Difficult Urban Geography Part 1: Narrow Streets

I did a complex Patreon poll about series to write about. People voted for general transit network design, and more posts about national traditions of transit in the mold of the one about the US. Then I polled options for transit network design. There were six options, and people could vote up or down on any. Difficult urban geography was by far the most wanted, and three more alternatives hover at the 50% mark. To give the winning option its due course, I’m making it a mini-series of its own.

There are cities that, due to their street layout, make it easy to run transit on them. Maybe they are flat and have rivers that are easy to bridge or tunnel under. Maybe they have a wealth of wide arterials serving the center, with major cross streets at exactly the right places for stations and an underlying bus grid. Maybe they spread out evenly from the center so that it’s easy to run symmetric lines. Maybe their legacy mainline rail network is such that it’s easy to run interpolating buses and urban rail lines.

And then there are cities that are the exact opposite. In this post I’m going to focus on narrow or winding streets and what they mean for both surface and rapid transit. The good fortune for transit planners is that the city that invented urban rapid transit, London, is a prime example of difficult urban geography, so railway engineers have had to deal with this question for about 150 years, inventing some of the necessary technology in the process.

Rapid transit with narrow streets

The easiest ways to build rapid transit are to put it on a viaduct and to bury it using cut-and-cover tunneling. Both have a minimum street width for the right-of-way – an el requires about 10 meters, but will permanently darken the street if it is not much wider, and a tunnel requires about 10 meters for the tracks but closer to 18 for the stations.

Nonetheless, even cities with narrow streets tend to have enough streets of the required width. What they don’t always have is streets of the required width that are straight and form coherent spines. The labyrinth that is Central London does have wide enough streets for cut-and-cover, but they are not continuous and often miss key destinations such as major train stations. The Metropolitan line could tunnel under Euston Road, but the road’s natural continuation into the City is not so wide, forcing the line to carve a trench into Farringdon. Likewise, the District line could tunnel under Brompton Road or King’s Road, but serving Victoria and then Westminster would have required some sharp curves, so the District Railway carved a right-of-way, demolishing expensive Kensington buildings at great expense.

While London is the ur-example, as the city that invented the subway, this situation is common in other cities with large premodern cores, such as Rome, Milan, and Istanbul. Paris only avoided this problem because of Haussmann’s destruction of much of the historic city, carving new boulevards for aesthetics and sewer installation, which bequeathed the Third Republic a capital rich in wide streets for Metro construction.

Dealing with this problem requires one of several solutions, none great:

Deep-bore tunneling

London’s solution was to invent the tunnel boring machine to dig deep Tube lines, avoiding surface street disruption. With electric-powered trains and reliable enough TBMs to bore holes without cave-ins, London opened the Northern line in 1890, crossing the Thames to provide rapid transit service to South London. Subsequently, London has built nearly all Underground lines bored, even in suburban areas where it could have used cut-and-cover.

The main advantage to TBMs is that they avoid surface disruption entirely. Most first-world cities use them to bore tunnels between stations, only building stations cut-and-cover. The problem is that TBMs are more expensive to use than cut-and-cover today. While turn-of-the-century London built Tube lines for about the same cost per km as the Metropolitan line and as the cut-and-cover Paris Metro and New York City Subway, in the last half century or so the cost of boring has risen faster than that of shallow construction.

The worst is when the stations have to be mined as well. Mining stations has led to cost blowouts in New York (where it was gone gratuitously) and on London Crossrail (where it is unavoidable as the tunnel passes under the older Underground network). A city that cannot use cut-and-cover tunnels needs to figure out station locations that are easily accessible for vertical digging.

The alternative is the large-diameter TBM. Barcelona is using this technique for Line 9/10, which passes under the older lines; the city has a grid of wide boulevards, but the line would still have to pass under the older metro network, forcing the most difficult parts to be deep underground. The large-diameter TBM reduces the extent of construction outside the TBM to just an elevator bank, which can be dug in a separate vertical TBM; if higher capacity is desired, it’s harder but still possible to dig slant bores for escalators. The problem is that this raises construction costs, making it a least bad solution rather than a good one; Barcelona L9, cheap by most global standards, is still expensive by Spanish ones.

Carving new streets

Before the 1880s, London could not bore the Underground, because the steam-powered trains would need to be close to surface for ventilation. Both the Metropolitan and District lines required carving new right-of-way when streets did not exist; arguably, the entire District line was built this way, as its inner segment was built simultaneously with the Victoria Embankment, under which it runs. The same issue happened in New York in the 1910s and again in the 1920s: while most of the city is replete with straight, wide throughfares, Greenwich Village is not, which forced the 1/2/3 to carve what is now Seventh Avenue South and later the A/C/E to carve the southern portions of Sixth Avenue.

This solution is useful mostly when there are wide streets with absolutely nothing between them that a subway could use. The reason is that demolishing buildings is expensive, except in very poor or peripheral areas, and usually rapid transit has to run to a CBD to be viable. If the entire route is hard to dig, a TBM is a better solution, but if there are brief narrows, carving new streets New York did could be useful, especially if paired with improvements in surface transit.

Looking for station sites

Milan built its first metro lines cut-and-cover. However, lacking wide streets, it had to modify the method for use in a constrained environment. Instead of digging the entire street at a sloped angle and only then adding retaining walls, Milan had to dig the retaining walls first, allowing it to dig up streets not much wider than two tracks side by side. This method proved inexpensive: if I understand this article right, the cost was 30 billion lire in 1957-1964 prices, which is €423 million in 2018 prices, or €35 million per km. Milan’s subsequent construction costs have remained low, even with the use of a TBM for Metro Line 5.

The problem with this method is that, while it permits digging tunnels under narrow medieval streets, it does not permit digging stations under the same streets. Milan is fortunate that its historical center is rich in piazzas, which offer space for bigger digs. One can check on a satellite map that every station on Lines 1 and 2 in city center is at a piazza or under a wide street segment; lacking the same access to easy station sites, Line 3 had to be built deeper, with tracks stacked one under the other to save space.

I have argued in comments that Paris could have used this trick of looking for less constrained sites for stations when it built Metro Line 1, permitting four tracks as in New York as long as the express stations under Rue de Rivoli stuck to major squares like Chatelet. However, Paris, too, is rich in squares, it just happens to be equally rich in wide streets so that it did not need to use the Milan method. London is not so fortunate – its only equivalent of Milan’s piazzas is small gardens away from major streets. It could never have built the Central line using the Milan method, and even the Piccadilly line, which partly passes under wide streets, would have been doubtful.

Surface transit

Rapid transit benefits from being able to modify the shape of the street network to suit its needs. Surface transit in theory could do the same, running in short tunnels or widening streets as necessary, but the value of surface modes is not enough to justify the capital expense and disruption. Thus, planners must take the street network as it is given. The ideal surface transit route runs in the street median on two dedicated lanes, with boarding islands at stops; creating a parking lane, a moving lane, and a transit lane in each direction on a street plus some allowance for sidewalks requires about 30 meters of street width or not much less. Below 25, compromises are unavoidable.

Cutting car lanes

A lane is about 3 meters wide, so removing the parking lanes reduces the minimum required street width by about 6 meters. Contraflow lanes instead allow the street to have the same four lanes, but with a moving lane and a parking lane in one direction only. In extreme cases it’s possible to get rid of the cars entirely; a transit mall is viable down to maybe 12-15 meters of street width. The problem is that deliveries get complicated if the city doesn’t have alleyways or good side street access, and this may force compromises on hours of service (perhaps transit doesn’t get dedicated lanes all day) or at least one parking lane in one direction.

Single-track streetcars

Some city cores with very narrow streets don’t have double-track streetcars. A few have one-way pairs, but more common is single-track segments, or segments with two overlapping tracks so that no switching is needed but trams still can’t pass each other. Needless to say, single-tracking is only viable over short narrows between wider streets, and only when the network is punctual enough that trams can be scheduled not to conflict.

On longer stretches without enough room for two tracks or two lanes, one-way pairs are unavoidable; these complicate the network, and unless the streets the two directions of the bus or tram run on are very close to each other they also complicate interchanges between routes. New York has many one-way pairs on its bus network, even on wide and medium-width streets in order to improve the flow of car traffic, and as a result, some crosstown routes, such as the B35 on Church, are forced to stop every 250 meters even when running limited-stop. While New York’s network complexity is the result of bad priorities and can be reversed, cities with premodern street networks may not even have consistent one-way pairs with two parallel streets on a grid; New York itself has such a network in Lower Manhattan.

Bus network redesign

The best way to avoid the pain associated with running buses on streets that are not designed for fast all-mode travel is not to run buses on such streets. Boston has very little surface transit in city center, making passengers transfer to the subway. In Barcelona, part of the impetus for Nova Xarxa was removing buses from the historic core with its narrow streets and traffic congestion and instead running them on the grid of the Eixample, where they would not only provide a frequent system with easy transfers but also run faster than the old radial network.

However, this runs into two snags. First, there must be some radial rapid transit network to make people connect to. Boston and Barcelona both have such networks, but not all cities do; Jerusalem doesn’t (it has light rail but it runs on the surface). And second, while most cities with a mixture of wide and narrow streets confine their narrow streets to premodern historic cores, some cities have streets too narrow for comfortable bus lanes even far out, for example Los Angeles, whose north-south arterials through the Westside are on the narrow side.

What not to do: shared lanes

It’s tempting for a transit agency to compromise on dedicated lanes whenever the street is too narrow to feature them while maintaining sufficient auto access. This is never a good idea, except in outlying areas with little traffic. The reason is that narrow streets fed by wide streets are precisely where there is the most congestion, and thus where the value of dedicated transit lanes is the highest.

In New York, the dedicated bus lanes installed for select bus service have sped up bus traffic by around 30 seconds per kilometer on all routes Eric Goldwyn and I have checked for our Brooklyn bus redesign project, but all of these figures are averaged over long streets. Within a given corridor, the short narrows that the transit agency decides to compromise on may well feature greater time savings from dedicated lanes than the long arterial stretch where it does set up dedicated lanes. This is almost certainly the case for the Silver Line in Boston, which has unenforced dedicated lanes most of the way on Washington Streets but then uses shared lanes through Downtown Boston, where streets are too narrow for dedicated lanes without reducing auto access.


  1. Martin

    Just adding that if you are lucky enough that the ground is solid enough rock, you can just use dynamite for both tunnels and stations. My understanding is that this is typically cheaper than cut-and-cover (maybe except on basically undeveloped land). Though you have to build pretty deep.

    • Alon Levy

      The same link I gave or the Milan method also explains what Stockholm did for T-Centralen, which was a weird four-track caisson that was neither cut-and-cover nor mined.

  2. adirondacker12800

    Milan is fortunate that its historical center is rich in piazzas, which offer space for bigger digs.

    Not always. DC’s Metro has Farragut West and Farragut North because one station would have disturbed Farragut Square.

  3. adirondacker12800

    Not always that either. Plans can change over the next few decades. That park between Chrystie Street and Forsyth Street is great for the Lower East Side. They did it so the Second Ave could come through, the Sixth Ave Station under Houston at Second is lower so the Second Ave could pass over it. Plans can change over the decades. Last thing I saw about the Second Ave that far south that the station would be under the Sixth Ave lines “to reduce community impacts” what about the community impacts of having to go down three levels instead of one? Or never getting it because it would cost too much. Having the technocrats saying “we’ll do this” then doing it had it’s problems but so does endless accommodation.

  4. Michael James

    London’s solution was to invent the tunnel boring machine to dig deep Tube lines, avoiding surface street disruption.

    I suppose that is one way to put it! But the invention of the tunneling shield was 70 years earlier, and also long before trains, and was by a resident Frenchman Marc Isambard Brunel, who took out the first patents in 1818 and then applied it to build the first Thames Tunnel at Rotherhithe. This was after the first attempt, using traditional digging methods, had run into sand and mud and was abandoned. About 30y after it was completed* it was purchased by a London train company and trains were run thru; even later it became part of London Overground and is used for this today.

    Of course Marc was the father of Isambard Kingdom Brunel who became Britain’s most celebrated engineer. He is described as British but actually he is perfectly Anglo-French in that his mother was English (Sophia Kingdom who only narrowly escaped execution by the Revolutionary Terror) and father French; more than that, Marc ensured his son got the best education and sent him to France, including Lycée Henry IV (same salubrious school–opposite the Pantheon–where Emmanuel Macron finished his high schooling). After Henry IV he was due to go to École Polytechnique but at this point the French decided he was British and not eligible! Brunel the younger also worked with his father on the Thames Tunnel.

    Curiously another famous engineer has a similar story. Joseph Bazalgette designed and built the London sewers beginning 1858. His family hailed from the tiny town of Ispagnac in the Gorge du Tarn, as do all people in the world with that surname (they seem to be a bunch of over-achievers). His appointment was facilitated by Isambard K. Brunel.

    I am sure there are lessons in this for Brexit.
    *The Thames Tunnel still had its problems due to that awful geology and the decision to bore it at a relatively shallow depth. It didn’t have any catastrophic collapses but it did have floods. Almost a century later when they put M4 under the Seine, from Leftbank to Cité, they had similar problems for the same reasons–not deep enough, presumably because of its cut-and-cover track and stations on both banks. To solve that they used liquid nitrogen to freeze the soil during tunneling.

    • Jhonny

      The cologne city archive was destroyed by a cave-in a few years ago that was officially blamed on a single worker stealing parts…

  5. ckrueger99

    It’s ironic that Philadelphia today has a large surface tramway network only because the Center City portion of it piggy-backed on the construction of the heavy rail tunnel under Market Street. The shared tunnel is too small with too tight turning radii even for today’s electric buses, let alone the GM diesel ones of the bustitution era.

  6. Ross Bleakney

    “an el requires about 10 meters, but will permanently darken the street if it is not much wider…”

    Really? I’m looking at the Canada Line on Cambie, and it looks like it is about 3, maybe 4 meters at most (https://goo.gl/maps/eg2VYUkRqUU2). It doesn’t look much wider than a car, really (if you look at all those pictures).

    Of course the platforms and the tracks take up a lot more space. So you are right, it definitely darkens the street. But to me that is a low priority in most cases. If surface running doesn’t make sense (because of the cross traffic), deep bore tunneling is too expensive, and cut and cover is too disruptive, it seems like a logical way to go.

      • Ross Bleakney

        Are you saying that the amount of space taken up by the system *at the surface* is 10 meters?

        Or are you saying that the width of the track well above the ground is somewhere around 10 meters?

        Those are two different things, and an important distinction. If an elevated line takes up *one lane* of an urban setting, but requires three or four lanes up above, that still makes it appropriate for many, many settings (such as the one in Vancouver). That means it takes up significantly less space than even surface running, thus effecting surface travel substantially less. The only thing that might match it is single track streetcar/ single lane bus, but those approaches have all sorts of disadvantages.

        I’m not arguing over when it makes sense or when it doesn’t. I’m not really arguing anything — I’m merely asking for technical clarification. How much space — on the surface — does elevated transit take? How much space does it take to put up that single concrete column?

        Followup questions: Does that vary? Didn’t it used to be much bigger, but now it has shrunk?

        • Alon Levy

          I’m saying that well above ground it’s around 10 meters. This is relevant when it comes to right-of-way width and shadows.

          • Ross Bleakney

            Sure. But I keep asking this, because to me it is way more important: How much land does it use at the street level? By my estimation it is one lane. But again, I could be wrong (or it could need to be wider in other instances). But if it really is just one lane (or even less) then it makes it the appropriate choice in many instances.

            For example, Seattle is expanding its light rail system. In several places this will include running elevated rail. This will likely either be on the road, or right next to it. Taking one lane from the road is an issue, but a small one. It is just one. Running on the side is also a small issue if you only need to take a little bit of space. But if you require taking a lot of space (e. g. 10 meters, or even 6) then the dynamic changes. Then it means taking two or even three lanes.

            The point being that taking only one lane, while providing fast, frequent, affordable mass transit seems like a great choice in many instances. The alternatives are simply worse. Surface running has to deal with crossing streets, and typically takes up two lanes. Surface running in one lane can’t be very frequent (if the lane is long). You can run the train or bus in traffic, but that is almost always a bad idea (as you mentioned). Cut and cover is very disruptive — I’m guessing more than elevated (I wouldn’t mind more clarification and some discussion of the trade-offs).

            I just think it makes sense to go elevated in many instances, shadows be damned. Speaking of which, I can think of two cases where shadows are a really minor issue. One is where tall buildings already block out the sky. Of course in those instances, you may be able to afford to go deep bore (and you may not want to take even one lane). The other instance is where the street is simply unattractive. This gets back to the Seattle example. The plan is to run elevated along this corridor: https://goo.gl/maps/SbSG5aPBPs32. I have never heard anyone mention shadows, probably because no one cares about shadows for a road like that. No one walks along that road, really. There has been some discussion about whether surface or elevated makes more sense for that stretch (as well there should be) but to my knowledge no one mentioned shadows. Even with the areas that have more pedestrians (https://goo.gl/maps/F9ESAcEaKf72) it hasn’t been mentioned. I think it is a really minor issue compared to the other trade-offs.

          • Ross Bleakney

            Also, I think the width up high is rarely important in terms of right-of-way. If the street is too narrow, sure. If the buildings on either side are ten meters apart, then you just can’t do it. But if you have a street that is 15 meters wide and has four lanes and wide sidewalks, it seems like you could run an elevated train right down the middle. The lane with the pylons become turn lanes (as you change the road from four lanes to three). The space above road (where every car is running) is meaningless to everyone except the tallest of trucks.

            By the way, I’m not trying to be argumentative (well, maybe a little) but I’m honestly trying to learn more about the engineering aspect of transit. Most of the rest of this is fairly obvious when you think about it, but the engineering details aren’t.

        • Tonami

          From my understanding of what Alon describes as space requirement. He’s not so much talking about space taken by columns on the ground but by the elevated track width. If a street is 10m or less, it can barely accommodate an el without blocking demolishing some portion of the buildings to contain the 10m wide tracks or blocking out all the light from the street below.

          The three google maps examples you linked to including the Canada line all have 24m wide streets excluding the side walk and 32m width from building to building. Those 3 examples don’t have any difficulty accommodating an el. It’s streets like this https://goo.gl/maps/SxUmQ3AqLPD2 in London that will have difficulty. That street in question is about 5m excluding the sidewalks and 12m between building. Putting an el on this street will cover the street with 100% shadows with the tracks barely touching the buildings on both sides.

          Even looking at a short section of Wupertal’s Suspended monorail in a really tight street, it’s still 15m street width between buildings https://goo.gl/maps/PnuTJ68zMEu. If it were a regular el and not a monorail, that entire street will be as dark as the completely covered section under the station.

          • adirondacker12800

            It’s streets like this in London that will have difficulty.

            …… Don’t it put there? It seems there is a Northern Line station a block away, would they need one?

          • Ross Bleakney

            I agree. But my point is, sometimes it isn’t the street width that is the problem. The street is wide enough, but carries lots of cars. The city just doesn’t want to give up that space. They don’t want to add two lanes, dedicated only to transit, since that would mess up automobile traffic. That is why so many streetcars in the U. S. (and buses) end up running in mixed traffic.

            But elevated uses less space. But how much space does it use? No one seems to be able to answer that (this is my fourth time asking). From what I can tell, using two posts means each one is pretty skinny (based on your example as well as the El in Chicago). Even with the cement barrier I’m guessing two feet on each side. They essentially just take up space on the sidewalk (like a potted plant or tree — just not as pretty). One single post (like in the Vancouver example) is a bit wider, but still not that bad. I’m guessing two meters, if that. But again, I don’t know. Nor do I know if a single post can curve, and then sit over the street. This would make it appropriate in some cases.

            Unfortunately, this is more than just idle curiosity. In Seattle, planning has begun on a very large expansion of our light rail system (which is basically a light metro). Decisions are being made about where, exactly, stations will be placed. Many of the ideas sound terrible (for reasons I won’t get into). But there are some options that haven’t been considered, but should be (in my opinion). We could send a train west on Market Street, as shown here (https://goo.gl/maps/d6ektZTU7fv). If you look at this from the ground level, you can see that the street is quite wide, depending on how you measure it. It is between four to six lanes, and the sidewalk itself is reasonably wide (meaning the distance between buildings is fairly large). One option would be run on the surface. But that takes two lanes. That is mostly just parking, but at the intersections (where there are curb bulbs) something would have to give. Either the train takes up some street space, or some sidewalk space. You also have the problem of intersections (accidents, delays, etc.). On the other hand, if you ran an elevated line, you simply take up less space. You could run it down the middle of the street, and turn the four lane street into a three lane street. The station itself might be an issue, but that is about it. Another option would be to run it on one side of the street. That wouldn’t take up any traffic space at all, from what I figure. You put the pylons in the parking area, and that’s that. You basically jump over the narrow sections. You would have shadow issues, but not if it was on the south side of the street (where the buildings typically block the street). There might be some objection to putting in a station around there, but I doubt there would. Nor do I think anyone would mind the track in general.

    • Alon Levy

      Yes, and the way San Jose chose to build the large-diameter bore suggests they didn’t really understand what the point of this technique is. The drawings still show a station cavern.

      • EJ

        Other than *maybe* the downtown SF core, pretty much every BART station is grossly overbuilt. That’s one reason for BART’s eye-watering construction costs.

          • Paludicola

            Give Philadelphia a little credit! We built a mostly four-track subway (although it took more than sixty years to lay all of the express tracks) with ample mezzanines at every express station and at all of the stations of the local-only South Broad Street segment for roughly $ 200 million dollars per mile in the first half of the 20th century that, as of FY 2018, runs eight trains per hour and has an average of 111,575 daily riders. (That figure includes whatever misbegotten few souls ride the Broad-Ridge Spur)

          • adirondacker12800

            Ya have the hole open a mezzanine may be cheaper than filling it back in.

      • Comradefrana

        “The drawings still show a station cavern.”

        Do you mean the station entrance shaft(s)? Otherwise I can’t find a drawing that shows anything I would call a ‘station cavern’ in the official San Jose BART extension plans.

  7. Michael James

    One can check on a satellite map that every station on Lines 1 and 2 in city center is at a piazza or under a wide street segment; lacking the same access to easy station sites, Line 3 had to be built deeper, with tracks stacked one under the other to save space.
    I have argued in comments that Paris could have used this trick of looking for less constrained sites for stations when it built Metro Line 1, permitting four tracks as in New York as long as the express stations under Rue de Rivoli stuck to major squares like Chatelet.

    Just a thought (bubble): Haussmannian Paris facilitated the building of the Metro* but I have still wondered of its relationship with the sewers that were put down at the same time–by Eugène Belgrand, Bazalgette’s French equivalent (and who would have been Brunel’s fellow student at the École Polytechnique if the Franco-Brit had been allowed to attend). Every book on Haussmann has maps of the streets he built or widened but alas I have never seen one of the sewer system. Including, frustratingly, Donald Reid’s Paris Sewers and Sewermen (1991). It must exist–in one of those specialty libraries in Paris maybe musée Carnavalet–because Belgrand complained that the existing sewers were unmapped and a big survey was his first task.

    Of course every street had a sewer but Belgrand built three super-sewers that were collectors for all the others, and these followed natural valleys to descend east to west, at minimum 3% grade, converging near Asniers (at that point just dumping everything into the lower Seine). So except in a few places where the Metro had to cross the super-sewer, I guess it wasn’t an issue for the smaller sewers. Well, I don’t really know as I can’t find any hard info on how deep they were. Note, even the non-super-sewers were big; designed by Belgrand so men could stand up in them for maintenance purposes (the pre-19th century sewers were difficult to access and the resultant build up of stuff, most of all building sand which would not flush out, caused big problems); the new sewers were also used to carry potable water mains plus non-potable riverwater–from Canal St Martin (btw Paris is apparently the only major city that has this dual system that saves the best quality water)–and gas pipes for the gas street lighting and eventually the electrical wires for street lighting, all of this attached to the ceiling.
    Anyway you see my thinking here: with cut-and-cover they possibly could avoid disturbing/ rebuilding/ relocating the sewers, but going deeper …

    Oh, another thought re your desire to double that RER tunnel for B & D: you know when, on the RER-B between Chatelet and Gare-du-Nord, one gets a distinct whiff of sewer? I am guessing it travels over a super-sewer and the pump action of the train’s movement causes some air exchange. And btw, the next major improvement to Paris’ sewers was under Jacques Chirac period as mayor, and I suppose some major work was done when they dug the giant hole for Chatelet-les-Halles and the giant tunnels for RER.

    *Incidentally, looking at the timelines I find it curious that there was less than 30 years between the end of Haussmann’s remodeling of Paris (though which was incomplete and was continued by others at various times after him) and the beginning of construction of the Metro. That’s less time than when I first went to live in Paris and now! And Chirac was still mayor.
    Speaking of subterranean tunnels bored with a sewer TBM, Musk’s much-hyped hole in LA has its grand public opening tonight.

    • Michael James

      Musk not only used a second-hand TBM previously used to bore sewers in Oakland, for his Hawthorne-LA new tunnel opened yesterday, but he is also adopting Eugène Belgrand’s multiple-use concept: “for water and utility pipes”. Weirdly, the primary article, in Bloomberg, then syndicated all around the world, talks of “the company’s technology” as if there is something new here. Either there is journalistic incompetence (or more charitably, no time to think to produce 400 words for the news maw, this is possibly a PR release) or a kind of conspiracy. Musk didn’t hide that he had purchased this sewer-TBM. He named it Godot –presumably to contrast its slow snail-like, or perhaps blind mole-rat, pace compared to the super-duper replacement his Boring Co. would produce. He hasn’t modified it in any way and it produced the tunnel in completely standard fashion in a normal timeframe. Maybe he and his engineers could have picked up a few ideas from this pilot tunnel to optimise the process a bit, but actually there is nothing about this anymore.

      So the “incredibly profound” breakthrough was not maglev skates or sleds to whisk cars at 155 mph (250 km/h) in his 1.6km tunnel, but a Tesla driving on some hastily laid platform-tracks, at 55km/h. (Almost like a Parisian rubber-tired metro train!) Some of the riders complained the ride was so bumpy that they emerged with motion sickness. But this does make marginally more sense than any of his earlier proposals that had a entry/exit issue, and it still only makes sense for a single entry and exit, ie. at both ends, perhaps to do what frustrates Musk so much in LA: to whisk you in your car across the vastness of the city at 240km/h without traffic jams. Of course what happens at the exit when cars are arriving at the pace of 240km/h to rejoin the 55km/h (or stopped) reality of surface LA, is not clear …

  8. Jhonny

    I find it a bit strange that you don’t manage the “Stadtbahn” solution.

    True it was only theorized about in the 1920s and implemented after WW2 but in the not insignificant numbernumber of cities that have narrow streets downtown and wider streets further out tunneling downtown andand going to the surface further out is at least a solution that suggests itself, whatever its merits

    • Alon Levy

      It’s another way of tunneling, really; it works if you have a street that you can tunnel under, but otherwise you run into the same tradeoffs that London, New York, and Milan have run into.

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