Five days a week, twice a day, 25 million commuters flow into the UK's major cities and travel home again, like a giant lung breathing in and out. Transport capillaries turn into veins and the veins turn into arteries, creating a sea of half the country's employable population on the move - alert and focused in the morning, relaxed and more open to advertising on the way home.
Eric Newnham, global chief executive of Kinetic, describes the mobile workforce as "a super Coronation Street" - a consistent, mass audience that other media would give their right arm for. "Commuters are repetitive," he says. "They find the most efficient journey and stick to it, and this allows us to relate to them over time."
But although the audience is mass and their habits predictable, communicating with commuters is not as simple as it first appears. For example, advertising revenue on the London Underground may match that of The Mail on Sunday at £100m per year, but 67% of London commuters never use the Tube to get to work.
Newnham, a seasoned commuter with 35 years of working life on his CV, travels by train between Didcot Parkway and Paddington twice a day. Even on his straightforward 55-minute commute, he encounters ads on station billboards, in free and paid-for papers, on the sides of buses and taxis, in phone boxes and on sandwich boards. And all this before he has sat down at his desk to read City AM.
"You have to think about what makes a city tick," he says. "You might have a naïve point of view that the Underground dominates commuting in London, but if you live south of the river, the south-east rail network takes over. The key to a good strategy is micro-targeting urban villages."
Commuting trends include the vogue for cycling to work, an increase in the number of city centre trams and a rise in long-distance commutes, with one in 25 commuters now travelling more than 60 miles (each way) to work. But the most significant change in the out-of-home market over the past five years has been the increase in digital sites, which are now creeping out from the Underground to adorn the sides of roads as well.
Newnham says: "In September, we had Ocean Outdoor's new cross-route digital site across the M41, the first to straddle a motorway (see box, right). The recession seems to have relaxed local authorities' planning restrictions, and if this is allowed on one motorway, you can argue the case anywhere. Ocean's site and the Westfield sites are the first big digital roadside panels and this is the start of many more to come."
But while digital media has made OOH inventory "richer and tidier", neither media agencies or media owners are up to speed with the technology. "There needs to be a big catch-up," says Newnham.
"There is a lack of understanding about how the commuter sees things and a one-size-fits-all approach in the agency world. Planners should serve different creative content over the course of the day, and there should be far more collaboration with media owners in press, TV and online."
Digital allows outdoor to become the star of campaigns it would previously have been excluded from, such as the strategy to fix Heathrow Terminal 5's PR disaster, which used digital formats on the London Underground to post daily updates to show the terminal was running smoothly. Newnham says: "Digital allows you to reach people when it is most important. Since commuters have regular habits, being able to tell a story over a period of time is really useful."
Mobile is also becoming part and parcel of outdoor, as the rise of wi-fi zones makes download speeds faster and allows advertisers to send greater volumes of rich data. The "perfect interface", says Newnham, is giving consumers information offline to allow them to use online more efficiently.
Over March and April, for example, hairdresser Essensuals ran a call to action on posters inviting customers to text in for a 20% discount and used locational knowledge of the phone to text back details of their nearest salon and a mobile internet link to a map of their nearest store.
Looking ahead to how this might influence the outdoor trading model, Newnham imagines situations where clients are given free media in exchange for new customers introduced, with the outdoor agency rewarded by the success of the campaign. For example, a magazine such as The Economist could introduce a subscription number into its call-to-action ad, the consumer receives a link to the subscription homepage via text and the agency takes a cut of the new subscriptions generated.
Newnham says: "Call to action is going to be big, and we might be able to find the perfect world where we don't charge clients for media at all, but take a cut of transactions instead. There are many conversations about cost-per-acquisition and outdoor would have been excluded until now. This is an ideal world to go into, but you need to know what works and what doesn't before you take the risk."
One related opportunity, following the introduction of mobile coverage on the Glasgow Subway - a first for the UK, negotiated for O2 customers by Arqiva - is the rise of interactive tube ads.
Newnham says: "If you have time on your hands and a mobile phone, the tube card is a great call to action." And as rail provider National Express East Anglia tests digital screens between train carriages, Newnham predicts digital screens will appear between carriages on the Underground, as in Shanghai, where state broadcaster CCTV owns digital inventory on buses and the subway.
In America, CBS Outdoor's OOH screen networks carry content from CBS television stations, blurring the boundaries between TV and outdoor. If outdoor sites carry more time-specific ads showing moving images, Newnham believes outdoor planners could start to function like TV traders, buying "airtime" and assigning the right clients to the right daypart: for example, Unilever in the morning and Carling in the evening.
In an ideal world, targeting commuters would be a seamless experience, with media owners collaborating to follow travellers' paths from breakfast TV to a cover-wrap on Metro to the bus and roadside ads they see on their route to work. "It is a planner's dream," Newnham says. "The key is to be topical and timeworthy and know how the commuter lung works - there is no catch-all solution."
Top six sites for targeting commuters
1 West Cross Route, Shepherds Bush
Ocean Outdoor's new West Cross Route site is the first of its kind: a set of giant LED screens across a four-lane dual carriageway near the Westfield shopping centre, west London. One slot in the advertising loop costs £15,000.
2 Chester Road, Manchester
Clear Channel's four backlit 96-sheets at the junction of Chester Road and the Mancunian Way target traffic travelling between central Manchester and Old Trafford. Two-week campaigns cost between £10,000 and £12,000.
3 Five Ways Exit Corridor, Oxford Circus
CBS Outdoor's Five Ways site dominates the corridors of one of the UK's busiest Tube stations. The sites allow advertisers to reach 2.8 million people over a two-week period - £88,550 well-spent.
4 Premiere 450 billboards, Glasgow
The architecturally designed Sail structure on Crown Street is one of JCDecaux's six Premiere 450 billboards in Glasgow. A two-week campaign costs £6,500 per billboard, delivering 6.1 million commuter impacts.
5 The Colossus, Clapham Junction
Titan's Clapham Colossus is Europe's biggest backlit site, located at the UK's busiest railway station. It costs £126,000 per month plus production costs, but demand is so high it is sold out until 2010.
6 M6 Towers, Birmingham
Signature's two mega six-sheet towers on the M6 are each visible to about 670,000 drivers travelling in and out of Birmingham each week. T-Mobile and Stella have recently booked £18,000 two-week campaigns.