They require massive investment, affect a large population, take many years to plan and build, and have profound economic, social and environmental impacts.
Oxford University's Saïd business school's professor of major programme management, Bent Flyvbjerg, sees megaprojects as a completely different class of infrastructure projects in aspiration, lead times, complexity and stakeholder involvement.
Roads of national significance, arguably, are New Zealand's version of megaprojects.
Before the current programme, the most recent experience of this phenomenon in New Zealand was the 1970's Think Big programme of the Muldoon National Government, which led to the fast-tracking of infrastructure projects with dubious economic benefits.
Then, as now, flawed political visions and weak justifications were advanced to justify large infrastructure projects.
The National-led Government's ideological commitment to road building has seen these "significant" roads trump other transport investments in rail and public transport since 2009.
Conveniently, the global financial crisis was used to justify unprecedented increases in spending on roads, which continued in subsequent budgets despite the strong economy and an urgent need to create a more resilient multi-modal, land transport system.
The best outcome of the Waterview connection is a direct motorway link between the CBD and the airport.
However, it may attract more traffic and eventually develop congestion quickly. As many have argued, both the CBD and airport nodes need a rail link as well.
The maths of megaprojects can be out of step with benefits? According to Flyvbjerg, the costs of poorly handled megaprojects frequently increase significantly between their initial costing and their completion.
For example, earlier this year Transport Minister Simon Bridges was advised the proposed Warkworth to Wellsford motorway's projected cost had nearly doubled from $494m in 2009 when it was first announced to between $1.4 billion and $1.9b in 2017. The indicative route has yet to be confirmed.
Another project on the Auckland western ring route, the SH16-Lincoln Road interchange was delayed nearly three years and ran $45m over budget.
The debate is not just about the escalation of cost, but overestimation of benefits as well. Two criteria used by the NZ Transport Agency in deciding the ranking of transport projects are effectiveness, and the very nebulous "strategic fit". The latter, according to the agency, measures how a problem, issue or opportunity is aligned with government policy - hardly a robust or independent assessment.
The evidence that the roads of national significance produce sustainable economic growth, beyond the construction phase and inevitable agglomeration "benefits" (the development made possible by the new transport linkages), is very sparse.
In the case of the Kapiti Expressway, with expected travel time savings not being achieved, the main justification now is safety benefits.
Flyvbjerg identifies four reasons why megaprojects appeal to politicians. A technological rationale refers to engineers and technologists' enjoyment in challenging the boundaries of technology, as evident in the Waterview and Transmission Gully projects.
The economic rationale refers to the appeal large projects have for those who benefit from the expenditure.
This includes contractors, suppliers and investors, but also businesses, local councils and workers who benefit from increased employment in the construction phase.
The east-west link in Melbourne is an example, which is continuously supported by the State Government and construction companies.
The aesthetic rationale refers to the pleasure derived by professionals involved in designing signature infrastructure.
And the political one is the most important of all four, as it attracts the politicians and policymakers to initiate large infrastructure projects because they are visible to the people.
They attract the positive attention of media and garner more public support for funding of more road projects.
Scotland is one of the best practice case studies in planning, delivering and funding infrastructure projects like the Queensferry Crossing (the replacement road bridge across the Firth of Forth) for the M90 in Scotland.
In contrast to New Zealand's roads of national significance, these best practice case studies projects are identified in a national spatial planning strategy or framework, are based on independent analysis, and align local government aspirations with a national strategic direction. In this way, these projects are not hijacked or advocated by one political party.
At present New Zealand lacks long-term planning and robust, independent assessment. Government transport policy, so heavily focused on road building, does not foster resilience to natural hazards and climate change, undermines our greenhouse gas emissions targets and is not responsive to emerging and future travel patterns.
• Associate professors Christine Cheyne and Imran Muhammad are senior lecturers in planning at Massey University's school of people, environment and planning, and were supervisors for Dr Babar Chohan who graduated in May with a PhD on roads investment and economic growth.
