Distributing and assigning trips in a Transport Assessment

Trip distribution and assignment is the stage of a Transport Assessment that converts a number of trips into flows at named junctions, and it is where a reviewer can most easily unpick a conclusion. Generation tells you that a scheme produces 38 two-way vehicle trips in the morning peak. Distribution tells you where those trips are going. Assignment tells you how many of them turn right at the junction the authority is worried about. Each stage needs its own evidence, and a mitigation package is only as sound as the weakest of the three.

Why distribution is a separate exercise from generation

Trip generation is an empirical question answered from surveyed sites. Distribution is a question about the geography of the area, and no database answers it.

Two schemes with identical trip rates can place entirely different loads on a junction. A housing site west of a town, with employment to the east, sends most of its morning departures through the centre. The same site east of the town sends them away from it. The trip rate is the same number and the junction result is not.

So distribution has to be evidenced from local data about where people travel, and then assignment has to be evidenced from how they would reach those destinations given the network. Where an assessment states a distribution as a set of percentages with no source, that is the first thing a highway officer will query, and rightly.

Census and survey sources for journey patterns

The usual starting point for residential development is the census origin and destination data on travel to work, which gives flows from the area containing the site to workplace areas. It is free, published and reproducible, which makes it defensible even where it is imperfect.

But its limitations need stating rather than hiding. It covers commuting, which is only part of the morning peak and a smaller part of other periods. It is collected at a point in time, and the most recent collection in England and Wales took place during a period of pandemic restriction, which affected recorded travel to work patterns for many people. And the geography is coarse for a small site.

Other sources, used alone or to supplement it:

• Surveys of the existing use, where a site is being redeveloped, including registration plate matching to establish real routes rather than assumed ones.
• Pupil postcode data from the local education authority for school proposals, which is the best distribution data available for any land use.
• Household interview surveys for retail and leisure, which establish a catchment directly, with the survey area and sample agreed in advance.
• The authority's own strategic transport model, where one exists, which already contains a calibrated distribution.

State which source you used for each land use in the scheme. Mixed use proposals need separate distributions for each component, and blending them produces a result that fits none of them.

Gravity models and what they assume

A gravity model distributes trips in proportion to the attractiveness of each destination and inversely to the cost of reaching it, usually distance or travel time. It is the right tool for a large scheme in a complex network and the wrong one for 25 houses.

The assumptions worth exposing when you use or review one:

The measure of attraction. Employment floorspace, jobs, population or retail floorspace each give different answers, and the choice should relate to the trip purpose being modelled.

The deterrence function and its parameters. A model calibrated on long-distance commuting will spread trips further than one calibrated on local journeys, and the calibration data should be named.

Zoning. Coarse zones hide short trips inside a zone, which is exactly where a local junction assessment needs detail.

Whether the model is calibrated at all. An uncalibrated gravity model is a set of assumptions with arithmetic attached, and a proportional distribution to published census flows is more transparent and usually more defensible on a minor or small major scheme.

Assigning trips to the junctions assessed

Assignment converts distribution into turning movements. Four steps, each needing agreement.

1. Agree the study area and the junctions to be assessed at scoping, in writing. Adding a junction later means redoing the work.
2. Identify the realistic routes to each destination zone, taking account of signal delay and congestion rather than distance alone. Drivers do not use the shortest route if it queues.
3. Split the distribution across those routes and convert the result into turning counts at each junction, in each assessed period.
4. Add the trips to the baseline flows for the same period, giving the with-development case to be tested against capacity.

The common error is assigning all trips in a direction to a single route when two exist. That overloads one junction and understates another, and reviewers spot it by checking whether the inbound and outbound flows at each junction reconcile.

Where a scheme is small enough for a Transport Statement rather than a full assessment, the same logic applies at a reduced scale: a short distribution table, a route diagram and the resulting turning movements at one or two junctions.

Committed development and background growth

The baseline against which the development is tested has to include growth that is already expected, and this is where double counting creeps in.

Two sources of future flow exist. General background growth, usually taken from the national trip end dataset for the relevant area and years. And committed development, meaning schemes with planning permission or allocated and likely to come forward, which generate specific trips at specific junctions.

Those two overlap, because the national growth forecast already assumes housing and employment growth in the area. Adding full background growth and all committed development on top of it inflates the baseline and makes the proposal look less harmful, which an authority will challenge even though it helps the applicant.

The usual resolutions are to adjust the growth forecast assumptions to remove the development the committed schemes represent, or to use the growth forecast alone where committed development is modest, or to use committed development alone over a short horizon. Agree the method at scoping and record it, because this single decision moves the junction results more than most design changes do.

Also agree the assessment years. The opening year and a future year, commonly five or ten years after opening, with the design year matched to the local plan period where the authority asks for it.

Sensitivity testing the distribution

Distribution is the most uncertain input in the chain, so test it rather than defend it.

Useful tests: a distribution skewed towards the critical junction, to see whether the capacity conclusion survives. A higher car mode share than the travel plan assumes, which answers the objection that the plan is optimistic. A different split between two competing routes. And on sites where a new access changes route choice, a reassignment test showing existing traffic rerouting.

Present the sensitivity results in the same table format as the base case, with a sentence on whether the conclusion changes. Where it does change, the honest answer is to design for the sensitivity case or to propose mitigation triggered by monitoring. A Travel Plan with monitoring, targets and a remedial measures fund is the usual mechanism, and it is far more persuasive when it is tied to the specific uncertainty the sensitivity test exposed.

Presenting flows so they can be checked

A reviewer should be able to follow the chain from trip rate to junction flow without asking for a spreadsheet. That means showing the working at every step.

• The trip rates used, per unit, by period, with the source. The companion question of how those rates were chosen is covered in the note on selecting trip rates inside a Transport Assessment.
• Total trips by period, arrivals and departures shown separately.
• The distribution as a percentage table by destination zone, with the source named for each land use.
• A route diagram for each destination group.
• Turning movement diagrams at each junction, for each period and each scenario, with the scenario labelled unambiguously.
• A reconciliation check: total trips assigned equals total trips generated.

And that last line is worth building into the drawing. Assessments routinely assign more or fewer trips than they generated, usually through a rounding or spreadsheet error, and the check catches it before the authority does.

Where flows are presented only as modelling outputs, include the input files or at least the full turning counts. A Transport Assessment that reports only capacity results asks the reviewer to accept the distribution on trust, and the result is a request for information and a lost month.

Illustrative example: a 90-dwelling scheme on the northern edge of a market town. Census travel to work flows from the containing area give 46 per cent of commuting trips to the town itself, 22 per cent to a city 14 miles south, 11 per cent to a business park east and the remainder dispersed. Southbound trips split between the A road and a signalised town centre route in a 70:30 ratio, tested at 85:15 as a sensitivity. The critical junction remains within capacity under both, and the travel plan carries a monitoring trigger tied to the southbound split.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. For specialist technical reports, qualified professional input may be required, and distribution and assignment work should be prepared or checked by a transport planner. We do not guarantee planning approval. To see which transport documents a scheme is likely to need, enter the site postcode on PlanScope.