Reading signal junction model outputs at planning stage
Three numbers decide whether a highway officer accepts your junction: degree of saturation, mean maximum queue and practical reserve capacity. Here is what each one hides.
A signal junction model produces three numbers per approach that decide whether a highway officer accepts your access strategy: degree of saturation, mean maximum queue and, for the junction as a whole, practical reserve capacity. Everything else in the output file is supporting detail. If you can read those three and explain what they mean on the ground, you can hold a conversation about mitigation rather than waiting for a consultee response.
What follows assumes a standard signal modelling package of the type used across the United Kingdom, run for existing, future baseline and future with-development scenarios in the assessment years agreed with the highway authority.
What the model is being asked
The model takes turning flows, saturation flows and a signal arrangement, and calculates how much of each approach's green time is used. It is a deterministic calculation, not a simulation, and it assumes the demand arrives as modelled.
That assumption is the model's main weakness and the reason officers look past the headline numbers. Real traffic arrives in platoons, drivers block junctions, buses stop in lanes and pedestrians push buttons. A model showing every arm comfortably within capacity can describe a junction that visibly queues.
So the sensible framing in a report is that the model tests whether the signal arrangement can serve the demand in principle, and that the observed behaviour and the queue storage check tell you whether it will work in practice.
Degree of saturation and the accepted limits
Degree of saturation is the ratio of the demand flow on a lane to the maximum flow that lane could discharge in the green time available, expressed as a percentage.
The conventional practical limit in the United Kingdom is 90 per cent. Below it, queues clear within the cycle and the junction behaves stably. Above it, queues carry over from one cycle to the next and delay rises steeply for small increases in demand. Authorities set their own expectations and some accept higher values on constrained urban arms, so agree the threshold before you model rather than after.
Read it by lane rather than by arm. An arm at 85 per cent overall can contain a right-turn lane at 102 per cent, and the right-turn lane is where the problem will show. Present the results by lane and let the reviewer see the detail.
Watch the assessment period too. A peak hour split into segments will usually have one segment materially worse than the average, and that segment is the one to quote.
Mean maximum queue and blocking back
Mean maximum queue is the average, across the modelled periods, of the longest queue reached in each. It is reported in passenger car units, which convert to roughly six metres of road each for a mixed traffic stream.
The question that matters is whether that queue fits. Three checks are worth doing on every arm.
- Compare the queue length in metres against the distance to the next junction or access upstream.
- Compare the right-turn queue against the length of the right-turn pocket, because an overflowing pocket blocks the ahead lane.
- Compare the queue on the exit side against any downstream constraint, because a blocked exit starves the junction of capacity regardless of its own signal timings.
A model that reports acceptable saturation while predicting a queue longer than the available storage is telling you the junction fails. Say so in the report rather than quoting the saturation figure and moving on. Officers notice.
Practical reserve capacity explained
Practical reserve capacity is a single figure for the junction, derived from the worst lane's degree of saturation. It expresses how much additional traffic the junction could accommodate before that worst lane reaches the practical limit, as a percentage of current flows.
A value of zero means the worst lane is operating exactly at the practical limit. A positive value means spare capacity. A negative value means the limit is already exceeded, and the magnitude tells you roughly by how much traffic would have to fall for the junction to recover.
Two cautions. Practical reserve capacity is driven entirely by the single worst lane, so a junction with one constrained right turn and seven comfortable approaches reports a poor figure that misrepresents the whole. And it assumes flows grow proportionally across all movements, which development traffic rarely does. Quote it, then explain what is driving it.
Cycle time, stages and pedestrian phases
The signal arrangement is an input you control, within limits, and it is where most of the available capacity improvement sits.
Longer cycle times increase capacity because a smaller proportion of each cycle is lost to intergreens. They also increase delay for every user, including pedestrians, and long cycles at junctions with high pedestrian demand encourage crossing against the signals. Authorities in urban centres increasingly set a maximum cycle time for that reason.
Stage arrangement matters as much. Separating conflicting movements into their own stages buys safety and costs capacity. Running a filter arrangement recovers some of it. Pedestrian phases, whether all-round or parallel, consume time in every cycle in which they are called.
Model what will actually be implemented. A model that assumes a cycle time or a stage arrangement the authority will not operate, or that omits a pedestrian phase that exists on the ground, is not testing the junction that will be built.
| Output | What it tells you | What to check it against |
|---|---|---|
| Degree of saturation | Whether the lane can discharge its demand | The authority's practical limit, by lane and by segment |
| Mean maximum queue | How far traffic backs up | Available storage to the upstream junction |
| Practical reserve capacity | Spare capacity at the worst lane | What movement is driving it |
| Total delay | Comparison between options | The equivalent figure for the baseline |
Where mitigation buys capacity
Mitigation options fall into three groups, and they differ in how much programme and money they consume.
Signal changes are the least disruptive: revised stage arrangements, adjusted timings, filters, and modern detection to make the operation more responsive. These need the authority's agreement and often a change to the signal controller, and they cost far less than civil works.
Geometric changes buy more but cost more: an extra approach lane, a longer right-turn pocket, a widened exit. These bring in a highway agreement, land within the highway boundary or outside it, and a longer programme. Test whether they fit the highway boundary before modelling them, because a modelled improvement that cannot be built is worse than no mitigation at all.
Demand measures are the third group, and they are the ones most often dismissed too quickly. Reducing the trips the development generates at the peak is capacity you do not have to build. A properly resourced Travel Plan with monitoring and a mechanism for further measures is treated more seriously by highway authorities than it once was, particularly where the site has genuine alternatives to driving.
Reporting results without drowning the officer
The reviewer wants to check your work quickly. Structure the reporting so they can.
Put the summary tables in the main text of the Transport Assessment and the full model output in an appendix. Report by lane and by segment. State the assessment years, the growth assumptions and where the flows came from. State the signal arrangement modelled and confirm it matches what the authority operates or would agree to operate.
Then add the two things most reports omit. A short paragraph comparing predicted queues against available storage, arm by arm. And a plain statement of what the development changes: the difference between future baseline and future with development, rather than the absolute figures alone. A junction already failing without your scheme is a different case from one your scheme pushes over the limit, and the officer needs to see which it is.
Offer the model files. Where the junction lies on or near the strategic road network, expect a closer review and read the position set out in when National Highways objects to an application before you submit.
Junction modelling is work for a suitably qualified transport planner, and councils expect that. PlanScope's drafts support the written submission rather than the modelling.
This is general guidance, not legal advice. Check the position with your local highway authority or a qualified professional before relying on it.
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Frequently asked questions
What degree of saturation is acceptable at a signalised junction?
The conventional practical limit in the United Kingdom is 90 per cent. Below it queues clear within the cycle; above it they carry over and delay rises steeply for small increases in demand. Authorities set their own expectations and some accept higher values on constrained urban arms, so agree the threshold before modelling.
What is practical reserve capacity?
A single junction figure derived from the worst lane's degree of saturation, expressing how much additional traffic could be accommodated before that lane reaches the practical limit. Zero means the worst lane is exactly at the limit. Because it is driven by one lane and assumes proportional growth, quote it and then explain what is causing it.
How should mean maximum queue be interpreted?
It is the average, across modelled periods, of the longest queue reached in each, reported in passenger car units of roughly six metres. Compare it against the distance to the upstream junction, against the length of any right-turn pocket, and against downstream constraints. A queue longer than the available storage means the junction fails whatever the saturation figure says.
Does a model showing spare capacity mean the junction works?
Not necessarily. The calculation assumes demand arrives as modelled, whereas real traffic arrives in platoons, drivers block junctions and buses stop in lanes. Treat the model as a test of whether the signal arrangement can serve the demand in principle, then use the queue storage check and observed behaviour to test whether it will in practice.
What mitigation options are available at a signal junction?
Signal changes are least disruptive: revised stages, adjusted timings, filters and better detection, all requiring the authority's agreement. Geometric changes buy more capacity but bring in highway agreements and land. Demand measures come third and are often dismissed too quickly, since trips avoided at the peak are capacity you do not have to build.
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.