Imagine a motorway between Starttown and Finishcity. When it all flows it takes exactly 3 hours to get from start to finish for a car, a little longer for a lorry, and a little shorter for a motorcycle, so 3 hours ± 15 minutes. It has 3 lanes in each direction for its entire length.
Things go well for a time, but demand eventually creeps up due to Finishcity being named the City of Culture, and so we start getting regular tailbacks due to “Sheer weight of traffic.” All of the traffic now tends to have an extra hour to get there in rush hour. The number of cars per hour arriving at Finishcity at peak rush hour times doesn’t go up, it just seems that rush hour is now two hours long instead of one hour. Pollution and fuel consumption is also up as every vehicle is burning fuel for 4 hours instead of 3.
a system of fixed capacity has an optimum flow rate. Pushing more stuff through will not result in an increase in that flow rate, it will just make everything going through that system take longer to get through.
the longer something is in progress, the more cost there is (petrol in cars, effort in knowledge work) to service that same need.
So how do we solve the problem?
Well we can try to increase capacity, but that will take at least 2 years, mean more demand - all of the work traffic in addition to what we already have, and actually reduce capacity for the 2 years we are working on the roads.
Shape the Demand?
We can put a toll booth on the front of the road, charging road users to use the road in peak hours (a bit like train tickets being more expensive in commuter times), thus reducing all but essential demand during the peak hours. Or we could use traffic lights on the slip roads to join our motorway, thus causing queues back to the roundabouts at the junctions, and causing drivers to baulk at joining the queue and use a different route instead.
So we’ve shaped our demand, and that has helped, and journey times are back to 3 hours most of the time. The neighbouring A roads are taking a bit of a hit on traffic increase, but at least the motorway is flowing again. Phew.
A lorry has jack-knifed, and blocked all 3 lanes northbound! The whole road is stopped! What do we do?
Well we now have increased demand, we need to get fire engines, ambulances and police to the scene as soon as we can, but how can we do that when there is no way through the traffic?
Luke, use the hard shoulder!
The what? you mean that our motorway that had all of those traffic problems that could have been solved by having an extra lane, had an extra lane all the time but people weren’t allowed to use it? We are saving 25% of our entire capacity to allow us to have an emergency capability? This is what we really do on our roads. We value the emergency capability so much we throw 25% of our capacity at it.
In a Kanban system we will sometimes have an Expedite lane. This is like the hard shoulder. No-one can use use it unless they meet some pre defined conditions and rules. They are very expensive, but the cost doesn’t show on the work items that go down the expedite lane, the cost is hidden and paid by every other piece of work that is put on hold while the expedite item is worked on. The same is true on the motorway, the cost of having the 4th lane protected for emergencies is in the traffic that has to queue in the other 3 when the 4th sits idle.
Hang on a minute...
Don’t some motorways have peak running for hard shoulders now? Yes they do, and this is more like how we do expedite in real life. We don’t keep 1/4 of our team members sitting idle in case something expedite class comes in, they all work on standard work items until they are needed to work on expedite. The cost is then only bourn when an exceptional event happens and the traffic is moved out of that lane to make an on demand emergency capability.
In real life one way to do this is to have a rule where some of (usually your best) people cannot ever pick up a work item of their own, but can pair or swarm on a work item someone else ‘owns’. That way if expedite work comes in, you can move your best people onto that without having to block some other work up, unless they need help of course. It also means that your best people are working with and helping your other people more often, which is a motivator for both sides as one gets to show mastery, and the other gets to increase their mastery (see Dan Pink’s book Drive for more on that).