Consumers are captive in hospitals and airports. However, they are also in a rush and need to grab things on the run in transit hubs.

One of the newest areas is Westfield's retail hub in the World Trade Centre in New York.

While many trains stations in Asia double as retail hubs, the concept is still being worked on elsewhere around the globe.

New York's World Trade Centre Transportation Hub includes the Santiago Calatrava-designed main hall, the Oculus, which has opened this month.

Twelve years and $US4.4 billion ($5.7 billion) in the making, the hub is not yet complete, including a retail section.

However, on completion, it could be the core hub for Lower Manhattan.

It's more a product of space. Asian countries are generally warmer so subterranean activity is popular to escape humidity and tropical downpours. The same thing is happening in colder climates.

However, in areas of Europe, the underground train stations are small and offer convenience food rather than an array of services such as those found in Hong Kong, Singapore or Japan.

Shopping in airports is a major growth opportunity for retailers, and is being well-supported by consumers. Airport retailing poses a threat to CBD and large regional shopping centres.

Colliers International reports strong demand from quality domestic and international retailers for airport terminal space.

With offshore travel strong and domestic markets improving, the number of people moving through airports has reached record levels. Retailers are taking advantage of those numbers and opening flagship stores in terminals.

Melbourne Airport will have a more modern international terminal as a revamp of the T2 precinct is under way. It will include new retail outlets and a food court.

Sydney Airport houses about 200 stores and an upgrade continues under its 20-year master plan.

Meanwhile, major supermarket operators are considering smaller format stores close to train stations/transport hubs with high traffic thoroughfare.

A new CBRE report shows one-fifth of brands from the Americas, and Europe, the Middle East and Africa are targeting travel hubs (such as airport and railway terminals) as an emerging format for expansion.

Recent research by Knight Frank shows that with Australia's capital cities forecast to add 3.2 million citizens in the next 10 years, new mass transit systems and faster connections to markets will be needed.

Sydney will lead the way as transport infrastructure projects improve accessibility, create business clusters, drive real estate opportunities, trigger demographic movement and push a need for higher densities.

Greater Sydney is home to more than 4.8 million residents and more than 470,000 businesses, most of which rely on the city's transport infrastructure system in terms of employment movement and the transfer of services.

With a further 820,000 residents expected to call Sydney home in the next 10 years (ABS medium series projections), Sydney's infrastructure network will need to continually evolve to keep pace with the anticipated population growth.

Knight Frank's associate director, research and consulting, Paul Savitz, said transport was critical to the efficient functioning and quality of life of Sydney and its residents.

"Poor or reduced accessibility can be a major constraint on the success and quality of places, their neighbourhoods and communities. Central to Sydney's future are a number of key infrastructure projects," he said.