When transport impact is ‘severe’

When transport impact is ‘severe’

Since the launch of the National Planning Policy Framework (NPPF) in March 2012 the statement that “Development should only be prevented or refused on transport grounds where the residual cumulative impacts of development are severe”  has created substantial uncertainty. The definition of ‘severe’ has been open to interpretation ever since and now, nearly five years on, we are starting to see the depth of evidence to be able to interpret the NPPF in a meaningful way to projects.

Although still officially undefined by the NPPF and subsequent Planning Practice Guidance (PPG), we have investigated the meaning of ‘severe’ through numerous appeal decisions and our own experiences as expert witnesses. Many things appear at first inconsistent, but this highlights the overarching need to treat each site individually and fundamentally understand the transport context of the development proposals.

The dictionary definition of severe is in 3 parts:

“Severe adj. 1 (of something very bad or difficult) very great. 2 strict or harsh. 3 very plain in style or appearance”.


In terms of NPPF, we assume the first definition applies and have encountered the following scenarios where it crops up:

Infrastructure Development Plans: where infrastructure is listed in IDPs, and is being delivered by developers, or if infrastructure is intended to deliver growth, inspectors have found that this is not severe. Furthermore, inspectors have concluded that where highway authorities have identified junction improvements to deliver a Local Plan, they cannot justify additional mitigation from individual developments. However, where there is no strategic approach to transport infrastructure, inspectors have found that is it not acceptable to just make a financial contribution to an undefined scheme, as this could lead to a severe impact.

Traffic impact: many transport planning consultants and development control officers focus on Degree of Saturation, Practical Reserve Capacity and Ratio of Flow to Capacity to determine traffic impacts and severity. Whilst this is important, inspectors have focussed on queue lengths (and blocking back to previous junctions), delay and locational context. Inspector Stuart Nixon identified that it is not the magnitude of development traffic that leads to a severe impact, but to assess the final residual implications and establish if these are adversely severe. In fact, by its very definition, some impact is acceptable within the NPPF.

Traffic routeing: many inspectors have commented on the routeing options available, and whether all vehicles from the development have to pass through congested routes, contributing to a severe impact. If there are a choice of routes, then this is considered not to be severe.

Accessibility: inspectors have concluded that walking distances, travel times and topography all contribute and influence accessibility, but there is no clear and definitive guidance that is always, strictly applied. The NPPF seeks that development “maximises the opportunity to travel by sustainable modes”; meaning that measures should be provided to encourage and enable it sustainable travel to occur.

Design: Manual for Streets is the starting point for design on non-strategic roads, even when local design guidance is based on Design Manual for Roads and Bridges (i.e. pre-dates Manual for Streets). Inspectors have also cited that where existing junctions did not conform to modern day standards, and improvements were proposed to bring them up to standard, these improvements cannot be in conflict with the severe criterion.

Conversely, where junctions were considered severe in geometry and in potential accident terms without development, with additional traffic (and in the absence of mitigation and testing) the cumulative impacts were also considered severe.

Air quality, noise, health and wellbeing also contribute to inspectors’ conclusions on the overall planning balance to inform their decision, but these are rarely considered as the determining factor.

Our findings from review of the inspectors' reports highlights the importance of focussing on the sensitivity of surrounding land uses, understanding committed and proposed transport infrastructure improvements, as well as exploring practical route choice options, and the need to reflect this within our Transport Assessments.

We know towns and cities across the country have finite road space and increasing congestion issues but new homes and jobs are needed in all authorities; accessibility to travel by sustainably modes is, therefore, relative to other sites that could provide the same land use, and as transport planners we facilitate travel by sustainable modes even if it is not taken up from day one.

It has also reinforced the need to focus on the safety of all users in response to the interpretation of inspectors, as we continue to look at whether there are existing issues that can be addressed, consistency in the design process and standards being used as well as balancing the needs of vulnerable users.

Finally, we look for certainty on the delivery of mitigation measures, and to make sure that these are directly linked to the impact – although these do not necessarily need to achieve “nil-detriment” – to demonstrate that the cumulative, residual impacts are not severe.