When people talk of road emissions they are usually only talking about the total fuel emissions used by all those who drive on a particular road. While at first glance this might seem reasonable, it excludes a lot of other emissions directly attributable to building a new road and therefore hides the true impact that building a new road has on carbon emissions.
Increasing road capacity increases CO2 emissions in five main ways:
- during construction, notably land clearance and preparation, embodied carbon used in the production of concrete and other materials, and emissions from contractors' vehicles and other activities;
- during operation from road maintenance, servicing, lighting etc;
- from road users during the lifetime of the scheme particularly the tailpipe emissions of the traffic which has been generated or 'induced' by the presence of the road itself, including from changes in traffic speed*;
- consequential effects of the roads on settlement and activity patterns, notably when they enable developments that increase car-dependent lifestyles, increase car ownership (and increase the embedded carbon from vehicle manufacturing) and new patterns of warehousing and freight logistics; and
- synergetic effects such that the impact of each single road improvement on its own may be small but the combined effects of many, in the context of prevailing transport policies, pricing and management, give a greater total than the sum of their parts.
*Note: User emissions usually only covers fossil fuels burnt whilst driving on a stretch of road. They do not cover the emissions linked to electricity generation for electric vehicles
There can also be issues of how accurate estimates are, given that National Highways / Department for Transport routinely overestimate traffic increases without a road expansion which allows them to then model a lower uplift due to a new or widened road. This has the effect of underestimating carbon emissions from induced traffic.