A good idea.

However, the cynic in me envisages a scenario whereby all the usable underground space has been exploited, hence returning us to the status quo and seeking to build skyward again.

Rob Cheeseman, Derby, UK

I lived in Luxemburg for a few months in 1981. They built a multi-storey car park underground whilst I was there, and replaced all the grass, shrubs and flowers obove it once they'd finished. I've also been in underground multi-storey car parks in Germany. I can't see what the problem is in this country.

David Leslie, Perth, Scotland

Is there any room down there, amid the spaghetti of sewers, tube tunnels, service tunnels, ducting etc. And what about the danger of flooding? I thought global warming posed a risk of floods in future years even above ground.

Barry, Wallington, UK

I noticed a lot of urban motorways in tunnels in other European cities such as Brussels, Santa Cruz and las Palmas in the Canary islands and Oslo. The traffic flows freely belwo gorund and the surface has large public squares or maybe small, quiet suburban streets with lots of car parking.

Luke Nicolaides, London, UK

Tunnesl are good in some ways, but just wait until you have a crash, even a small one. For a big one, just think Mont Blanc and you see the problems.

John, Knutsford, UK

"We could, for instance, drop the dual carriageway that currently blights the north side of the Thames into a tunnel below"
Actually, we couldn't, because the District Line is already there under the Victoria Embankment between Blackfriars and Westminster, along with one of Bazalgette's main sewers.
I doubt the security services would like the idea of a tunnel underneath Buckingham Palace Gardens.
The Post Office should be made to reopen their underground railway, to keep their vans off the streets. Maybe TfL could fund it, as it would help reduce congestion. Unfortunately the southern end of the Kingsway tram tunnel under Waterloo Bridge was blocked when they built the Aldwych underpass, so reopening it isn't possible.
Those bits of the Tube which run in open cuttings (much of the Circle, District, Met and H&C lines) in central London could be covered over relatively easily with parks or public spaces.

Nick, London,

It is not Madrid Calle 30. Actually, the name is M-30, one of the peripheric motorways of the capital and undoubtedly the busiest. Now, parks and green areas cover the once noisy and polluted road. A wonder of the modern engineering, its 60 km include some of the longest and safest urban tunnels in the world. And it is not in its finishing touches. It was completed and inaugurated by the Mayor of Madrid last year.

Fernando, Madrid, Spain

It would be a little hard to tunnel under the embankment - the only reason it is there is because it's built on top of the sewers.

But why not pedestrianise Oxford Street and run a free underground hop-on, hop-off service along the abandoned Royal Mail line. It would take a load off the Central line and all the busses could terminate at Oxford Street.

Damian, London, UK

Perhaps there's no money to be made from roads going underground. Festooning what was once a beautiful capital city, with banks of ugly camera gantries and charging and fining people might even be a thing of the past.....I don't think so,with our pigmy politicians myopia and thirst for power and riches.

Josh Martin, Oxford, Gt. Britain

There is a simple reason why we don't dig. Cost.

Ground works are (a) very expensive and (b) risky. The problem is that you never know what you might find (underground rivers, undocumented wells or sewers, old building foundations, UXBs, archaeological sites, contamination).

Soil removed is also expensive to "dispose of" and may be treated and taxed as waste (even more so if contaminated).

Even when you have committed yourself to the cost and risk you are faced with the problem that London has looked downwards for so long. Crossrail was notoriously hampered by the fact that the Government refused to disclose to Cross-Rail what lay beneath Westminster. They had to submit a route blind and be told to go back and try again without knowing what the problem was.

Nice idea though. Burying traffic is a better idea than previous efforts to echo Hong Kong by transferring pedestrians to elevated walkways (as the London Wall walkway attempts)

Simon, London,

London is one of my favourite places in the world but I've never seen the problem with building upwards. Unlike some other European capitals, quite a lot of London was built post-war and doesn't particularly need preserving.

tony, rochester,

Absolutely. How else do we separate cars and people? Currently London is suffocating with congestion, made worse by putting in as many traffic lights as possible on the logic of making roads safer. The net effect is stationary vehicles and a not so pleasant experience for anyone on foot. If key arterial roads were put under major green spaces (Clapham Common, Hyde Park, Blackheath, ...) this would allow the roads to run fluidly, solving some of our congestion issues, whilst providing safer and fresher open green spaces. And with space such an issue, why not build arterial roads on top of railway lines? A little imagination and London could far better use its fantastic inheritance. But then again, with design by committee, is that ever likely to happen?

Harry, London,

While it's an interesting idea that is certainly worth looking at, you might want to avoid mentioning Boston's "Big Dig" when making your case. Massive cost overruns, years of delays, and horrendous transport disruption made Bostonians a very disagreeable bunch for a decade or so - and they weren't that agreeable to begin with!

C Heathcote, Tonbridge,

A 23 mile tunnel to get from Whitechapel to Paddington? No wonder the post takes so long.

Jonathan, London,

The charm of London was its lack of skyscrapers and not being a clone of so many other cities, with their giant tombstone skies.
Going underground has good possibilities. I have always enjojed shopping underground.

margie, victoria, australia

Why not copy the Japanese and the UAE and build a few artificial islands to relieve your space problems? Things like airports and prisons take up a lot of room, and don't need to be on the mainland. Transfer a few prisons and or/airports to artificial islands, knock down the ones taking up space on the mainland and replace with parks and housing. You could recoup some of the costs of building the islands by selling formerly occupied public land to developers, and it would create a lot of direct and indirect jobs -- both in building the islands and in building on the formerly occupied land. .

MaryJ , San Francisco , Calif.

I lived in Toronto for 14 years. PATH is really quite amazing. Go for it, London!

Peter Orme, Nanaimo, BC , Canada