The above I think is the most difficult technical issue in planning right now. It deserves a conference. It deserves its own joint research programme by the professional institutes working together, it deserves a joint NIC/CCC report, it deserves a targeted ERSC research programme.
Its a hard nut technically to crack
The Chinese have a saying ‘ if you want to get rich build a road’ They have a point
Going back to the origins of classical economics with Cantillon and Turgot, roads got your agricultural surplus out to cities and your industrial surplus out to markets. It is impossible to build new or expanded cities without new roads. Roads make cities; what matters is how the roads are used.
However a century of planning experience shows that if you just build roads for car borne commuters they clog up reducing their ability to get you rich through supply trade (and killing cities in the process). Hence the classic planning concept from Benton MacKaye of the ‘Townless Highway and Highwayless Town’ .
We also have the complex business of induced traffic. Building roads can induce traffic from elsewhere on the network and from public transport (the Down’s Thomspon Paradox). In some cases closing roads can reduce traffic (Breasses Paradox) Each of these are network effects of transport choice and land use. They don’t apply to every road link everywhere and they all depend on the network (or otherwise) of public transport in an area.
What we don’t have a clear picture of is what changed road patterns would look like in a zero carbon strategic plan. Which is just part of the wider question of what changed public transport and land use patterns would look like.
What we do have is a series of strategic way markers.
We know for example that with the right patterns (regional scale BRT and cycle networks etc.) you can get car modal share down to 40% or less, which with growth of population and households in a strategic growth region means that ‘in theory’ the modal shift would absorb all of the ‘residual’ growth of car traffic. If strategic car use is then fixed the issue then is the direction of growth.
Some localised road improvements would be necessary, you cant expand Crawley for example and still rely on its 18th C Farm roads which count as West Sussex’s B Road network. A large new town within the orbit of Cambridge for example would clearly require new roads to get its logistics in and out. The problem is ensuring those new roads don’t clog up with car commuters, which may require new forms of traffic management (such as ramp control – with compulsory engine cut off- out of major housing areas accessing on roads to strategic roads, well before the strategic road ramps), public transport prioritisation (zooming past the deliberately created car blockages) and targeted road-pricing. This means rethinking concepts like prioritising reducing congestion, and planning for car ‘levels of service’ from strategic housing sites, and towards planning on how to creatively create it to disincentive car use on strategic roads at peak periods. The overiding target should be level of low carbon service and travel times not the metric of congestion. Low travel times should be a negative in any benefit cost calculation if it is at the expense of busting a carbon budget.
Where to study? Well Homes England have lots of strategic sites and no sustainability plan (no plan at all really) so lets start there, forcing Highways England to be a strategic partner on the study (junior of course) would also be a nice discipline and learning exercise for them.