Support for Roe 8 has grown since September, according to fresh polling, as an emboldened Colin Barnett forges ahead with a Roe 9 tunnel option he dismissed as under-done just months ago.

An exclusive ReachTEL poll of 1525 residents across WA on Thursday night revealed 51.4 per cent support for Roe 8, up from 44.9 per cent in mid September when respondents were asked about the entire Perth Freight Link project, including Roe 8.

Opposition remained steady at 30.4 per cent, compared with 31.1 per cent in September, while the proportion of undecided respondents fell from 24 per cent to 18.2 per cent, meaning the vast majority of people who are making up their mind are backing the new road.

Liberal voters have increasingly adopted the party line, with their support surging from 67.1 per cent to 79.7 per cent, while the proportion of conservatives opposed slipped from 13.7 per cent to 10.6 per cent.

Roe 8 remains deeply unpopular among respondents who vote Labor (28.9 per cent support, 54.3 per cent opposed) and Greens (15.1 per cent support, 69.9 per cent opposed).

The results, which bolster Mr Barnett’s claim that the “silent majority” of voters support the Roe 8 extension to take trucks off Leach Highway, come as the Premier yesterday doubled down by committing the Liberals to a Roe stage 9 extension to Fremantle via a tunnel.

As recently as October 12, Mr Barnett said it was unlikely a route would be decided before the State election, paving the way for a Labor scare campaign in the key seat of Bicton about Palmyra houses being compulsorily acquired for an above-ground option.

Mr Barnett allayed those fears yesterday, committing the Liberals to a 5km stage 9, including a 3km tunnel from the intersection of Stock and Winterfold roads to High Street, Fremantle.

The State would pay $181 million and the Commonwealth the rest of the $903 million cost.

Play Video Labor has slammed the Liberal's new election promise as a waste of money. The West Australian Video Labor has slammed the Liberal's new election promise as a waste of money.

But the Premier added fresh uncertainty about the so-called “last mile” from High Street to Fremantle port, claiming there were no plans to duplicate Stirling Bridge or tunnel under the Swan River, two options which have been considered within government.

“There’s always going to be a bit of congestion about the port because the Fremantle port is an inner-city port,” he said.

Mr Barnett expected Roe 9 contracts would be signed in 2019.

Opposition Leader Mark McGowan, who has pledged to scrap the entire project, said the Liberals were “making it up as they go along” and the PFL remained “a road to nowhere”.

Greens MLC Lynn MacLaren called on the State to release a business case for Roe 9 — a requirement of the Federal funding.