this wasn't one
of those times. Lear and Joe and the others had
(21:43):
waited an hour and a half to get onto the Smiler,
then another half an hour while the technical fault was resolved.
Now they were perched precariously at the highest point of
the ride. A long minute ticked by. Then another excitement
gave way to waves of anxiety, boredom and annoyance. They
(22:08):
said over the tannel, we're experiencing technical difficulties. Bear with us.
They really weren't supposed to be poised and ready to
roll for such a long time. What was going on?
The Smilers engineers were trying to figure that out too.
The Smiler has a simple computer system designed to prevent
(22:30):
trains from smashing into each other at certain points on
the track. There are proximity switches which register when a
train enters a block of track and when it leaves
it again. If the system thinks a train is on
a particular block of track, it will automatically prevent the
train behind it from being released into that block. That's
(22:51):
what was happening here. Of course, the empty train hadn't
left the block. In fact, it was still gently penduluming
backwards and forwards, settling in the valley ahead of Leah
and Joe's train. So the smiler's computer prevented the occupied
train behind it from moving forward, leaving Lear and the
other passengers waiting and waiting at the top of the
(23:13):
lift hill. Meanwhile, on the ride's control panel, the zone
stop alarm was activated, notifying the engineers that one train
was being held because the train ahead hadn't left its
block of track. Hmm, well, the occupied block was the
one including the vertical chain lift, where the previous train
(23:37):
had stalled. It was natural to assume that if a
second train had stalled, that would be where it was.
One of the engineers went out to have a look,
but there was no train at the bottom of the
vertical lift. As far as he was concerned, then the
smiler's collision prevention system was just malfunctioning. Probably it hadn't
(23:57):
reset itself from the train which stalled fifteen minutes before.
That might seem like a big assumption, but this was
one of the engineers who arrived after the fifth train
had been added. He didn't know about it, or if
someone had mentioned it, it had slipped his mind. So
he thought there were four trains, and he knew where
(24:19):
four trains were LEAs at the top of the lift hill.
And three in the station. If it turned to look
over his shoulder and looked carefully, he might have seen
the fifth train valid at the other end of the
ride through the tangle of roller coaster track. But he
didn't look, or if he did, he didn't see. At
(24:46):
the bottom of the vertical chain lift was a control
panel to reset the block sensors. The engineer performed that reset,
telling the computer to forget its erroneous belief that there
was a train stuck on the track. Unfortunately, it wasn't
the computer which had an erroneous belief, it was the humans.
(25:09):
Retails will return in a moment. Leah's train has now
(25:30):
been held at the top of the chain hill for
six long minutes. There's still a chance the smiler system
won't release the train without an explicit override from the humans.
So now the critical decision is with the engineers back
in the station. But they don't explicitly discuss whether there
(25:53):
are four trains running or five. They don't send anyone
out to visually confirm that the whole track is clear, nor,
it seems, do they look carefully at the CCTV screens.
The smiler does have lots of CCTV screens very on
brand for a dystopian roller coaster experience, but the ride
(26:15):
is such a tangle of track that it's hard to
see clearly. Right now, the valid train is partly obscured,
and the engineers don't look carefully enough. Maybe they don't
look at all. They're all just assuming the computer must
be wrong, so they decide to override it. The smiler's
(26:39):
system still won't release one train into a section where
it knows another train is stationary, but it doesn't know
about the stationary train anymore because its memory of the
obstacle has been deliberately reset. Leah recalled it just set
back off without any warning. At last, Leah's train starts
(27:01):
to roll down from the lift hill and accelerate into
loop after loop after loop. Twenty six seconds away is
a stationary train. They were fine, We were going around
loops and everything. We just came round this corner and
I saw this car and I'm like, oh my god,
this isn't good. Paul Neil, the clinical psychologists who pose
(27:29):
the question about when we should follow the formula and
when we should use our heads, didn't have an easy answer,
but he did suggest a rule of thumb. If you,
as the human, know something that the algorithm doesn't know
that the algorithm can't know, that's a reason to think
about overruling the computer. But when investigators later looked into
(27:52):
the causes of the Smiler accident, they noticed that the
engineers had a patchy knowledge about how the ride's false
alarms actually worked. This meant they were often puzzled when
the alarms went off. They'd do some safety checks and
reset the system, but exactly why the alarm had sounded
was sometimes a mystery, so the engineers regarded the alarm
(28:16):
system as capricious and unreliable. If the engineers had been
better trained, they'd have understood the logic of the alarm
system and would have been able to think clearly about
when to override it and when not to. Paul Meel
warned us, if you're going to overrule the computer, you'd
better have a reason to think you know something that
(28:39):
it doesn't. But to do that, you need to have
a firm understanding of how the computer works. The Smilers
engineers didn't. As Leah and Joe and the others looped
upside down and saw the empty train ahead of them,
(29:02):
they had only a moment to brace for the shattering impact.
The train wasn't going especially far. You'd probably walk away
from a car crash at that speed, but unlike cars,
roller coasters don't have crumple zones. Leah was right at
the front with no protection, and as metal smashed into metal,
(29:26):
the two trains locked together and rocked backwards and forwards
twelve times, with the legs of the front row passengers
crushed between them. The next thing I remember was the
screaming and the blood. There was so much of it.
The metal of the safety bars had folded into our bodies,
(29:46):
said Leah later. All I wanted was to hold Joe's hand,
but when I looked down at it, I could see
there was no way I could. His little finger was
hanging off his left hand, and the middle finger on
the other was broken. All the front row passengers had
horrendous leg wounds. Both Joe's kneecaps were shattered. Leah's injuries
(30:10):
were even worse. I started getting pain. I couldn't feel
my toes, Leah remembered, and I started to look at
my legs. My left leg was all pushed up. They
were snarled up in a tangle of metal at a
forty five degree angle twenty feet above the ground, and
(30:30):
the Smile operators were apparently still confused because it took
seventeen minutes before anybody called for the emergency services. Up
on the coaster, Leah nearly died from blood loss, but
an air ambulance flew in blood for an emergency transfusion
(30:52):
twenty feet above ground. Later, she stopped breathing, but the
medics reaching her from precarious ladders and hoists managed to
resuscitate her. Once she was released from the train. Hours later,
Leah was flown to hospital, slipping in and out of consciousness.
(31:15):
She woke up twenty four hours later. I realized two
things were missing, my boyfriend and my entire left leg.
The doctors told her sorrowfully that they had faced a
choice her leg or her life, so they took the leg.
(31:37):
The woman sitting next to her on the roller coaster,
Vicki Balch, also had her leg amputated. Joe Pew was
elsewhere in the hospital. He and several other passengers also
suffered severe injuries, all because the Smilers engineers hadn't double
checked when the computer told them there was a train
(32:00):
out on the track. Given what she's been through, Leah
Washington is doing incredibly well. She's something of an Instagram influencer,
posting videos of herself learning to walk, learning to run,
(32:20):
learning to ski, or selfies of her looking fabulous in
a bikini. They look like a lot of Instagram self is,
in fact, except that she's looking fabulous in a bikini
with an artificial leg. Joe never regained the use of
one finger, but his knees were covered eventually, and the
pair are still together. Leah is now missus Washington Pew
(32:46):
after she and Joe tied the knot in twenty twenty four.
But that happy ever after ending so nearly didn't happen.
At that moment when Leah was unconscious and trapped in
crumpled metal twenty feet above the ground, when a team
of paramedics on hoists were trying to get her breathing
(33:08):
started again, it was touch and go. The medics later
told her if the weather had been any colder, she
wouldn't have made it. In a world where we're surrounded
by automatic systems, statistical formulas, algorithms, and computers, we're going
(33:32):
to have to get better at deciding when to trust
them and when to overrule them. There will never be
a hard and fast rule for when to do that,
But if you're thinking of ignoring the computer, it's wise
to have a logical reason to think you know better.
The Smiler engineers didn't. When investigators picked over the disaster,
(33:57):
they concluded that there was nothing wrong with the roller
coaster itself, nor with the automatic alarm system. It was modern,
well designed, and functioned exactly as intended. The problems emerged
from the working practices of the engineering team. It was
partly the fact that the engineers didn't fully understand their
(34:20):
own safety alarms, and it was also that nobody in
particular was in charge of the process of resetting the
Smiler after the very first fault alarm. There were the
ride operators, the first engineering team, and the second engineering team,
all trying to make decisions. There were loose, informal conversations
(34:41):
in which nobody had a full overview of what was
going on. There was no written process for working systematically
through a checklist before resetting the ride, and there was
no formal assignment of responsibility as to who had the
authority to override the automatic system and under what circumstances.
(35:06):
This lack of a formal process was critical. Without it,
the engineering team could just assume that the track was clear.
With a formal process, someone would have been clearly responsible
for checking. Fifteen months after the accident, a judge called
(35:26):
it an obvious shambles involving lack of communication and double checking,
which could and should easily have been avoided by a
written system of working, including a single overall supervisor. Automatic
(35:47):
systems can help us avoid accidents and make better decisions,
but Paul Meele's question will never go away. When should
we use our heads instead? Maybe the trick is in
the question. It's okay to use your head if you
really are thinking. Very often we aren't. As I waited
(36:14):
in line for the smiler with my twelve year old
son snaking through the maze of concrete under the black netting,
it was hard not to think about Leah Washington and
her accident. Maybe all this was going through my son's
mind too. As the que edged forward and his knees buckled.
(36:38):
I'm not sure I want to do this. Come on,
I said, he came all this way. It'll be sad
if you don't do it. So we did it, and
it was amazing. We had a great time, and he
was so proud of himself. But there was one moment
(37:06):
when we got to the front of the line, when
we were offered a place on the front row. We
looked at each other and shook our heads. No, not
the front row, thanks, We'll sit a little bit further back.
(37:43):
Cautionary Tales is written by me Tim Harford, with Andrew Wright,
Alice Fines and Ryan Dilly. It's produced by Alice Fines
and Marilyn Rust. The sound design and original music are
the work of Pascal Wise. Additional sound design is by
Carlos San Juan at Brain Audio Bend and daff Haffrey
edited the scripts. The show features the voice talents of
(38:06):
Melanie Guttridge, Stella Harford, a Hemborough, Sarah jupp As, Sam
and Roe, Jamal Westman and Rufus Wright. The show also
wouldn't have been possible without the work of Jacob Weisberg,
Greta Cohene, Sarah Nix, Eric Sandler, Carrie Brody, Christina Sullivan,
Kira Posey and Owen Miller. Cautionary Tales is a production
(38:28):
of Pushkin Industries. It's recorded at Wardoor Studios in London
by tom Berry. If you like the show, please remember
to share, rate, and review. It really makes a difference
to us and if you want to hear the show,
add free sign up to Pushkin Plus on the show
page on Apple Podcasts or at pushkin dot fm Slash
(38:48):
plus eight
Host
Tim Harford
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