Bypass vs bridge: Ashburton’s 15-year transport saga

Ashburton's second bridge has been 15 years in the making.

Initially planned to ease increasing traffic volumes, the 2021 Canterbury floods highlighted its critical role for Ashburton and the South Island.

It's not there yet, but the Government will fund the bridge while the Ashburton District Council covers the new connecting road.

Still, some believe the Chalmers Ave route for the second bridge isn't the best option, and a bypass is the way to go.

Comments like "the only solution is to put a highway bypass on the outskirts of town and leave Ashburton traffic to get where they want to go" appear every time the project hits the news.

Bypass options were considered in the early days but discarded due high costs and a lack of NZTA support.

Mayor Neil Brown, on the council since 2004, has seen the second bridge progress from concept to construction, which is set to start next year.

Looking back, Brown remembers considering the options in 2010-11 and that the Chalmers Ave option was the most cost-effective.

"The 12 options were ranked, and Chalmers Ave ticked the most boxes to deliver what was required.

"It's also the one that NZTA supported and suggested if council didn't support it then they wouldn't be funding their share.

"It was the logical decision to go with Chalmers Ave."

A bypass was considered expensive in 2011 and today it would probably cost $1 billion and cause major disruption, Brown said.

"You would be splitting farms in half, centre pivot irrigators, people's houses in the way, and then another designation process, it would take years to get all of that done."

Diverting SH1 to a new bypass route would also see the council, and the ratepayer, inherit the infrastructure - the 97-year-old bridge, and the traffic lights - along the existing SH1.

"That would be a huge added cost going forward."

He still believes Chalmers Ave is the right option.

There is now light at the end of the tunnel (not one of the options considered), and with construction of a second river crossing in Ashburton in sight, there is no turning back now.

Genesis

In 2005 Transit New Zealand (now NZTA) and the Ashburton District Council commissioned Opus to identify transportation demands within the Ashburton urban area through to 2026 and recommend measures for improvements.

This led to the Ashburton Transport 1 Study. It highlighted the future problem of State Highway 1's ability to cope with increasing traffic, through the Ashburton urban area, particularly at the Ashburton River Bridge.

A second bridge was identified as the solution for local traffic.

In 2010, a report explored 13 options for the second bridge including bypasses to the west and east of the town, and various options through the town.

Chalmers Avenue to Grove Street was initially the preferred option, with Chalmers Avenue to the east of Tinwald the second option.

A second report in 2011 focused on eight options.

They included an eastern bypass from Fairton to Winslow with the 26km route estimated to cost between $69-84m or a shorter 9.3km route from Seafield Road to Laings Road for $48-58m.

The report noted a bypass and the removal of through traffic was "likely to have negative impacts on Ashburton businesses" and that NZTA funding was unlikely.

The council identified the Chalmers Ave options as the preferred route.

Connecting to Grove Street met plenty of public opposition and the council resolved to focus a Chalmers Avenue to east of Tinwald options - an urban and a rural option.

The bridge route was fought by the Ashburton Bridge Action group, which produced now-former councillors Dianne Rawlinson and Alasdair Urquhart.

A decision was made in 2013 to go with the Chalmers Ave route linking up with Grahams Road and seek land designation.

A public hearing was held in March 2014, where submitters, including affected landowners and stakeholders, presented to independent commissioners.

Their recommendations were considered at a council meeting in May, where the Notice of Requirement for the designation was confirmed, when mayor Angus McKay used his casting vote, leading to an amendment in the District Plan.

Of note, at the time the construction of the bridge was intended to begin in 2026.

From business case to building

Work on a business case to secure Government funding started at the end of 2018.

The council budgeted $7.5m in its 2021-31 long-term plan for was an estimated $37m project.

The council submitted the detailed business case to NZTA to help fund a 'nationally significant' $113.6m second bridge in 2022.

The business case also reiterated the bypass options were not progressed due to affordability concerns and it only benefitted through traffic "which accounts for only 20-30% of the demand on the existing SH1 Bridge".

This posed the risk a bypass would carry a relatively low volume of traffic and "not solve the inherent problems".

The second bridge project was included in the National Land Transport Plan released by the Government in September.

NZTA and the council reached a funding agreement in December, with construction expected to begin in 2026.

Chalmers Ave "not SH1"

Chalmers Ave won't become a new SH1 route.

Brown said the second bridge is a local road that will benefit SH1 by providing an alternate crossing for the local traffic and freeing up the flows on SH1.

"Traffic isn't going to divert from SH1 to go through town to use the second bridge. It will stay on SH1.

"A lot of those vehicles [that will use the second bridge] are already using Chalmers Ave now as they come over the SH1 bridge and turn down South Street."

Chalmers Ave used to be a two-lane road but the onset of cycle lanes reduced it to one.

The roundabouts on Chalmers Ave were upgraded last year to handle the increased traffic volumes and heavy traffic already using the road.

Brown said the rest of the road is programmed to get a new asphalt seal.

"We just have to wait for any underground works to be completed before we do it."

Cost comparison

The second bridge was a hard and long sell to get approved for NZTA funding.

Initial estimates put the Chalmers Ave second bridge at $30-35m, but the latest 2024 estimate put it at $130m.

A Fairton-Winslow bypass was estimated at $69-84m so would now exceed $250m.

A comparison can be the Woodend bypass, a total of around 6km of new SH1 highway the Government will fund and a previous cost estimate in 2022 was $120m.

That's $20m per km, which would have the 26km Fairton-Winslow route cost $520m, excluding the added extra land and bridge costs.

Four-laning SH1 option

Putting the second bridge alongside the existing SH1 bridge is another popular option.

It had community support because as a SH1 bridge, it would have been government-funded but NZTA opposed having three bridges in "such close proximity", Brown said.

The other factor was the push for four-laning SH1 from Rolleston to Ashburton.

Four-laning SH1 was set to be considered by NZTA In 2017 when a business case looking at traffic solutions on SH1 between Christchurch and Dunedin was ready to go before the NZTA Board.

Instead, the National Government announced its roads of national significance policy that included four lanes from Christchurch to Ashburton.

That had the business case shelved only for National to lose the election later that year and the roads of national significance were scrapped by the incoming Labour-led coalition Government.

The business case, released under the Official Information Act, shows the preferred option was not four lanes.

It included a second bridge in Ashburton and a series of "2+1" lanes, essentially alternating passing lanes between Christchurch and Ashburton.

It suggested West Street/SH1 be four lanes between Havelock and Moore Streets and signalled required upgrades at the Walnut Ave/SH1 intersection (traffic lights installed in 2022) and the Tinwald corridor at the Lagmhor/Agnes Street (traffic lights installed in 2024).

LDR is local body journalism co-funded by RNZ and NZ On Air.