Theory of Constraints in the Wild: Queuing for the Tube
Posted: December 8, 2016 Filed under: flow | Tags: theory of constraints
Tube queuing after the Tottenham match at Wembley
When a WWT colleague invited me to the Tottenham – CSKA Moscow Champions League match Wednesday, he treated me to more than simply world-class soccer — he unwittingly gave me a chance to see Theory of Constraints in action: after the match, while we waited for a Tube ride.
Theory of Constraints tells us that every system has a single constraint — which is anything that limits a system’s performance relative to its goal — that governs its throughput. A particularly effective way of dealing with a constraint (aka bottleneck) is Eli Goldratt’s Five Focusing Steps, which I talk about this way:
- Identify the constraint
- Optimize the constraint
- Subordinate everything to the constraint
- Add supply to the constraint
- Goto 1
If you’ve ever been in London during rush hour, Step 1 is easy: The constraint is the supply of train cars. At busy times, London simply has more people needing to ride than it has trains to take them. Step 2 is fairly straightforward: We optimize the constraint by ensuring that the constraint is
- always busy (Londoners are particularly adept at squeezing as many people as can fit into the car, so this is never a problem!), and
- is only ever doing “value-adding” work, which in this case is moving passengers forward (as opposed to failure demand, which doesn’t typically happen in the Tube in the form of redoing work — going backward — but does often take the form of people getting their bags or themselves stuck in the doors).
That’s where Step 3 comes in. Only after we’ve properly satisfied the first two steps, we want to subordinate — which is simply to adjust the speed of things arriving to the constraint and departing from it to match the cycle time of the constraint. For the Tube, that means making sure that when a train stops, the platform has enough room for the passengers to spill out and leave the station. But the more vital subordinating occurs upstream — such as after the 62,034 fans emptied out of Wembley Stadium and headed for the nearest Tube station. That’s what happened the other night, as the photo shows: Tube security personnel created multiple upstream “batches” of people by holding up signs with either a red Stop or green Go (an indicator of capacity, a kanban). Only when the batch of people saw the green could they advance to the next “gated” area and finally onto the platform. This is exactly the “pull” behavior that an explicit work-in-progress limit encourages in knowledge systems: Rather than pushing as much work forward toward the constraint, we wait to advance new work until we get the signal, which indicates we are now under our WIP limit and therefore have capacity.
The result was a very orderly movement of people through the station and into the trains, avoiding the evil of overburdening and its demon spawn of waste, unpredictability and delay (and in this case, possible physical harm). This was good enough for the problem at hand; Step 4, add supply, is likely too expensive because it means adding trains to the line for what is essentially an exceptional case (a mid-week soccer match at Europe’s second-largest stadium). In any case, the result is testable, in that we can review the three TOC measures to see if any improved:
- Throughput (should go up)
- Operational Expense (should go down)
- Inventory/WIP (should go down)
Presumably, the London Tube officials have indeed tested whether the practice of subordinating has improved the situation. My guess is that the practice has increased throughput, while holding steady their operational expense and WIP (which is irrelevant in this case). It has probably also improved human safety.
Seeing flow in the physical world — especially when we’re personally involved! — is usually obvious. Knowledge work, on the other hand, is by its nature invisible, so we need special lenses to see bottlenecks in order to manage and improve flow. Thankfully, we have aids such as kanban boards and cumulative-flow diagrams that make visible the flow of work — work in progress, throughput and average approximate delivery time. It’s also helpful to be aware of physical manifestations — such as post-match queuing for the Tube — to train ourselves to look for flow, inhibitors to it and ways to manage it.