Wollongong and Liverpool city councils have joined Sutherland Shire Council in calling for the Woronora River bridge on Heathcote Road at Engadine to be duplicated.
The three councils, representing a total population of more than 700,000, oppose the state government move - due to start at the end of this year - to spend $73 million widening the bridge while retaining just one lane in each direction.
Sutherland Shire Council has highlighted the support in a powerful submission on the environmental impact statement (EIS) for the proposal.
The submission includes the most detailed data ever made public on the horrific crash history of the road, including the location of every fatality.
The analysis reveals 23 people have been killed and 556 injured over the last 23 years, from 1996 to 2019.
Eight of the fatalities have occurred between New Illawarra Road, Lucas Heights and Princes Highway, Engadine.
Between 2009 and 2019, two people died on, or within 500 metres of, the bridge, six people suffered serious injury, five moderate and three minor, with nine non-casualty tow-aways.
The council submitted widening the bridge, while keeping just one lane in each direction, would actually make the risk of a head-on crash greater because motorists would be less likely to slow down.
If the widening project did proceed, there should be median lane separation on the bridge to minimise head-on collisions, the submission said.
The council also argued that a project costing $73 million should be the product of a business case as stipulated in Treasury guidelines.
"[The Treasury document] indicates a good business case should: 'Convince through arguments that are optimally supported by hard data, including accurate costing of alternatives and expected benefits'," the council submitted.
"Advice from TfNSW suggests no adequate cost estimate has been prepared for council's resolved option of bridge duplication.
"If this statement is accurate, then council is entitled to ask how the current preferred option of widening was selected?"