The Cahill Expressway in Sydney, the city’s first expressway, opened in 1958, connecting the Eastern suburbs to the Harbour Bridge. After the Harbour Tunnel opened in 1992, traffic was halved, the section’s Eason or being eliminated. Looking at a map, you can see the Harbour Tunnel and Harbour Bridge approaches join north of the Harbour, and basically form an upside-down V-shape, with the Circular Quay section forming a cross, the segment turning the upside-down V into an A.
Traffic counts for the Cahill Expressway at Circular Quay are given for 2012 as about 20000 average annual daily traffic in each direction. While certainly non-trivial, this is also not a lot for two lanes in each direction, equivalent to a four-lane arterial. And when the system is working, all of this traffic has alternative routes, as the route is topologically similar to the classic Braess Paradox.
The Braess Paradox observes that under certain circumstances an additional link increases total travel time, and is dysfunctional, because of the difference between the costs that travellers pay and the costs they impose by congesting others. While it is hard to prove such cases in the real world, there is no reason for this link to exist in the post-Tunnel configuration except as a backup when the Harbour Tunnel is closed or constricted to divert traffic to the Harbour Bridge.
If this section of the Cahill were to be removed, many of its access and egress links could be removed as well, creating additional space and sunlight in the constricted central business district. Southbound traffic would decide north of Sydney whether to diverge for the East or West and then take the Bridge or Tunnel, with no recourse except for city streets. Northbound traffic from the East would take the tunnel to cross the Harbour or exit onto city streets. The operators of the tunnel should be pleased.
Suppose the Circular Quay section were closed. The expressway lies on the upper deck of a double-deck elevated structure, with an elevated railway (the under-rated John Bradfield‘s City Circle, completed in 1956) immediately below. So the whole structure cannot easily come down. Instead the expressway deck can be repurposed, much like New York’s High Line and other infrastructure reuse projects, as a pedestrian overlook (there is already a sidewalk) on the north side, with the south side hosting restaurants and open-air cafes with a gorgeous view of the Harbour.* I am sure urban designers could come up with some lovely watercolour renderings.
While all of this undoubtedly requires study and many, many consultant contracts, it is really easy to test the actual traffic effects (and would make a nice Master’s Thesis project). Close the ramps for a few weeks “for repairs”. This must happen from time-to-time anyway. Perhaps there is a ‘natural experiment’ coming up, or recently passed, when this happened. Monitor traffic elsewhere in the system. Evaluate the consequences.
The hypothesis is that traffic conditions are no worse overall (system travel time is unchanged or lower), though selected links may in fact be worse off while others are improved. Given the reduction in merges and diverges, I suspect more links are improved than worsened.
If this hypothesis is borne out, there is less total travel (fewer vehicle kilometres traveled) in the city, travel is faster, and most travellers are better off.
In recent decades there has been a trend for cities to close obsolete freeway sections. San Francisco famously took down the Embarcadero Freeway for instance, opening up the waterfront. Seoul removed the Cheongyecheon freeway and restored a river. There have been others. While removal of this section of the Cahill is not likely to have the same effects, as the elevated railway will remain, it still could be beneficial. Proposals to demolish the entire Cahill, which bisects major parks the Botanical Gardens and the Domain have also been discussed, though burying them under air rights park seems a far simpler and less controversial proposal, and less like to strand the Harbour Tunnel.
* A single lane passage for emergency vehicle could be maintained if necessary.