It's the way we use new infrastructure that determines its green credentials

The Infrastructure Commission for Scotland last month called for "a presumption in favour of enhancing, re-purposing, or maintaining existing infrastructure over developing options for new infrastructure". Given its twin ambitions of inclusive economic growth and net zero carbon, that sounds fitting. After all, new infrastructure is carbon intensive, right?

In looking at life-cycle carbon footprint calculations for buildings, Dr Schwartz and colleagues at UCL recently found that 75% of emissions come from the operational stage (from heating, for example) and only 25% from construction and demolition. Aversion to new-build risks missing the point: it is how infrastructure is used that counts most. New thermally-efficient offices and housing may be very worthwhile. The equivalent is true of course for transport infrastructure, depending on how it impacts on patterns of travel behaviour.

To be fair, the Scottish Infrastructure Commission calls for an emphasis on active transport and public transport – that is to say, usage. For road investment, it wants to see an emphasis on safety, resilience and reliability rather than increased capacity.

It is hard to imagine broadly similar guidance not appearing for the rest of the UK in due course. The toughest problem, I suspect, is going to be getting a meaningful handle on the inclusive piece (for which read, variously, 'levelling up', regional re-balancing and addressing the needs of places 'left behind'). Adding into the Green Book a requirement to have regard to such issues will not hack it. The weight of assured, time-honoured, familiar, transport benefits will always lie in the busiest places and where economic output – and so incomes and productivity – are highest.

The demand for new infrastructure, rather than enhancements and re-purposing existing facilities, is hardly likely to diminish, I suggest. This is because today's infrastructure (road, rail, ports, runways in the south east) is so intensively used that all changes and enhancements carry an added cost from disruption effects. And we have become remarkably reluctant to accept outcomes which, while providing net overall gains, leave some groups visibly losing out. This makes gaining public acceptability hard.

But let's test out how some key infrastructure projects might work out: are they still worth doing against what I am taking to be the new key criteria: inclusive economic growth and achieving net zero carbon?

Take a pair of long-standing road proposals in the South East – the Lower Thames River Crossing (£6bn) and the £3.5bn (an rising) Oxford Cambridge Expressway.

The LTRC scheme connects congested key growth areas in Kent and Essex at the expense of losing as much ancient Woodland as the whole of HS2 Phase 1, but, like other estuarial crossings, can be shown to reduce vehicle miles (a shorter route across).

The Expressway was deemed a national priority as recently as 2017 by the National Infrastructure Commission, essentially because it 'unlocks' large-scale housing development in an area of continuing strong demand growth.

Neither would score well on carbon reduction in use and neither could lay claim to addressing any version of economic inclusivity – both suffering from being in an area of economic strength in the wider South East. In both cases, there is the possibility of a parallel rail-based alternative, which if electrified, and especially if it accommodated freight, could have much better carbon reduction credentials. In any event, the draft strategy from Transport for the South East calls for a network "using digital technology to manage transport demand, encourage shared transport and make more efficient use of our roads and railways". So they are on-message.

Or let's take the PM's ambition to establish a bridge (a tunnel might be more likely) across the Irish Sea between SW Scotland and Northern Ireland. Now that will take concrete and steel to build (so lots of embedded carbon), as will the new access links needed to make it work. But besides being a boon to the Northern Ireland economy (a big win for inclusive economic growth in UK-wide terms), if developed with the new key criteria in mind, it could also lead to a switch out of short-haul air travel, and also a switch from road haulage to rail-freight. (We all thought this would follow from the channel tunnel but it didn't). Still, just maybe a scheme with usage characteristics of lasting benefit.

Or take Runway Three at Heathrow. Yes, there is a carbon case for this scheme provided the extra capacity it creates is used to eliminate stacking – both low-altitude aircraft queueing to land and those in a taxi-queue to take-off. There would then be a great environmental win from airspace/ground-space usage shifts. Just one problem: a complete change in airport and aviation commercial regulation would be needed. No shareholder could vote to spend multiple £bn on R3 unless it is guaranteed a commercial return.

Jim Steer is Director of Steer