The junction capacity section of a transport assessment
Junction capacity modelling decides most highway objections. Here is how to scope the junctions, agree growth and committed development, and read the capacity and queue outputs a case officer will question.
Junction capacity is the part of a transport assessment that decides whether a highway objection lands, and it is the part most often scoped badly. A junction capacity assessment for planning tests whether the network can absorb development traffic at the busiest hours of a future year, using surveyed flows, agreed growth and a model of each junction's geometry. Get the assessment years and the committed development list agreed with the highway authority first, because every number downstream depends on them.
Choosing the junctions to model
There is no national screening threshold any more. The Guidance on Transport Assessment was cancelled in 2014 and Planning Practice Guidance on travel plans, transport assessments and statements took its place, which leaves the scope to be agreed locally.
In practice the highway authority will ask for any junction where development traffic forms a material share of existing flows, any junction already operating close to capacity, and the site access itself. Authorities express the trigger differently, usually as a percentage of two-way flows or as an absolute number of additional vehicle movements in the peak hour. Ask for the figure in writing at scoping rather than guessing.
Two traps recur. The first is stopping at the nearest signal when the real constraint is a priority junction two links away that queues back into it. The second is modelling only the highway authority's list when a neighbouring authority, National Highways or Transport for London also has an interest, which happens whenever a strategic road or a red route sits within the study area.
PlanScope flags a Transport Assessment for schemes of 50 or more dwellings or 2,500 square metres or more of floorspace. Those are the product's own prompts rather than a legal test, and the council's validation list and the highway authority's own thresholds take precedence.
Base year surveys and the neutral month rule
Counts have to represent a normal week. That means avoiding school holidays, the weeks either side of Christmas, and August, and it means recording the date and weather so a reviewer can judge the sample.
A standard package is a manual classified count at each junction across the weekday morning and evening peak periods, usually 07:00 to 10:00 and 16:00 to 19:00, with turning movements split by arm and by vehicle class. Queue length surveys taken at the same time are worth the extra cost, because they are the only way to calibrate a model against observed behaviour rather than trusting the defaults.
Where counts are older than about three years, expect to be asked to refresh them or to justify the reuse with a comparison against a permanent count site. Surveys taken during the pandemic period carry an obvious health warning and most authorities will not accept them without a validated uplift.
Growth, committed development and double counting
Background growth normally comes from the Department for Transport's TEMPro dataset, applied between the survey year and each assessment year. Committed development is added on top, taken from a list of consented but unimplemented schemes supplied or confirmed by the authority.
But the two overlap. TEMPro growth already reflects planned housing and employment in the trip end forecasts, so adding a large committed site on top of an unadjusted growth factor counts the same traffic twice. The usual fix is to apply an adjustment within TEMPro to strip out the growth attributable to the committed sites being modelled explicitly. Say in the report which approach was used and why.
Trip generation for the proposal itself will normally come from the TRICS database, with the site selection criteria, the filtering and the resulting rates reproduced in an appendix. A reviewer who cannot reconstruct your rates from the appendix will treat the whole chapter as unverified.
Reading capacity outputs
Priority junctions and roundabouts are reported as a ratio of flow to capacity. Signal junctions are reported as a degree of saturation and a practical reserve capacity. Both describe how much of the available throughput is being used in the modelled peak hour.
| Junction type | Headline measure | Conventional practical limit |
|---|---|---|
| Priority (T junction, crossroads) | Ratio of flow to capacity | 0.85 |
| Roundabout | Ratio of flow to capacity per arm | 0.85 |
| Signalised | Degree of saturation | 90 per cent |
Those limits are conventions rather than standards, and they matter less than the direction of travel between scenarios. What a case officer needs from the chapter is a clear comparison of base year, future year without development and future year with development, arm by arm, so that the increment attributable to the proposal is visible. A junction already at 1.05 without the development is not the applicant's problem to solve, and the report should say so plainly.
Queues and blocking back
Mean maximum queue in passenger car units is the output that turns a number into an argument. A right-turn arm running at 0.9 with a 12 vehicle queue behind it will block a side road, a bus stop or a farm access long before the ratio reaches one.
Check the queue against the physical length of the link. If the modelled queue exceeds the distance to the next junction, the model has stopped being valid, and the honest answer is either a linked model or a microsimulation rather than a footnote. Inspectors notice when a chapter reports comfortable ratios while the site visit shows queues past the upstream junction every evening.
Mitigation that can actually be built
Mitigation only counts if it is deliverable. Widening an approach into a verge owned by a third party, or relying on a signal upgrade that the authority has no programme for, is a scheme that will not survive a highways consultation.
Work through four questions for each measure.
- Does the land sit within the adopted highway or within the applicant's control?
- Can it be secured through a section 278 agreement under the Highways Act 1980, a condition, or a planning obligation?
- Does it degrade conditions for pedestrians, cyclists or bus users, which Active Travel England will examine on larger majors?
- Does the model show the improvement under the same assumptions as the do-nothing case?
Demand management sits alongside physical works. A Travel Plan with monitored targets and a named coordinator carries weight where an authority accepts modal shift, though few will let you discount modelled trips for it without evidence from a comparable site.
Agreeing the parameters before you model
Put the scoping note in writing and get it acknowledged. It should fix the junctions, the survey dates, the assessment years, the growth method, the committed development list, the trip rates and the mitigation principles. Everything after that is arithmetic.
For smaller schemes the same discipline applies at reduced scale, and a Transport Statement may be all the authority asks for. The difference is scope rather than rigour.
Transport Assessments are a report type where councils expect a suitably qualified author, normally a transport planner with access to the modelling packages and the trip rate database, and professional review is strongly advised before submission. A PlanScope draft can carry the policy framing, the site description, the sustainability narrative and the structure of the chapter, with the drafting flagging every point where survey data and model outputs still have to be supplied. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
This is general guidance rather than legal advice, and highway authority requirements vary, so confirm the scope with the authority before commissioning surveys. Browse all 66 PlanScope report types to see what else a major application is likely to need.
Frequently asked questions
How many junctions should a transport assessment model?
There is no national figure. Since the Guidance on Transport Assessment was cancelled in 2014, the study area is agreed with the local highway authority at scoping. Expect to cover the site access, any junction where development traffic forms a material share of flows, and any junction already close to capacity. Get the list confirmed in writing.
What does a ratio of flow to capacity of 0.85 mean?
It means the arm is using about 85 per cent of its modelled throughput in the peak hour. The figure is a long-standing industry convention for the point at which queues start to grow quickly, not a policy threshold. What matters more is the difference between the future case with and without the development.
When should traffic counts be carried out?
In a neutral month, avoiding August, the weeks around Christmas and school holidays, on a normal weekday. Most authorities want morning and evening peak period counts with turning movements by arm and vehicle class. Counts older than around three years usually need refreshing or a justification based on permanent count site comparisons.
How do you avoid double counting committed development?
TEMPro growth factors already include planned housing and employment, so adding committed sites on top without adjustment counts the same trips twice. The usual method is to adjust the TEMPro trip ends to remove growth from the sites being modelled explicitly, then add those sites separately. State the method used in the report.
Can a transport assessment be prepared without a transport planner?
Councils normally expect a suitably qualified author, because the junction modelling and trip rate work need specialist software and judgement. A draft can set out the policy framework, site description and structure, but the modelling chapter and its conclusions should be prepared or reviewed by a transport planner before submission.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.