More About Mistake-Proofing

After yesterday’s post about trucks crashing into the famous 11foot8 bridge and mistake proofing, I got the feeling I should drive home my key point that the problem isn’t with the driver, it is with the environment.

As of this writing,  Jürgen has recorded 154 crashes of overheight vehicles into the bridge.

And I’ll put even money that if all of the data were known, this process would pass any test for statistical control and we are getting what we should expect from a stable system. It might not be what we want, but it is what we should expect. (All images are copyright  Jürgen Henn,

So addressing the individual incidents probably isn’t a solution. In any case, it is unlikely that any driver will repeat the mistake.* For the TWI folks – this isn’t really a Job Relations type of problem. It might feel like it is, but it isn’t.

In the Factory

I was working with a company with a similar problem. Their inspectors kept missing defects. The response was often to say “I don’t see how she could have missed that!” and even write them up for failure to do something that wasn’t particularly well specified.

But the fact on the ground was, like the bridge, the misses weren’t confined to any particular individuals, any particular shift, any particular anything. People missed things all of the time because the expectations greatly exceeded the limitations of what humans can do for 12 hours (or even one hour). It isn’t an inspection problem. (The reliance on inspection vs. upstream controls is another topic for another day.)

People Work Within a System

It is all too easy to fall into the “bad apple” fallacy and seek out someone who was negligent. It feels good, like we did something about the problem. But the problem will happen again, with someone else. Then I hear frustrated managers start to make disparaging comments about their entire workforce that “doesn’t care” about quality.

I challenged a quality manager to do that inspection job for two hours – not even a complete shift – under the watchful eye of the inspector whose job he was trying to do. Funny – he was a lot slower to assign blame after that experience. He couldn’t keep up.

Deming was pretty clear about the ineffectiveness of exhortation as a way to get better performance. “Be more careful!” might well work for one individual for a short time. “Making an example of someone” might well work for a group for a short time. But there are norms, and the system will return to those norms very quickly. There are simple limits to what humans can focus on and for how long.

The Bridge is a Metaphor

To be clear, the bridge represents a working system, but it is different than what we would find in a company. This is public infrastructure, and the truck drivers that get featured on the videos are not part of a single organization.

This means that you have more control than the city engineers in Durham do. You can establish procedures, ask questions, train people, have them practice, alert them to the Gregson Street Bridge on their route. You can make sure your navigation system routes your trucks around the low bridges. You can support your people so they are less likely to even end up in the situation. All of these are system changes – and that is what it will take to change the outcome.

Change the System: Raising the Bridge

In late 2019 the city of Durham, in coordination with the railroad who owns the bridge, did actually raise the bridge by 8 inches. It is now 11 feet 16 inches (3.76m).

And that is a legitimate approach. Rather than trying to create infallible humans, what can we do to make the system less vulnerable to fallible humans.

While that likely reduced the number of trucks that hit the bridge…

*Caveat: There is one video where a truck seems to avoid the bridge, then circle back around and hit it. And another video where a truck that hit this bridge then proceeded to run into another low bridge with the damaged truck.

Mistake Proofing – Getting People’s Attention

Besides being a great source of schadenfreude, Jürgen Henn’s website, offers some great insight in the difficulty of effective mistake-proofing.


The clearance between the road and the railroad bridge at 201 Gregson Street in Durham, North Carolina is officially 11 feet 8 inches (3.55 meters).

A typical rental box truck is about 12 feet high (3.65 meters).

The result:

Penske rental truck smashed under low bridge.

The more astute of you may have noticed the “OVERHEIGHT MUST TURN” sign just above the smashed truck.

Well before the truck approaches the intersection, it passes under a height sensor. If the vehicle is overheight, the OVERHEIGHT MUST TURN sign comes on and starts to flash, and the traffic lights turn red.

While this effort did mitigate the problem, obviously there are drivers who simply do not notice. Lots of them. Jürgen has cameras trained on the intersection and captures over a dozen crashes in a typical year. As a result, his website has attracted international attention. See the short documentary here:

Familiar Tasks = “In the Zone”

The human brain is an amazing thing. It also works by deceiving us. It creates the illusion of complete awareness of the things around us when, in reality, we are simply aware of a model our brains have constructed of what we perceive to be there.

It is also an amazing engine at engaging actions based on pattern matching. For example –

In sessions I facilitate, I routinely ask people if they can recall a time when they arrived home after work but realized they didn’t remember driving the route. Nearly every hand in the room goes up. That is pretty amazing because driving is an incredibly complex task. But, assuming you get a license in your mid-teens, by the time you are in your early 20s, most people don’t give it much thought. (There is a reason your insurance rates drop when you turn 25.)

It has become a hard-wired neural pattern, a series of habits, a programmed set of responses that are operating below below the conscious and deliberate thinking part of the brain. This is really good because this process is much faster and more responsive than the alternative. Think about how it felt to be driving when you were just learning – or how it feels to be doing anything new when you are just learning.

The downside to this amazing programming is that things that don’t jar us into consciousness often go unnoticed.

And the more familiar, the more expert, we are with the task at hand, the more likely this will happen.

Levels of Mistake Proofing

I like to talk about three levels of mistake-proofing. Four actually, if you count Zero as a level.

  • Level 0 – the task must be performed correctly from memory.
  • Level 1 – there is a discrete task of checking build into the job.
  • Level 2 – there is an active indication when a mistake is about to be made (or has just been made and there is still time to correct without consequences).
  • Level 3 – there is active prevention that gets in the way of making the mistake.

The OVERHEIGHT MUST TURN sign is a Level 2. The driver has already passed signs informing him of the bridge clearance about 100m before the bridge.

The height sensors are on the poles just past the speed limit signs.

Low clearance signs about 100m before the bridge. (Google Street View)

Then in the next block, there is another sign telling the driver to turn:

Approaching the last intersection (Google Street View)

And finally the light changes and the OVERHEIGHT MUST TURN sign illuminates.

And still they miss it.

(Direct YouTube link for the email readers:

To be clear, the vast majority of truck drivers don’t hit the bridge. But over a dozen a year do. (There were more before the flashing sign was put in.)

“Pay Attention!” = Fundamental Attribution Error

As we watch Jürgen’s videos it is easy to assign character attributes to the drivers who hit the bridge. If only they were better drivers. (The vast majority people who are asked to rate their driving skill will say they are “above average” by the way. Think about that.)

But it is human nature to get lulled into a bit of complacency when engaging a routine task- and driving is a routine task in spite of the complexity.

Now – think about the people in your organization. How many opportunities to they have to make a mistake every day… every hour… in the course of their work? How many of those possible mistakes have serious or expensive consequences?

They are experts at what they do. And they don’t make many mistakes. And they usually catch themselves before there is a problem. But the Law of Large Numbers says “Given enough opportunities, the unlikely is inevitable.”

Can anyone reading this honestly say they have never inadvertently run a stop sign? Yet accidents rarely occur. Now and then there is some panic braking, and rarely there is a collision.

Yet it is really easy to single out the person who:

Performs the work the same way, in the same conditions as everyone else.

Makes the same mistakes, just as often, as everyone else.

Just like everyone else, usually they are caught in time, or other conditions aren’t present for a bad outcome.

But this time everything lined up and BANG – the defective product got missed, or the leak wasn’t noticed before the sump went dry.

Then that person gets singled out for “not following procedure” when nobody follows procedure.

Job Instruction “Key Points” = Opportunity

In a Job Breakdown for TWI Job Instruction we assign a “Key Point” as something the learner must remember specifically because it:

  • Might result in a hazard – injure the worker or someone else.
  • “Make or break the job” – if something isn’t done a specific way, the task fails.
  • Is a “knack” or technique that makes it easier to do.

Those first two are red flags. You are asking your team member to memorize critical to safety and critical to quality tasks. My challenge: How many of those key points can you “mistake proof” out of the job breakdown?

Finally – rather than the sensors and flashing lit signs, there is this approach from “somewhere on the Internet.”

Warning Sign: If you hit this sign, you will hit that bridge.

That is awesome because even if the driver doesn’t see the sign, the BANG! may get his attention in time for it to register and stop. In the Durham, NC example above, apparently this wouldn’t work because there are legitimate reasons for trucks to come down the street up to the intersection just before the bridge. After that, it is really too late unless the driver is already aware that he has to turn.

Oh – and by the way – you can get souvenirs from the Gregson Street Bridge at  Jürgen’s store here: 🙂

There is a follow-up to this post here:

That Broken Bolt is Speaking to You






Normalized Deviance




How do Bolts Break?

A few possibilities came up.

  • The might be loose.


“What is the torque spec?”

“… I don’t know.”

Now we are getting somewhere.


Software Rules









US Customer Service – Chrysler Brand Site

Brief Description: How do I turn off the radio?


VIN: 1CHJXDG3Wxxxxxx

Mileage: 61

This is the response I got:

<> wrote:
Dear Mark,
Thank you for contacting the Jeep Customer Assistance Center.


Thank you again for your email.  Should you require additional
Customer Service Representative
Jeep Customer Assistance Center






And get a deeper, more personal, response back:

Dear Mark,
Thank you for contacting the Jeep Customer Assistance Center.









Team Member Saves




But let’s make sure we are celebrating for the right reasons.

The save isn’t what should be celebrated.



We usually just celebrate correcting the immediate problem.









Where and when was that moment?


Dig in, think about it.




Averages, Percentages and Math


The Problem with Averages


The Average Trap



Awesome, right?




On the one below, 11 points miss the goal


But the both have an average 5 points over the goal.




Using Averages Correctly










The averages will be different even if you haven’t changed anything.









The Problem with Percentages


Don’t Make Me Do Math



Here is my question:

“How will you know if you have achieved it?”





Indicators of a W.A.G.






Bottom Line Advice:





Be suspicious of round numbers that sound authoritative.



Scientific Improvement Beyond The Experiment




How I Think Science Works










How I Think Most Companies Try To Work


Somebody has a good idea and “proves it out.”









How the Scientific Approach Would Work






But Sometimes there is just a good idea.





oh… and this is how rocket science is done.

Edit to add:


DMAIC and Toyota Kata



My Background





The Improvement Process Flow



Defining the problem and establishing a charter for the team.

Establishing a Measurement that will define progress.




Define, Measure, Analyze, Improve, Control. DMAIC.







Relating DMAIC to the Improvement Kata

Here is how I relate DMAIC to the Improvement Kata.











Blending Toyota Kata and Six-Sigma Coaching


[preliminary social rituals]


[Green Belt reviews the charter and objective]






“What did you expect from that step?”


“And what actually happened?”


“And what did you learn?”




“Which one of those are you addressing now?”








“And what result do you expect?”




DMAIC is now more like:

DM [repeat AIC as necessary]


The Master Black Belt












Checklists: “Do.” vs. “Did you do?”

When operations or steps get omitted, a common countermeasure is to establish a checklist.

A typical checklist has a list of items or questions – sometimes even written in the past tense.

“Was the _______?”

There are a couple of common problems with this approach.

First, the time to actually, physically make the checks is not included in the planned cycle time. This implies we are expecting the team member to review the checklist and remember what she did.

The second issue is that the team member often does remember doing it even if it wasn’t done.

This is human nature, it isn’t a fault or flaw in the individual. It is impossible to maintain continuous  conscious vigilance for any length of time. There are techniques that help, however they require some discipline from leaders.

Overall, a checklist that asks “Did you___?” in the past tense is mostly ineffective in practice.

We make things worse when the checklist is used as a punitive tool and we “write up” the team member for signing off on something that, actually, didn’t get done. Most of the time it does get done, but everyone in this system occasionally misses something. Sometimes those errors get caught. This kind of “accountability” is arbitrary at best.

Where checklists work is in “what to do next” mode – referring to the check list, doing one item, checking off that it was done, then referring to the next item on the list. This is how it works in an airplane cockpit.*

CAPTAIN: okay, taxi check.

FIRST OFFICER: departure briefing, FMS.

CAPTAIN: reviewed runway four.

FIRST OFFICER: flaps verify. two planned, two indicated.

CAPTAIN: two planned, two indicated.

FIRST OFFICER: um. takeoff data verify… one forty, one forty five, one forty nine, TOGA.

CAPTAIN: one forty, one forty five, one forty nine, TOGA.

FIRST OFFICER: the uh weight verify, one fifty two point two.

CAPTAIN: one fifty two point two.

FIRST OFFICER: flight controls verify checked.

CAPTAIN: check.

FIRST OFFICER: stab and trim verify, thirty one point one percent…and zero.

CAPTAIN: thirty one point one percent, zero.

FIRST OFFICER: the uh…. engine anti-ice.

CAPTAIN: is off.

FIRST OFFICER: ECAM verify takeoff, no blue, status checked.

CAPTAIN: takeoff, no blue, status checked.

FIRST OFFICER ON PA: ladies and gentlemen at this time we’re number one for takeoff, flight attendants please be seated.

FIRST OFFICER: takeoff min fuel quantity verify. nineteen thousand pounds required we got twenty one point eight on board.

CAPTAIN: nineteen thousand pounds required, twenty one eight on board.

FIRST OFFICER: flight attendants notified, engine mode is normal, the taxi checklist is complete sir.

(This is also how it works when assembling a nuclear warhead, but I can’t tell you that.)

This is also very effective for troubleshooting. For example, I was working with a team in a food processing plant. The obstacle being addressed was the long (and variable) time required to change over a high-speed labeling machine and get it “dialed in” and running at full speed without stops and jams.

Some operators were much better at this than others. We worked to capture an effective process of returning the machine’s settings to a known starting point, then systematically adjusting it for the specific bottle, label, etc. It worked when they were able to slow down enough to use it. That was an instance of “Slow is smooth; smooth is fast.”

The act of reading out load, performing the action, and verbally confirming is very effective when it is actually done that way. Even so, people who are very familiar with the procedure will often take shortcuts. They don’t “need” the checklist… until they do.

Still, you have a sequence of operations, and it is critical that they are all performed, in a specific order, in a specific way.

What works?
I’d say look around.
If you are reading this, you likely have been at least dabbling, and hopefully trying to apply “lean” stuff for a while.

What is a basic shadow board? It is a “checklist” of the tools to confirm they are all there – and a lot faster because missing items can be spotted at a glance. At a more advanced level, companies move away from shadow boards and to having the visual controls outlining what should be where to perform the work.

Color coded tool holders.

If you kit parts, you can set them out in a sequence – a “checklist” that cues the team member what order they should be installed.

I could continue to cite examples, but here’s the point.

When things are being left out, there is a high temptation to say “Let’s make a checklist” and sometimes make it worse by saying “…and we’ll have the worker sign it off for accountability.” That is more often than not simply a “feel good” solution. You feel like you have done something, and I’ve even heard “Well, it’s better than nothing.” I’m not sure it IS better than nothing – at least not in very specific conditions.

Instead, you need to study the actual work. Don’t try to ask questions, just stand and watch for a while. (Explain what you are doing to the team member first, otherwise this is creepy. “Hi – I’m just trying to understand some of the things that might get in your way. Do you mind if I just watch for a while without bothering you?”)

What cues the team member which step to perform next? Does he have to know it from memory? Or is there something built into the way the workplace is organized?

Does he end up going back and doing things he forgot?
Does he set out parts and tools in order on his own so he doesn’t forget?
Does he get interrupted, by anyone or anything, that takes him out of his mental zone?
(I go through airport TSA security checkpoints at least twice a week. I have a routine. When the TSA agent tries to “help” by talking to me, my routine gets broken, and that is when I forget stuff.)

If you are coaching someone, it helps if you go there with them, help the see the details by spotting these things and “asking” about them; then taking them to another area and challenging them to see as many of these issues as they can. See who can spot more of them.

What you are seeing are obstacles that impact the team member’s ability to do quality work.

Checklists don’t help remove those obstacles.


*The checklist transcript here is a cleaned up version of the Cockpit Voice Recorder transcript from Cactus 1549, the US Airways A320 that successfully landed in the Hudson River after multiple bird strikes knocked out both engines. I used it here because it is authentic, and the accident was one where everything went right and no one was seriously injured.

Eliminating Key Points

TWI Job Instruction is a structured process for breaking a job or task into teachable elements, and a 4 step process for teaching that job to someone.

I am not going to try to explain everything about breaking down a job here, this post is primarily for people who use job breakdowns and job instruction already.

The process of breaking down a job involves:

  • Identifying the Important Steps – the sub-tasks that materially advance the work.
  • Then identifying Key Points within important steps. These are things which the team member must pay special attention to, or perform a specific way. The guideline is that a key point is something which:
    • Is a safety issue – could injure the worker, or someone else.
    • Would “make or break the job” – critical to quality or the outcome of the work.
    • Is a “knack” or special technique that an experienced team member would use to make the job easier to do.

Breaking down a job this way takes practice, but it is a great way to identify the elements that are critical to safely getting a quality job done, and ensuring that the team member understands them.

But I want to propose this is only the first step.

Every key point is something the team member must remember in order to to not get hurt (or hurt others); to avoid scrapping or damaging something; to perform at the level you need.

Take it to the next level.

Think of these key points and the training that they drive as temporary countermeasures.  They are stop gaps you have in place until you can do something more robust.

Take one key point at a time. Why is it necessary? Why is it possible to do this step any way but the best way?

Can you alter the work environment – the product, the process, the equipment, the visual controls to reduce the things the team member has to remember (and you have to remember to teach)?

Can you make it impossible to perform that step in any way other than the way it should be done?

If you can’t, can you make it impossible to proceed until the error is corrected, before any harm is done?

If you can’t, can you make it so obvious that it is impossible to miss?

If you can’t, can you put in a robust reminder? (Signs and placards generally DON’T WORK for this unless they are especially “sticky.”

Every key point is something you have identified as critical to doing the job directly. Therefore every key point should drive a focused effort to mistake-proof the work.

You want to have as few as possible… but no fewer.