Wellington Urban Motorway extension - Inner City Bypass - Tunnellink

 

Cut and cover tunnel for Wellington Inner City Bypass

With proposals for a second Mt Victoria Tunnel included under Let's Get Wellington Moving and more recently from the National Party's 2023 transport policy, I thought I'd start a series on the Wellington Urban Motorway, which won't be in any specific order, as I have a lot of material on it.  The first article from July 1992, in the former free newspaper "Contact" about the decision by Wellington City Council and Transit New Zealand to play the motorway extension (technically not a motorway but an arterial extension with a 70km/h design speed) into a cut and cover tunnel.  It was previously proposed to go under Willis and Victoria Streets, be at grade across Cuba Street and elevated over Taranaki Street, under Tory Street and Sussex Street. 1.1km of the road would be underground, enabling use of the land above it (although there appears not to have been much calculation of the benefit of doing this).  The proposal also included a "scrubber" to remove noxious particulates and gases from the tunnel exhaust.

The Cut and cover proposal would be coined "Tunnellink" subsequently (to link Terrace and Mt. Victoria Tunnels), but did not include duplicating either of the tunnels, as it was believed there was adequate capacity with those tunnels for some time (and noting that completing the extension would help improve tunnel traffic flow in one direction at least). 

Note the estimated cost in 1992 was only $135m ($281m in 2023 values) which is a far cry from estimates today for a similar project. Note also the depiction of a park in front of the carillon, which reflects that the Arras Tunnel and War Memorial were built around 15 years later than this proposal, but the Arras Tunnel is around the size of more than half of the cut and cover tunnel depicted above.

Financial headaches along the motorway

Financial headaches along the motorway

This second article from the Dominion on September 1993 focuses on the issues with proceeding with the new proposal, including obtaining planning approvals, and also has a short history about the Wellington Urban Motorway.  This included the redesign to be predominantly trenched and then the cut and cover tunnel.  The report also noted that after the extension gets built, a second Terrace Tunnel would be necessary (as the single lane southbound will remain inadequate, and northbound three lanes merge into two). The price for the second Terrace Tunnel was estimated at $63m ($131m in 2023) which also is significantly cheaper than today's construction costs.

Also reported were variations in BCR calculations. A BCR of 20:1 was estimated in 1992, but that had dropped to 2.33:1.  Bear in mind the funding threshold at the time was a threshold of 5:1. Ultimately it was that funding threshold that saw this proposal deferred in favour of the now built Karo Drive "Stage 2" Inner City Bypass. 

Bypass could cut accidents by a third

This final article  from the Evening Post includes a short piece about Wellington City Council backing the extension, but also a report displaying the entire route and claims it would have significant safety benefits by cutting reported accidents by one-third. Noting the design would have retained Ghuznee St as an offramp and Vivian St as an onramp (one way from Taranaki St only), eastbound onramp from Victoria St and offramp westbound, with an eastbound offramp towards Cambridge Tce and another to Dufferin St (for traffic heading towards Adelaide Rd), an onramp from Sussex St westbound (akin to the Arras Tunnel arrangement) and finally eastbound onramp and westbound offramp from Mt Victoria Tunnel.