Appeal evidence when the objection is highway capacity

When a refusal rests on junction capacity rather than highway safety, the appeal turns on a single policy test: whether the residual cumulative impacts on the road network would be severe. That is a high bar, and inspectors apply it as written. Congestion that is merely worse than today, or a junction that already operates over theoretical capacity before your development is added, does not meet it. Most capacity appeals are won on the arithmetic of the assessment years and the committed development list, not on the design of a mitigation scheme.

Separate the two grounds early. A visibility splay or access geometry objection is a safety matter decided on a different test, and the evidence base is different too, as our note on visibility splay evidence in safety refusals sets out. If the decision notice mixes both, deal with them in separate sections of your statement of case.

The severe residual impact test in national policy

The National Planning Policy Framework allows refusal on highways grounds in two circumstances: an unacceptable impact on highway safety, or residual cumulative impacts on the road network that would be severe. The August 2026 edition reorganised the Framework into policy-coded chapters in place of paragraph numbers, so quote the code as it appears in the edition current when you write and state the edition. The wording of the test itself has carried through successive editions.

Three words do the work. Residual means after mitigation and after any committed highway improvement. Cumulative means your development together with other development reasonably expected to come forward. Severe is not defined, which is why inspectors look for a demonstrated outcome rather than a modelled number crossing a line.

That matters because a ratio of flow to capacity of 0.92 on one arm of a priority junction in a future design year is a model output. The question the inspector asks is what that means on the ground: how long the queue is, for how many minutes of the peak hour, whether it blocks back to another junction, and whether anything unsafe follows. A transport statement that reports values without translating them into consequences invites the authority to supply its own interpretation.

Planning Practice Guidance on travel plans, transport assessments and statements is the procedural reference for scope and method, and it expects the scope to be agreed with the highway authority. Where it was not agreed, say why.

Base year and future year disputes

More capacity appeals turn on assessment years than on anything else.

Surveys taken in a school holiday, in August, over Christmas, or during roadworks on a parallel route do not represent a neutral base. Counts should be from a neutral month, and where they were not, the appeal needs either a correction factor with a stated source or a fresh count. Authorities know this and will raise it.

The future year choice is the second battleground. Standard practice is to assess the opening year and a design year some years beyond it, with background growth applied from the Department for Transport's national trip end projections. Two legitimate approaches produce different answers: growth factors applied to the whole network, or growth already captured in a list of committed developments. Applying both to the same flows double counts, and an authority's consultant will find that quickly.

Where the base year is more than about three years old by the time the appeal is heard, expect a challenge. The practical answer is a short validation count on the critical arms, compared against the original survey, with the difference stated plainly. A single day of counting costs far less than losing the appeal on the point.

Committed development assumptions

The committed development list is where cumulative impact lives, and it is usually the weakest part of a submitted assessment.

Build the list from the authority's own decisions register, with the application reference, the description, the number of units or floorspace, the decision date, and whether the permission has been implemented. Include allocations where the plan expects delivery within the assessment period. Then state the assumption applied to each: whether trips were added in full, discounted for implementation risk, or excluded with a reason.

A list that excludes a 400-home allocation two junctions away because it has no permission yet looks like avoidance. A list that includes every speculative scheme in the district inflates the baseline and makes the junction look congested without you, which can cut either way. Pick a defensible method, write it down, and apply it consistently.

The useful consequence of a properly built list is that it often shows the junction failing in the future year with or without the appeal scheme. Where that is the case, the development's contribution to the problem becomes a small increment, and a small increment is hard to call severe. Set that comparison out as a table of without-development and with-development results for each assessment year.

Trip rate challenges from the authority

Authorities challenge trip rates in predictable ways: the sites selected from the national trip rate database are not comparable, the sample is too small, the location type is wrong, or the survey dates predate current travel patterns.

Record the query parameters so the selection can be reproduced: land use category, region and area type filters, date range, site size range, parking provision, and the number of surveyed sites retained. Explain any site you excluded. Where the sample is thin, present a sensitivity test at a higher rate rather than defending the central case alone, because an inspector who sees the scheme pass at the pessimistic rate stops caring about the argument.

Internalised trips, linked trips and mode share assumptions need the same treatment. A claimed 40 per cent sustainable mode share on an edge-of-settlement site with an hourly bus needs evidence from the census travel-to-work data for the right output area, not a planning aspiration.

Model audit findings at appeal

Expect the authority to audit the model files, and expect the audit to find something. The common findings are geometry inputs that do not match the drawing, flared approach lengths taken optimistically, saturation flows left at default where the arm is narrow or on a gradient, pedestrian stage timings omitted at signals, and queue blocking between closely spaced junctions left unmodelled.

Run your own audit before submission. Check the coded geometry against the survey, check that signal timings match the controller settings obtained from the authority, and test what happens when optimistic inputs are tightened. Where an input is uncertain, model the range and report both ends.

Hand over the model files with the appeal documents. Refusing to release them, or releasing them late, reads as a concession and gives the authority a procedural complaint it can make in front of the inspector.

Mitigation that the authority will accept

Mitigation that requires land you do not control, or works outside the highway boundary, is not mitigation unless the agreement is in place. A section 278 agreement under the Highways Act 1980 is the normal mechanism, and a technical approval in principle for anything structural.

Keep the scope proportionate. Signal optimisation, a minor flare, a right-turn pocket or a change to a give-way layout can be delivered and secured by condition or obligation. A new roundabout proposed to solve a problem the scheme contributes 3 per cent of is an invitation to argue about reasonableness, and the tests in the Community Infrastructure Levy Regulations 2010 for obligations apply: necessary, directly related, and fairly and reasonably related in scale and kind.

Where the strategic road network is affected, Circular 01/2022 governs National Highways' approach and its response will carry weight. Deal with it directly rather than hoping the inspector discounts it.

Presenting the capacity case

Structure the appeal evidence around the test rather than around the chapters of your transport assessment. State the test, then answer it.

A workable order is: the agreed scope and what the authority accepted at the time, the base data and why it is sound, the growth and committed development method, the results as a without-and-with comparison by year, what the results mean in queue lengths and delay, the mitigation and how it is secured, and finally why the residual cumulative impact is not severe. Cross-reference the planning statement rather than repeating the policy argument in both documents.

Appeal decisions on comparable junctions are indicative of how inspectors have approached the question, not binding precedent, and our guidance on using appeal decisions as evidence explains how far to push them. Where the refusal came from members against an officer recommendation, the absence of professional highway evidence behind the reason is itself a point to make, and the prospects in that situation differ from a case where the authority's own engineer objected.

Transport capacity work is specialist. Councils expect a suitably qualified transport planner to author or review it, and professional review before submission is strongly advised. Enter your postcode on PlanScope to draft the supporting statements around that technical evidence.

This is general guidance, not legal advice. Check the current National Planning Policy Framework edition and the position with your local planning authority or a qualified professional before relying on it.

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