We Should Dig Tunnels for Streets, Not Roads
Good news! The year is 2037 and famously eccentric billionaire and shit poster extraordinaire Helen Husk has succeeded in her goal of making tunnelling very cheap, and now her nonsensically named venture, The Exciting Company, is busy relieving star-struck US mayors of their hard-earned transport budgets in order to move traffic jams and suburban sprawl underground.
So far the slightly less credulous town planners and politicians of Europe have shown more hesitance towards the idea of spending limited budgets on rebuilding motorways under cities. However, Husk's trademark formula of first principles engineering, vertical integration, scale, and grinding her employees into dust is starting to make this breakthrough tunnelling technology too cheap to ignore. So how can Europe, and specifically London, best use them?
Because of Husk's outspoken belief that anyone who sets foot on public transport will be stabbed, which arguably has something to do with her upbringing at the richer end of the most unequal society on the planet, The Exciting Company's tunnels have a diameter of 3.66m to dissuade anyone from trying to squeeze anything larger than an oversized pickup truck down them. Luckily though, London has a long history of squeezing trains with four-digit passenger numbers down tunnels with diameters as small as 3.55m. So this new generation of luxuriously large 3.66m tunnels should give the city a lot of options.
I think the most exciting use case has the potential to solve a fundamental tension that exists in cities today. Right now a lot of busy streets are unpleasant places to be because they are also busy roads. A street is a destination, a place people head to to shop, eat, drink and meander. A road is a way of getting to a destination. Attempts to combine the two are doomed to serve neither function as well as they could, with North American style stroads being the nadir of this phenomenon. Removing private cars from these roads gives a large benefit with minimal downside, the same is true of taxis. But people who complain about the removal of buses have a good point. A street with buses running down it can't realise its full potential. Pedestrians are constrained to less than half of the available space, while cyclists are often forced to an even smaller fraction of the available space often sandwiched, terrifyingly, between buses.
The ur examples of this problem in London are Regent Street and Oxford Street. They are the busiest streets in London, vanquishing cars from them can't be done quickly enough. Vanquishing buses would free up an awful lot of space, but it's not at all clear where the buses would go instead. Even if this problem can be solved, and I'm sure it can, the steets are full of people, and people need to get on buses. Sending confused tourists and suffering locals off to back streets to catch a bus is fundamentally undesirable.
But what if, instead of sending buses careening down streets full of unprotected people, we pedestrianise the streets, dig two tunnels under each of them, and send the buses porpoising underground instead. They could then re-emerge every few hundred metres to pick up passengers, then descend again. There would need to be two bus stops per pick up point, one for each direction, and these will take up space, but they can be staggered to make sure that they don't take up the majority of the street's width at any single point. When a bus gets to the end of a tunnelled section, probably at a junction, there should be a traffic light system that gives it priority as it merges back into the rest of the road network.
This system would definitely put limitations on the buses that could be used on tunnelled routes. To reduce the burden of ventilation they would need to be either trolleybuses or battery powered. Double decker buses are obviously out, but these buses would have to have a smaller cross section than the vast majority of single-decker buses too. To make up for the loss of double deckers, the city would probably want to replace them with articulated buses (again), and to make up for the fact that there will be no room for buses to overtake each other at bus stops they might want as many as three doors that open on each side so that passengers can board and disembark as quickly as possible and avoid backlogs.
At this point you may have noticed that these buses are starting to look a lot like trams. And in most cases that's what you'd probably want them to be. But there's no reason electric buses and trams can't share the space as routes require. But most importantly, under no circumstances should black cabs be allowed into the tunnels.
More generally this approach to tunnels could be summarised as digging under obstacles to give public transport quicker and more useful routes while freeing up surface-level space. So far I've only discussed this in the context of pedestrianised streets. But one could equally imagine digging under the Thames to free up valuable bridge space for pedestrians and cyclists. In fact in this case one might want to invert the logic of high street tunnels and allow public transport to go over bridges while sending cars into their unhappy tunnels. Parks are another kind of obstacle that could be avoided; you could replace a park-bisecting road with a tunnel for public transport while sending cars around the perimeter. Having said that, grassy tram tracks are among the most pleasing bits of public transport infrastructure in the world, so where buses aren't needed they might well be preferable, and would almost certainly be cheaper.
If we could dig these tunnels for £1/km all of these plans would surely be worth doing. If it cost £1bn/km they surely would not. For each street that aspires to be useful to pedestrians there is a price between these two numbers below which this scheme makes sense. For Oxford Street and Regent Street this price is probably quite high, but as the cost of tunnelling falls there are more and more high streets where this can be viable. Gracechurch Street/Bishopsgate is so utterly hellish for cyclists I'd suggest it should be next in line. But "lesser" high streets in Clapham, Brixton, Peckham, Hackney, Angel, and other places in London that I don't know well enough to list would benefit from this too at the right price. It should also be useful for other parts of the country, the continent, and indeed anywhere in the world that cares about walkability. And who knows, if it's successful enough, maybe even North America will follow suit.