Committed development and cumulative impact in a TA

The committed development list in a transport assessment decides the capacity conclusion more often than the modelling does. Two competent engineers running the same junction model will agree on the outputs. They will disagree about which other schemes to add, how much background growth to apply on top, and which year to test. Settle that list with the highway authority before you commission a single count, because reopening it afterwards means repeating the work.

Cumulative traffic impact is the only basis on which a highway objection can carry weight under the current National Planning Policy Framework, published on 17 August 2026. Development should be refused on highway grounds where there would be an unacceptable impact on highway safety, or where the residual cumulative impacts on the road network would be severe. Residual and cumulative are both doing work in that sentence.

What counts as committed development

Committed development is other development whose traffic has to be in the future baseline because it is reasonably certain to happen.

The core of the list is uncontroversial: extant planning permissions not yet built or not yet fully occupied, within the study area, that would use any of the assessed junctions. Beyond that core, four categories get argued about.

• Permissions granted but unlikely to be implemented, for example a 2018 consent on a site now marketed for a different use.
• Local plan allocations without permission, which some highway authorities insist on and others exclude as speculative.
• Resolutions to grant subject to a section 106 agreement that has not been completed.
• Schemes at appeal, or refused and within the appeal period.

The treatment of partially occupied sites also matters. A 300-dwelling site with 180 occupied contributes only its remaining 120 to the future baseline, and the 180 are already in your surveyed counts. Getting that split wrong is one of the commonest sources of inflated future flows.

Agreeing the list with the highway authority

Ask for the authority's own committed development list in writing at scoping stage, then interrogate it rather than accepting it.

For each entry, record the application reference, the quantum in the same units as your trip rate source, the status, the expected opening year, and the source of the trip generation. Where the authority's figure for a committed scheme comes from that scheme's own transport assessment, read it. Trip rates prepared four years ago on an older database may differ from what you would derive today, and a difference in the junction turning proportions can matter more than the totals.

Send the list back as a table with a short note on each disputed entry. Where the strategic road network is affected, National Highways will want the same engagement, and Circular 01/2022 sets out how its interest is expressed.

Two practical points. Fix the study area at the same time as the list, because a junction included late brings its own committed development with it. And date the agreement, since a list agreed in March and a decision in November will have drifted.

Background growth and where the factors come from

Background growth covers traffic increases not attributable to any identified scheme: changes in trip rates, car ownership, demographics and economic activity.

In England the default source is the Department for Transport's TEMPRO software with the National Trip End Model datasets, which produce growth factors by zone, time period and trip purpose between a base and a forecast year. Use the local authority or ward level zone appropriate to your study area rather than a regional factor, and state the dataset version in the assessment. Where the authority operates a strategic traffic model, its outputs usually take precedence, and the model run will already contain assumptions about development you need to understand.

Growth factors are not a single number. A factor derived for all purposes over 24 hours will understate the peak hour change in a commuter location and overstate it elsewhere.

Double counting and how to avoid it

This is the error that most often undermines an otherwise sound assessment. Background growth derived from the trip end model already reflects planned housing and employment in the zone. Add committed development trips on top of an unadjusted growth factor and the same dwellings are counted twice.

The orthodox fix is to run the growth factors with alternative assumptions, reducing the zonal housing and employment inputs by the quantum you are modelling explicitly, so the factor covers only the residual growth. Document the adjustment with the inputs you used, because a reviewer cannot check it otherwise.

The opposite error exists too. Where committed development has been excluded on the basis that growth factors cover it, say so explicitly and show the numbers, or an objector will assume it was overlooked. Our note on reading signal junction model outputs at planning stage covers how these baseline choices surface in the results tables.

Choosing the assessment years

Survey validity is the weak link in the base year. Counts taken in a non-neutral month, during school holidays or in an atypical week will be challenged, and the point is examined in traffic counts in neutral months and survey validity.

For a phased scheme, test the year each phase opens rather than full build-out alone, because the mitigation trigger usually attaches to a phase. And where the local plan runs to a specific end date, authorities often want that date tested as well, which brings the whole allocation question back into play.

Sensitivity testing that survives scrutiny

Sensitivity tests exist to show which conclusions depend on contested assumptions. Three are worth running on almost every assessment.

1. With and without the disputed committed development entries, so the decision maker can see whether the scheme's own impact or the cumulative position drives the result.
2. Higher and lower background growth, using an alternative dataset version or a growth factor derived without alternative assumptions.
3. Trip rate variation, typically the TRICS upper and lower quartile rates alongside the mean, and a test with reduced mode share for the proposed travel plan measures.

Where a junction moves from acceptable to over capacity only in the most pessimistic combination, say so plainly and explain which assumption carries it. That is a stronger position than presenting a single central case. Capacity outputs for roundabouts respond sharply to geometry inputs as well, as set out in roundabout capacity, entry width, flare and the inputs.

Where a disputed list changes the conclusion

The practical consequence of a contested list is rarely refusal. It is mitigation, and who pays for it.

If a junction is shown to fail in the future baseline before the proposal is added, the development is not the cause of that failure, and the test becomes whether its residual impact is severe. The proportionate response may be a contribution rather than a scheme. If the junction only fails once the proposal is added, the applicant is likely to be asked to deliver the works, with the cost and land requirement that implies.

So the argument about whether a speculative allocation belongs in the list is an argument about several hundred thousand pounds, and it should be made at scoping with evidence rather than at committee with assertion. Set it out in the Transport Assessment as a short section of its own, with the agreed list, the rejected entries and the reasons.

Enter your postcode on PlanScope to see which transport documents a scheme of your scale is likely to need.

This is general guidance, not legal advice: check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.