Highway objections can often become decisive in planning applications, particularly for larger schemes where modelling identifies congestion or capacity issues. But when does a highway impact actually justify the refusal of a planning permission.
This question was considered by the High Court in the case of Greystoke CB Ltd v Secretary of State for Communities, Housing and Local Government & Anor [2026] EWHC 1997 (Admin) and provides an important reminder that identifying a significant highway impact is not enough. Decision-makers must keep firmly in mind the specific threshold for refusal in the National Planning Policy Framework (“NPPF”) and must carry that threshold through into the final planning balance. Although the case was considered under the NPPF in force before the August 2026 revision, our view is that the outcome would have been the same under the NPPF issued on 17 August 2026 and the test now contained in Policy TR6.
The development
The case concerned a substantial employment development on agricultural land near Junction 11 of the M40 at Banbury.
Outline permission was sought for up to 140,000 sqm of B8 employment floorspace, together with ancillary offices, infrastructure, new accesses and associated works. Cherwell District Council refused permission in March 2024, initially giving 15 reasons for refusal, with highway congestion, capacity and safety featuring prominently.
The Inspector dismissed the developer’s appeal, crucially finding that:
- Predicted queues on the M40 slip road would remain within suitable limits and would not result in an unacceptable highway safety issue; and
- The residual cumulative impacts on the local highway network would be “significant” but not “severe”.
“Significant” does not mean “severe”
Paragraph 115(d) required significant impacts on the transport network in terms of capacity and congestion, or on highway safety, to be capable of being cost-effectively mitigated to an acceptable degree.
Paragraph 116 then established a separate and deliberately high threshold for refusing permission on highways grounds, where development should only be prevented or refused where there would be an unacceptable impact on highway safety, or the residual cumulative impacts on the road network following mitigation would be severe, taking account of all reasonable future scenarios.
In Greystoke, the Inspector nevertheless gave substantial weight in the planning balance to the adverse highway impacts.
Mr Justice Coppel concluded that paragraph 116 had to be confronted at the planning-balance stage. Having determined that the highway impacts were not severe, the Inspector could not simply treat significant congestion as a substantial adverse factor without grappling with the NPPF’s express direction about when permission should be refused on highways grounds. The decision was therefore unlawful and was quashed.
The judgment does not however, establish that a highway impact falling below the “severe” threshold must be ignored. Nor does it mean that an applicant is automatically entitled to permission once it establishes that an effect is “not severe”.
The NPPF provisions must be read together. Significant capacity or congestion impacts can still require mitigation under paragraph 115, and their nature and planning consequences may still be relevant. What Greystoke prevents is a decision-maker effectively using those same sub-severe highway impacts as the principal basis for refusing permission without confronting the policy restraint contained in paragraph 116. (Now to be read as Policy TR6 of the NPPF 17 August 2026)
Practical points for applicants and highway authorities to avoid delays and litigation
1. Define the highway issue precisely
Transport evidence should distinguish between:
- An unacceptable highway safety impact;
- A severe residual cumulative network impact;
- a significant impact requiring mitigation; and
- ordinary congestion or capacity impacts which remain material but fall below those thresholds.
Greystoke demonstrates the danger of finding that an effect is not “severe” but subsequently treating it in a manner which effectively produces the same outcome as if it were.
For applicants responding to highways objections, simply demonstrating some congestion is therefore not the end of the analysis. The important question is: what is the highway authority actually saying the impact amounts to against the NPPF tests?
2. Get agreement on modelling as early as possible
The case is also a useful reminder of the importance of agreeing modelling methodology. Disagreement about methodology late in an application or appeal can turn what should be a technical exercise into a fundamental planning dispute.
3. A financial contribution needs a credible mitigation strategy behind it
There is another important practical lesson which could easily be overlooked because the developer ultimately won the challenge. Greystoke CB Limited proposed a strategic transport contribution of £907,709, originally requested by Oxfordshire County Council, towards highway improvements intended to relieve congestion.
However, the Inspector found there was insufficient detail about what infrastructure the contribution would actually fund and insufficient up-to-date quantitative evidence showing how effectively it would mitigate the development’s impacts. The proposed obligation consequently failed to provide the necessary certainty and was also found not to satisfy regulation 122 of the Community Infrastructure Levy Regulations 2010.
The practical lesson being that a number in a section 106 agreement is not itself mitigation and where contributions are being sought and relied upon to make a development proposal acceptable there needs to be evidence to support what the mitigation is and how it will deal with the identified issue.
4. Test the residual position, not merely the “before mitigation” problem
Paragraph 116, now effectively Policy TR6, expressly focuses on residual cumulative impacts following mitigation. Transport assessments and committee reports should therefore make the sequence of reasoning explicit with a view to reducing the risk that an identified congestion problem becomes, almost by default, a highway reason for refusal without the NPPF threshold ever being properly addressed.
5. The planning balance
Where highways are an important adverse factor, the decision should expressly address paragraph 116 when carrying out the overall planning balance, rather than considering the paragraph in the transport section and then effectively putting it to one side.
The Secretary of State argued that paragraph 116 only prevented refusal where highways impacts were the sole reason for refusal. The Court rejected that narrow interpretation. The policy was engaged where highways were, in substance, a principal reason for refusing permission. Otherwise, the protection in paragraph 116 could readily be circumvented by adding another, relatively minor, planning harm to a highway objection.
Concluding thoughts
Significant congestion remains something which developers may have to mitigate. Highway authorities remain entitled to interrogate modelling and require proper evidence that proposed mitigation will work. Local development plan policies remain relevant. But where the decision ultimately turns on highway impacts, the national policy threshold for refusal cannot disappear from the analysis simply because other planning harms also exist.
Three questions should be asked and answered in full at the application stage:
- What is the identified highway harm?
- What mitigation is required and how will it address that harm?
- After that mitigation, is the there still an unacceptable safety impact or a severe residual cumulative network impact that means the NPPF threshold is reached to allow a refusal?
If the answer to the third question is no, Greystoke demonstrates why considerable care is required before highways become the determining reason for refusing the development.
Significant” is not synonymous with “severe”. The NPPF provides both a requirement to address significant transport impacts and a policy threshold restricting refusal on highway grounds. Decision-makers need to give proper effect to both. For all parties, the best strategy remains to tackle transport issues early, agree the modelling methodology, identify deliverable mitigation and quantify its effect. But if significant highway effects remain, the final question should always be: do they actually cross the NPPF threshold for refusing permission?