This is a guest post from Nick R

The great irony of urban motorways is that they use up huge amounts of the very thing they are intended to access: land.

All surface transport infrastructure naturally consumes land. However, while a street, busway or railway fits in a corridor 10 to 20m wide, even a small motorway corridor is at least five times as wide. And motorways can easily stretch to 200m wide across at interchanges. In addition to using up land, the negative effects of noise and severance extend well beyond the corridor itself. This obviously affects properties that are adjacent to main roads and railways, but with the scale of motorways they keep lowering amenity and land values for hundreds of metres either side. The only way to really avoid this sort of impact is to go underground, a very expensive proposition for any transport and especially expensive for large scale motorways and interchanges.

Nowhere is this consumption of valuable land more visible than at Spaghetti Junction in central Auckland, where hundreds of homes and entire neighborhoods were demolished and excavated to build the central motorway junction (CMJ). This impact is so visible and obvious its not surprising that Greater Auckland and its predecessors have covered the topic many times.

Time for a quick trip into history. Early motorway plans actually avoided the city and focused on the northwestern corridor as the main state highway running northward around the upper harbour.  But there were a few main reasons that the network was quickly changed to run right through town and carve up the inner suburbs:

• Harbour bridge tolls. Initially the harbour bridge was only conceived as an arterial between the North Shore and central Auckland, analogous to the widening of Great South Road in the other direction that was happening at the time. However in the late 50s there was very real concern that the planned motorway via Hobsonville and Paremoremo would take all the traffic off the brand new harbour bridge, killing off the toll revenue that was needed to pay for the thing. So the decision was made to delay the upper harbour route by decades and make the harbour bridge the main regional motorway north, which required a big motorway interchange nearby. Obviously the traffic generating effects of motorway building and related sprawl development wasn’t well understood at the time.
• Slum clearances. At the time the demolition of old housing stock in the then undesirable city fringe was seen as a good thing. In hindsight this goal has failed twofold: in the short term this depressed land values even further around Newtown, Grafton and Freemans Bay and converting Karangahape Road, practically overnight, from a prestigious uptown shopping strip lined with department stores and fashion boutiques to a sleezy red light district. In the long term we’ve seen the remaining slum suburbs around the city fringe rocket to the most valuable land in the country, and people re-realised the value of walkable griddy neighborhoods of compact housing in close proximity to the city.
• Motorway access to revive the city. Somewhat paradoxically, bringing the motorways into the central city were thought essential to revive it, once the suburbanizing dispersal pressure of the motorways themselves and the related building demolitions to widen city centre streets and create more parking took hold. It’s debatable whether motorway access to downtown helped mitigate these impacts, or if it made them worse.
• And finally something I call the Brave New World factor, a sort of optimism bias that big new revolutionary plans must be good because they are big, new and revolutionary. How could the plan fail if it was so magnificent?

With decades of retrospect, this all looks decidedly fishy, sociopathic even. You can look with bewilderment at the ‘what might have been’ plans for motorways pushed through central London, Paris and Manhattan… but then realise we actually did this in our city.

So what to do about it? Well the answer that tends to come up is to deck over the junction to effectively bury it, and build in the city back on top. This has an obvious appeal, there are plenty of underground motorways and railways around the world. They’re pretty normal even. One interesting thing is that motorways tend to be built as tunnels to begin with and not covered over, while rail yards seem to get decked over quite frequently, for example Federation Square in Melbourne, the downtown yards in Chicago, or indeed parts of our own Britomart.

I used to think that the motorways were threaded through the gullies around central Auckland because they were convenient depressions below road level to route them through. However, apart from some parts of Grafton Gully around the bridge this wasn’t actually the case. The main Spaghetti Junction and the route under Symonds, Queen and Karangahape was excavated, creating a trench for the noodles to tangle in. So covering it over would be more like cut and cover tunneling, with fifty years between the cut and the cover. So this is the approach I’m suggesting, encase the road links in concrete culverts to turn them into tunnels, effectively building structural walls either side, bridging over, then filling in to restore something close to the natural landform. This is more or less the same process they used to cut and cover the CRL tunnel in lower Albert Street, without having to do the excavation first, and now working inches from the foundations of skyscrapers and sewer pipes.

Now there are some obvious fishhooks in this approach. One is working in, around and over a live motorway which is difficult, although certainly not impossible as we seem to have done it constantly for years in various widening, raising and realigning projects across the Auckland motorway network. Waterview interchange is an obvious tunnel. Secondly, this would make open motorways into tunnels, which would need all the requisite exhaust stacks, fire control systems and emergency evacuation routes. Again a large cost but taking the Waterview tunnel as an example, its clearly feasible. Thirdly, management, maintenance and operations. Complex structures are expensive to run and look after. I’m sure Waka Kotahi’s highway network team’s first response to this sort of idea would be laughter, followed by stone faced resistance.

Let’s wave those away as surmountable issues and look at what might be. But first let’s deal with the issue of why this, and why now? Why might a megaproject like this be a good idea and what has changed since the last time it was proposed. The answer to that is the City Rail Link, specifically Karangahape Station. As you can see in this map of walkable streets and building footprints, there is an almighty motorway shaped empty hole making up a huge proportion of the area that should be in close walking distance to the station. Not only that, the lack of walking paths across the junction means that you can’t actually get a lot of buildings to the south and west of the station that you would be able to get to with a normal street network. Some of this sits under a viewshaft, but some sits under unconstrained city centre zoning.

So how could a masterplan look? Well if we use the rest of the City Centre as a guide, we’d want to cover about 20% of the area with roads, streets and lanes, connecting in all the severed streets. Then we’d want about 15% for parks, plazas or public open space. This leaves about 65% of the area for building footprints. Something like this would fill in all that city again, with everything being only a couple of minutes’ walk from the new underground rail station.

I’ve taken the approach of applying these different uses where they can fit easiest. In the following renders the red links shows what a 6m high enclosure of all the single level road lanes, where there is space either side and sometimes between each piece to support these boxing-in structure from below. In the red areas you could create street, lane or plaza above, or build buildings of 2 or 3 stories on raft foundations that don’t need to penetrate the road links below.

The orange parts show areas where you have multiple levels of road structure or high bridge areas, so where you’d need to have some long spans or cantilevers or something else just to cap over. So I’m assuming the orange sites are where you’d probably have to keep as open space or parkland with some street crossings, perhaps with the odd single level building like the podium shops over Britomart station.

Here is where things get really interesting though. Once you’ve mapped that all out there are actually quite a lot of large islands in between of bare land (and a few redundant paved areas), where you would be able to drive foundation piles down to bedrock for large tower buildings. These renders show sites that are affected by viewshaft height controls in purple, and the corner that isn’t limited in height in blue. I was really surprised how much open area there is in between the noodles of Spaghetti Junction.

Wow gee that’s amazing I hear you say. Pretty pictures, but dreams are free and this thing sure isn’t. How much would it cost to build this? To be frank I have completely no idea on how to even start estimating the cost. But we can make a decent stab at estimating the value, which will tell us how much it would have to cost to be worthwhile.

Starting with land value, this varies a lot across the City Centre but a good estimate of the value of a unencumbered, serviced, viable development site near Karangahape Road is $10,000 per square metre. It can less than half that in the crappy parts of the city centre, and several times that on prime downtown waterfront zones, so I’d call $10k a metre quite conservative. The overall area here is 127,000sqm, if we take off the 35% we’re going to use for streets and parks, that leaves the land value of the developable sites at around $850 million.

So, is $850 million dollars enough to enclose all the roadways of the CMJ and fit the systems to make them tunnels, then fill in above, and build all the streets and services and stormwater and everything else required? My gut feeling is it is not enough, but in the right order of magnitude. If the value of the land was $85m you’d say no way, if it was $8.5b you’d do it in a heartbeat. There’s no slam dunk case here, but something along these line might be worth looking into to reap the connectivity-land value uplift benefits of the CRL and help stitch the city centre back to its suburbs.

Looking at those renders, I can’t help but notice all the development site is clustered at the Newton Road end, and all the difficult infrastructure is at the other. That makes me think there is probably a way to do this with a better return on investment. Perhaps something like this, focus on the bit to the southwest of the tall viaducts, which is less constrained and where there are more pockets of open land. This would let you only build the culvert structures (green) over the single level motorway links, and fill in the ground level more easily as they are mostly in cuttings already. Build three fairly conventional bridges (yellow) to provide access to the site and undo the worst of the severance. Then build in between or over the green caps and add other linkages through the site.

This doesn’t cover over the junction entirely, but it does reduce the severance, open up development sites within walking distance of Karangahape Station, and build back in a lot of the missing city fabric. A quick estimate shows around 20 building sites on terrafirma with this smaller option, worth perhaps $700m if brought to market. This is perhaps a more viable proposition.

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