It is one of those jokes that appears in e-mail inboxes with tiresome regularity: Bill Gates says that if the car industry had kept up with the computer industry, we'd be driving £25 cars which did 1,000 miles to the gallon; General Motors responds by saying that if they built cars like Microsoft, they would crash about twice a day.

Life imitated art at the WWW2006 conference in Edinburgh last week, where Mary Ann Davidson, the chief security officer at Oracle (the largest database company in the world) started making comparisons between the computer industry and the construction industry, as part of a critique of current standards in software engineering.

"What if civil engineers built bridges the way developers write code?" she asked. "What would happen is that you would get the blue bridge of death appearing on your highway in the morning."

It's an interesting comparison - but to get useful answers, we have to ask it the other way round, as presumably that's what she's pushing for. So, what if developers wrote code the way civil engineers build bridges?

The first thing to notice about bridge components and bridges is that you can't really use them in more than one place. If another bridge across the Severn is needed, commuters in Fife might be quite irate if you borrowed the Forth Road Bridge to do the job. Code, on the other hand, is extremely easy to reuse. In software, you would just wrap the bridge-making code up in a procedure called makeSuspensionBridge() and call it again.

The practical result of this, whenever a software developer writes new code, is that it is usually because they need to do something that's never been done before - or, at least, not quite in that way - whereas bridges which use new construction techniques are few and far between. Words like "tried and tested" are music to the ears of engineers. Boring is comforting. So the first thing software built like a bridge would be is uninnovative - about the same as what you had before.

The second important thing about bridges is that they don't go up or come down overnight. The proposed Straits of Messina bridge from Italy to Sicily has been in planning for decades. It's about as cutting edge as it gets - when and if completed, it may well be the longest span suspension bridge in the world. The construction alone will take six years and cost 4.6 billion euros, and its lifespan is over a century.

Software, by contrast, is built fast and changes quickly. The voice-over-Internet program Skype went from first public beta to version 1.0 in less than a year; it's now at version 2.5 (beta, naturally) and has gained a raft of new features such as video conferencing, voicemail and SMS. Can you imagine a bridge in beta? "Save time on your commute! But make sure you dodge the holes in the deck..." Yet in six years, an age in internet time, an entirely different system may be in control in the VoIP space (hopefully one built on open protocols, but that's another article).

So, what would happen if developers wrote code the way civil engineers build bridges? The answer is that you'd end up paying millions and waiting decades for software not much different from what you had already, and then you'd have to use it for a lifetime. Fundamentally, no one wants software like that.

That's not to say that the software industry hasn't got quality problems; in some segments people are demanding better software, and the status quo is changing. The browser market is a good example here, as Firefox's reputation for security drives its replacement of Internet Explorer. But it took engineers from the 1879 Tay Bridge disaster until Tacoma Narrows in 1940 to really get the knack of bridge building. By that measure, Mary Ann needs to accept that we still have about 30 years to go.

Gervase Markham works for the Mozilla Foundation, a non-profit organisation dedicated to promoting choice and innovation on the internet. His blog is Hacking For Christ.

Read his previous article for Times Online here

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