The trip distribution section of a transport assessment

Trip distribution is the step in a transport assessment that decides where the traffic from your site actually goes, and it is the step highway officers read most carefully. Trip generation gives you a number of vehicle movements in the morning and evening peaks. Distribution turns that number into a pattern of origins and destinations, and assignment puts that pattern onto real roads and junctions. Get it wrong and every junction capacity result downstream is wrong too.

A Transport Assessment that presents solid trip rates and then waves a hand at distribution invites a holding objection. The authority cannot test your conclusions if it cannot see how you split the trips.

What the distribution section has to prove

Three things. That the pattern you have used reflects how people actually travel to and from that kind of development in that place, that the data behind it is current enough to rely on, and that the busiest junctions on the network have been loaded with a fair share rather than a convenient one.

The section should open with the method in a paragraph, not a page. Say which dataset you used, which zone system you applied it to, and what you did where the dataset did not fit. Then show the resulting percentage split by direction or by zone in a table, and show the same split as arrows on a plan. Officers work from the plan. Consultees check the table.

Planning Practice Guidance on transport assessments expects the scope to be agreed with the highway authority before the work is done, and distribution method is exactly the sort of thing that belongs in a scoping note. Agreeing it early costs one email. Arguing it after validation costs a determination period.

Choosing the data that sets the pattern

For residential schemes, census journey to work data remains the default starting point in most of England. It gives you a matrix of home location against workplace location at output area or middle layer super output area level, which you can aggregate to the zones that matter for your site access. It is a travel to work dataset, so it describes the commuting peak rather than the school run, the supermarket trip or the weekend pattern.

That limitation matters more than people admit. A 60-unit scheme on the edge of a market town generates plenty of movements that have nothing to do with anybody's employer.

Where the commuting matrix is too coarse, the usual supplements are:

• Existing survey data from the site or a comparable site nearby, including automatic traffic counts and turning counts already collected for the junction modelling
• TRICS multi-modal surveys for the same land use in a similar location type, used for mode share rather than for distribution directly
• Local authority transport model outputs, where the authority holds one and will share the zonal pattern
• Catchment evidence for non-residential uses, such as a school's admissions area or an employer's staff postcode profile supplied by the occupier

For retail, leisure and other trip-attracting uses, the pattern is driven by catchment rather than by commuting, so a gravity approach or a competing-facility catchment study will beat the census every time.

Gravity models and observed patterns

A gravity model distributes trips in proportion to the attractiveness of each destination and in inverse proportion to the cost of getting there, with cost usually expressed as generalised journey time. It is defensible for a use with no existing equivalent nearby, and it is the honest choice when you do not know the pattern.

Observed data beats a model whenever you can get it. If there is a similar development two miles away with a car park and a willing owner, a licence plate or entry-exit survey gives you a real pattern for the cost of a day's fieldwork.

The trap is presenting a gravity model as though it were observation. Label it. Say which zones you included, which impedance function you used and what you calibrated against, even if the calibration was a single observed corridor split. A reviewer who can see the assumptions can accept them. A reviewer who cannot see them assumes the worst.

An illustrative worked split

Take a 45-dwelling scheme on a B-road east of a town centre, with the strategic road network reached by a roundabout 1.2 km to the west. This example is illustrative only. The census matrix for the surrounding output areas shows most employment destinations lying west and south-west. You adopt a westbound split of roughly three quarters in the morning peak, with the remainder heading east towards a smaller employment area, then sensitivity test at a 60:40 split because the census predates a large employment allocation to the east. The junction results hold under both. That sensitivity test is the paragraph the officer will quote when recommending approval.

Getting trips onto the network

Assignment takes the zonal percentages and routes them through the network to the junctions being modelled. On a small scheme this is manual and perfectly acceptable: you pick the sensible route to each zone, apply the percentage, and add the flows at each arm.

Two rules keep it defensible. Route on generalised cost rather than distance alone, because drivers use the quicker road even when it is longer. And show your routes on a plan with the flow numbers on each arm, so the authority can check arithmetic rather than take it on trust.

Where two routes are close in journey time, split the flow between them and say so. Where a route passes a sensitive frontage, a school or an air quality management area, expect to be asked about it whether or not your model sends traffic that way.

Peak hours and the directional split

The network peak and the development peak are not always the same hour. A primary school peaks at 8:45am and again at 3:15pm. A distribution unit peaks before the road does. State which hours you have modelled and why, and where the two peaks diverge, model both or explain why the worst case is covered.

Directional split follows from the land use. Housing runs out in the morning and in at night, typically in the region of a 30:70 out-in ratio reversing in the evening, and the precise figures should come from your trip rate source rather than from habit. Mixed-use schemes partly cancel, which is a real benefit worth claiming, but only where the uses are really linked and the internalisation assumption is stated.

Testing the distribution for sensitivity

Every distribution is a forecast, so test it. The standard approach is to rerun the critical junction with a materially different split, usually shifting 10 to 15 percentage points from the dominant direction to the next one, and report whether any arm moves from within capacity to over it.

If nothing changes, say so in one sentence and move on. If something does change, you have found the point your case turns on, and it is better to find it yourself than to have a consultee find it during the consultation period. The same discipline applies across the evidence base, whether you are setting survey hours in a parking survey or phasing vehicle movements in a construction management plan.

Where highway officers push back

The recurring objections are predictable enough to design against. Using a national average pattern where local data exists. Using census data without acknowledging its age or its commuting bias. Sending traffic away from a constrained junction on a route no driver would take. Omitting committed development that will change the pattern before your scheme is occupied.

Committed development is the one that catches people out. The authority holds the list, it changes, and it should be requested in writing at scoping rather than assumed from the public register.

Distribution is also where a highway objection is most often answered. If the objection says the scheme puts unacceptable traffic through a junction, the reply is usually a better-evidenced split rather than a bigger junction, and that reply lands faster when the original section set out its method clearly.

A transport assessment is a report councils expect to see from a suitably qualified transport planner, and professional review is strongly advised before submission. PlanScope can draft the structure and the policy framing around your modelling outputs, then you can send it for professional review before it goes in with the application. Browse all 66 PlanScope report types to see what else the site 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.