New roads generate traffic - CPRE

It says the impacts of road building are manifold.  It says evidence from 13 road schemes is consistent with the conclusion that they generate traffic with average increases over the short run (3-7 years; seven schemes) were 7%  and in the long term (8-20 years; six schemes) 47%.

It says these were increases over-and-above background traffic growth (measured by county and regional trends), and in most cases were across a screenline, to rule out reassignment effects. Exclusion of schemes where screenline data was unavailable reduced these averages, but the difference was small.

The report also says that schemes affect the environment with impacts on the  landscape and biodiversity being long-lasting, and schemes leading to increases in carbon emissions.

It adds that there are questionable economic impacts too, with a check on 25 road schemes justified on the basis that they would benefit the local economy, finding only five had any evidence of any economic effects.

It adds that effects on road safety were , at best, "a mixed effect" with eight schemes showing a reduction in collisions relative to the counterfactual, and seven showed an increase in collisions, over the short time period of (at most) five years following scheme completion.

The whole report is here.