For anyone who has watched John Key in action for any appreciable length of time, the news that he would be using his showcase speech to the National Party's annual conference to make an announcement about roading was initially puzzling.

Even more so, given the weekend get-together was taking place just three months before a general election.

The Prime Minister's promise to fix some longstanding danger spots and bottlenecks on the roading network in the regions hardly seemed the kind of stuff to create excitement on the conference floor on a Sunday morning.

Neither did it seem substantial enough to galvanise the 600 or so delegates who had been lectured continuously on the need to knock on every front door in their electorates between now and election day and identify National supporters and sympathisers and make sure they vote on or before the big day.

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On swift reflection, however, the $212 million package could be seen as part of a very clever strategy.

That is not just because it is a classic example of vote-buying - something that Key and Gerry Brownlee, his Transport Minister, repeatedly denied while struggling to keep straight faces when confronted with the obvious.

Neither is it because this blatant example of pork-barrel politics - as the Greens' Russel Norman noted with some bitterness - is cynicism in the extreme in using the proceeds of National's partial state-asset sales to shore up the party's support in provincial New Zealand.

No, the roading announcement was also part of a wider strategy to convince voters that the Greens will be the driving force of any post-election coalition between that party and Labour.

Labour seems to be increasingly paralysed by the division between MPs who put a priority on economic development and those who want environmental concerns to be very much part of that development.

National is seeking to exploit this division. Bill English used strong language in his speech to denounce the Greens' leadership as "vindictive and dangerous" but also "focused and energised" - unlike Labour's leadership.

Sensibly, Labour leader David Cunliffe did the only thing he could - try to trump National by saying that party was merely returning some of the road funding it took from the regions.

National's end-game to paint a Labour-Greens coalition as either unworkable or effectively run by the Greens may have faltered this time, but National will not be giving up easily. Expect more of the same.