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Agile Case Study: Kingston Bridge

Updated: Dec 23, 2023

Old Postcard of Kingston Bridge

The story, all names, characters, and incidents portrayed in this story are fictitious. No identification with actual persons (living or deceased), places, buildings, and products is intended or should be inferred.

The Mayor of Kingston Town

It is the early middle ages and Tom Sawyer - the newly elected Mayor of Kingston Town - is sat with his team; pondering how to achieve his big election promise: a bridge across the river. Local residents are all-too-aware of Richmond's own plans to do the same. Much is at stake. At present there is no bridge across the Thames between Staines and London Bridge. Whoever completes their bridge first will surely substantially grow their Town into a major market hub. His Town Planner has two proposals on the table; and it's going to be a long night!

The Bridge Proposals

  1. A Pontoon Bridge - the cheapest and fastest plan would see some older boats chained together at Kingston Reach, with a rudimentary wooden road surface laid across them. The Planner points to the benefits of a rapid prototype. With a Pontoon, Tom can test whether (in fact) there is any real demand for the bridge at all. He can also most certainly beat Richmond to the prize of being first. This - in itself - might kill off the competition altogether; with Kingston becoming the only Market Town in the region. His Business Chief favours the option. On the downside, a pontoon will be vulnerable to flooding and could turn out to be completely undersized for the demand; particularly if merchants wish to bring heavy loads.

  2. A Stone Bridge - his Engineer recommends this option. The whole build will take over 3 years; particularly given the extensive surveying of the river floor required and the lengthily driving of concrete piles. All this before a stone is even laid! He really worries that Richmond will get there first and/or his own term in office might even end before the bridge is complete! And what if no-one wants to use it... and it then stands idle as a hideous white elephant that has bankrupted the town!? On the upside, such a bridge would be capable of dealing with almost any weather, any level of demand, and any loads placed upon it - and last for hundreds of years. A real legacy!

Choosing the Right SDLC

Tom's problem - in a nutshell - is whether to go agile or waterfall. And you can see - it is a genuinely difficult choice! The Agile option looks genuinely compelling, as the first prototype could be thrown up so fast. You could always replace it later with a larger pontoon or even a fully wooden bridge. Then only move to a stone or cast iron option later; once the case is clear. On the other hand, if you did decide (in the end) that a stone bridge was needed, you would have spent a lot more overall and arguably lost a lot of time in getting started on the right project. So what would you do? Close your eyes. Don't read on. And say out loud your choice!

The Cheshire Cat giving directions to Alice

Big Design up-front

What Tom actually decides is that he doesn't have enough information to make the right decision! Most wise! He asks his Business Chief to undertake more extensive surveying of local merchants; so as to better gauge their likely levels of demand. He asks his Engineer to assess flood risks and visit Staines and look into their own experiences with (renovations to) the old Roman bridge there. Some weeks later, the results come back and Tom knows what he needs to do. Stone bridge it is. And the rest, as they say, is history.

Agile 'Discovery' Phase

Tom's story is not so very rare, really. This sort of dynamic plays out time and time again in the real world. Much as it would be great to run your entire life as if every day is your first - where you emerge blinking into the sunlight, with a zero-requirements Kanban board each morning - most human endeavour would benefit from some proper facts, analysis, and design up-front. Most businesses call this a 'discovery phase' - the moral of the story, if you like!

Tom might have concluded, from the discovery phase, to go pontoon bridge (if, for example, the merchant feedback had been less positive). From that point, the rest of the project would have been agile in nature. Either way, this discovery phase was waterfall. Whatever you might say! I call this "tri-modal": Waterfall to the end of design, then agile thereafter. The reality is most agile projects are like this.

> I explore this concept in more detail in my "tri-modal projects" article


A silly little story, I know. But deliberately chosen. Waterfall, by origin, is fundamentally grounded in civil and electrical engineering and emerged from the world of complex, multi-year, safety-critical aerospace programmes. Not so very different from putting up a stone bridge. Setting up a new market with new products in a new town centre, by contrast, is absolutely the world of agile: Uncertain requirements that change frequently (and would benefit from iterative prototyping). Making the choice on the right SDLC can (and really should) be a difficult one.

> For a tool to help you decide, check out my Agile-o-meter

© David Viney 2023. Licensed for re-use with attribution under CC BY 4.0

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The <a href="">Agile Case Study: Kingston Bridge</a>, by <a href="">David Viney</a>, licensed under <a href="">CC BY 4.0</a>


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