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Is your bottleneck morphing?

Many simulation tools and references exist to find the bottleneck within the production process to find ways to increase capacity and reduce waste. In most academic cases, we are trained to look for the stagnant inventory around a facility, and you will likely find the bottleneck. As a bit of caution in the training, we are taught that inventory might not always be in clear sight on the inlet or outlet side of a production unit. Instead, it might be stored as work-in-progress (WIP) in a variety of locations and forms. Within this caution, we are taught that this inventory might not always be transparent and visible to the naked eye.

Wait, if the inventory isn’t always in clear sight, do we even know where the bottleneck is? Is the variability of our product mix possibly hiding the bottlenecks? Could our bottleneck shift based on production flow scenarios? Instead of fixating on claiming one specific area is a bottleneck, can the bottleneck morph?

Process bottlenecks, or constraints, are the part of a process in which the nature of the flow changes significantly. The name refers to how the speed of pouring a liquid changes when it enters the narrow neck of a bottle. -

An experiment on morphing bottlenecks at Hartsfield-Jackson Atlanta International Airport

Getting off of an international flight at the Hartsfield-Jackson Atlanta International Airport in Concourse E in late December a few years back, my objective was to get to a terminal in Concourse B to carry me home. As I was laying out my strategy to quickly get to my departing terminal, I got to thinking about where the bottleneck was for thousands of people that were either coming or going. I decided that I would look for where the bottleneck could be hiding within the airport. I wanted to observe categorical examples that were disrupting the fluidity of the production unit intended to move me and all of the others through the airport. I wanted to observe process circumstances that could be morphing the bottleneck.

The experiment was enlightening. Along the way, I encounter different styles of flow restrictions that could move the bottleneck upstream and downstream through the process steps. I experienced potential bottlenecks everywhere. Things like boarding the escalators, congestions around Pascals, and just everyday shenanigans of the Hartsfield-Jackson Atlanta International Airport got me to personally experience the flow.

I imagined that each sub-area along my transition route was within some holistic process flow map. I then categorized the commonalities of these experience restrictions to understand how the bottleneck can morph into different areas of a production process depending on the scenarios. Take a walk with me through the Atlanta airport.

Types of resistance experienced

Upstream — It began with all of us getting off of the plane. During this time we are sometimes selfish focused on our restlessness and focused on the duration that we have to to get to the next seat. As we wait, those already in the transition area of the airport know our plane and many others have people getting off. However, they just haven’t experienced the mass of people that are coming. This is sometimes referred to as the Bullwhip Effect or Forester Effect, where our deboarding delay will ripple through the supply chain. In this example, the bottleneck could be within upstream areas.

Off the plane.

Jay Forrester described and explained such an effect for the first time (thus the name). Such an effect is related with biased attitude of managers to the changes in demand: when demand increases, it will be still increasing; when demand decreases, it will be still decreasing. - Anna Kowalczyk, Faculty of Management, Czestochowa University of Technology

Recurring event — As it pertains to the traffic of a traveler, a recurring event is a condition that one will experience at the same time, can typically plan for, and typically the same length of time regardless of the volume of individuals. This event could be the transitioning of my baggage from the international flight to my domestic flight or the time experienced clicking all of the right buttons going through Custom’s questionnaire. Those that fly a lot can anticipate these recurring events and know how to maneuver efficiently. Somewhat a Jedi-like power we think we may have. However, we do still participate in the recurring activity along with everyone else.

Through security now.

Auxiliary options — Typically, the Hartsfield-Jackson Atlanta International Airport has dozens of shuttles in service at any given time taking thousands of people from one concourse to another. You may experience seeing shuttles that go offline or others that come online to absorb the load. I am assuming that there is some algorithm behind the scenes that say if the shuttle’s timeliness is above some threshold, more shuttles come online to handle the load. If you pay attention by looking outside the window as you zip around underground on the shuttle, you will see some shuttles in an idled state. With the ability to turn on and off, these auxiliary options can affect the volumetric flow.

On the shuttle.

Hidden restrictions — Depending on what shuttle car you are on and which concourse you are getting off on, you sometimes have to walk against the grain of other people to then make a 180-degree turn towards the escalator. Completely dependent on where people are distributed and dispersed on the shuttles, these types of hidden restrictors depend on the surrounding circumstances. It's not every time you get off a shuttle you experience this type of restriction, and it isn’t routine for each shuttle car or concourse. It’s under certain circumstances you experience a hidden restriction.

Off the shuttle.

Ramped metering - Some escalators come from the basement where the shuttles are located and bring individuals up to the concourse level. These escalators are fairly narrow and at a constant speed. This ramped metering is transferring me and other travelers individuals up to their concourse at a constant speed. You typically cannot walk as you ride these escalators due to someone daydreaming as they enjoy the ride. Additionally, there is rarely enough room to get around this restriction. As you remain no less than four escalator steps behind this restriction, it is rare to avoid experiencing the metering as the escalator brings you up to the concourse.

Up the escalators.

An escalator can never break: it can only become stairs. You should never see an Escalator Temporarily Out Of Order sign, just Escalator Temporarily Stairs. Sorry for the convenience. - Mitch Hedberg

Congestion from Options — Have you gotten to the top of the escalator in Concourse B and smelled the gloriousness of Pascals? The smell of fresh greens and cornbread is a must for me. Within the same area, there are a variety of other restaurants, dining tables, and coffee shop options. Depending on the time of the day, this could be a quick to-go box or a convoluted mess just to get to the line. This congestion is restricting the flow of the production process to continue moving us. People are trying to get some Pascals, debating on what type of coffee they want, or just trying to stiff-arm their way through this congested mess. This collection of individuals is trying to find their next production step.

Top of the escalator.

Systemic congestion — Systemic congestion is felt and observed throughout the entire system. There is not one simple source and sometimes it originates from holistic stress placed within the airport. Think about the last time you went through it during the holiday season. Regardless of the time of the year, systematic congestion has an overarching pattern associated with it. Consider things like holidays, days of the week, or times of the day when the airport is more dense than usual. If you plan your travels to avoid certain congestion, you are planning to avoid examples of systematic congestion.

Proceeding through the holiday chaos.

Unplanned event - As people are getting from their initial location to their next destination, we all experience unplanned events as unplanned bottlenecks. For example, if you need to swing by a bathroom you soon experience that all of the toilets, except one, are unexpectedly out of service. This unplanned event is causing a momentary stop of the masses, disrupting the flow as maintenance tries to solve the problem. There are many other examples of unplanned events that we have all experienced, but I don’t have time to write them all. However, we can all relate.

Bathroom break.

Look… squirrel — I know we have all experienced walking down a concourse and someone takes a hard left across all incoming traffic to go to their terminal. They see a bathroom they can quickly get in and out of or realize they just walked past their terminal by accident. The merging of people cutting across the grain is a form of scatterbrained inefficiencies made at a moment’s notice. As people are walking, they emulate the saying look squirrel and disrupt the continuous flow of people. Happening too often, next thing you know everyone has come to a complete stop as someone just past their terminal.

Walking through the concourse.

Phantom congestion - Have you been walking down a terminal and all of a sudden it appears that the entire mass has just stopped and then restarted? Looking around for the reason, you see nothing. There have been a few times I have been at the front of the flow to witness the catalyst of this phenomenon. I believe they tend to start when someone or something arbitrarily stops in the middle of the flow. Consider the mobile carts stopping in the middle of a walkway or a large family stopping because the dad thinks they might have left their credit card at Pascal's. These phantom slowdowns come and go, popping up everywhere in your transition. You frustratingly observe the restriction and the gap in production in front of them.

I can see my terminal.

Downstream traffic — I finally get to my terminal B, and, go figure, there is a line. Progress stops due to a malfunction at the boarding gate or someone who had their battery die on their cell phone while attempting to use a mobile-boarding option. Regardless, this downstream delay compounds the working inventory of people and makes us piled up as we wait to board. The working inventory is backing up due to this downstream issue.

Come on people, we all just want to get home.


I do not have a fancy acronym yet like TIMWOODS to simply remember my experienced examples of potential bottlenecks. However, transitioning from Concourse E to Concourse B showed me that the bottleneck can shift based on conditions, environment, and circumstances. We can design and evaluate our supply chains to unlock the potential of our bottleneck process, however, we must maintain respect for how bottlenecks can morph. We may have a process that over some time is our primary bottleneck within our manufacturing processes, but we must understand that it can change location based upon things such as product mixes, crews, or behaviors. We must respect and prioritize improvements within our processes that morph and their influencers that are bottlenecking our entire process.


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