The author/s of the first of the two articles referenced above seem intent only in espousing the wonderful imported concept of "induced demand". The second article takes a rather more thoughtful approach, and within the opening paragraph mentions several relevant factors affecting the apparent phenomena.
Just jumping on the "Nature abhors a vacuum" wagon and damning all new roading as bad, is unhelpful. There are many factors that need to be considered, not the least of which would be the movement to remote working for many office employees. Conditions that exist in other more populous and mobile countries cannot simply be interpolated and assumed to apply to conditions in NZ.
I have recently made several trips out of Wellington using the new Transmission Gully expressway as far as Levin and further. There is absolutely no doubt that the money was well spent. And why ? Because the new route bypasses all the small towns and the congestion they caused. Plus a great deal of the local traffic between those towns has now been removed from SH1.
This same "miracle" improvement could easily be implemented in the Wairarapa, but apparently there are not enough important people living there. So cheap out and don't solve the problem as lifestylers and urban refugees boost the local commuting population. A road tunnel has been mooted for 50 years or more, but never seriously considered even when the machinery has been sitting idle in NZ. Thus the Rimutakas will continue to be a commuter nightmare in perpetuity.
Of course, inner city traffic and suburban traffic in the major cities is a different issue, and can be treated as such. With population growth, an abundant supply of cheap vehicles, and the distance between industrial areas and residential areas, it may be possible to make a case for induced demand. But the cure does not lie in stopping improvements to the roading network.
Cheers...