Trip generation and trip distribution: the two steps

Trip generation is the estimate of how many journeys a development will produce or attract in a given period. Trip distribution is the separate exercise of deciding where those journeys start and end, and assignment puts them onto specific roads and junctions. A transport assessment does all three, in that order, and a highway officer will scrutinise each on different grounds. Confusing them is why some assessments get long comments back.

The numbers matter because they drive everything downstream: junction capacity modelling, mitigation, contributions and sometimes the principle of development.

What trip generation counts and over what period

Trip generation is normally expressed as trips per unit of development in a defined period. For housing the unit is the dwelling. For commercial floorspace it is 100 m² of gross floor area. For a school it might be pupils, for a care home, bed spaces.

Rates come from surveyed sites held in the TRICS database, which records multi-modal counts at real developments across the country. You select a land use category, apply filters for location type, site size, region and survey date, then take an average rate from the resulting sample.

The periods assessed are usually the weekday morning and evening network peaks, with a separate Saturday peak for retail and leisure. Daily totals are reported too, mostly for context and for travel plan target setting. Which peak matters depends on the use: a primary school produces a sharp arrival peak that may not coincide with the network peak at all, and assessing the wrong hour makes the whole exercise academic.

The comparison the officer cares about is the peak hour on the network, not the peak hour of the development. Report both.

Net trips after the existing use is credited

Very few sites are empty. Where a lawful use exists, its trips can be credited against the proposal, and the net figure is what is assessed.

Three conditions attach to that credit. The existing use must be lawful, which means implemented and not abandoned. It must be capable of resuming, so a building demolished ten years ago is a weak basis. And the rate applied to it should come from the same source and the same filters as the proposal, rather than being taken from a generous category to inflate the credit.

Vacant premises are the contentious case. A unit empty for two years still has a lawful use, and an established planning position is that the trips associated with that use can be taken into account. Officers sometimes resist a full credit where the use has been vacant for a long time. Set out the lawful use, the evidence for it, the date it last operated and the rate applied, then let the officer form a view on the record rather than in correspondence.

Where the site is a change of use rather than a new build, the net position often turns out to be neutral or negative, which is the strongest argument available. Say it plainly and early.

Distribution and assignment to the network

Distribution answers where the trips come from and go to. The common methods are journey-to-work data from the census for residential and employment uses, a gravity model weighted by population and distance for retail and leisure, and observed data from a comparable operating site where the applicant has it.

Census journey-to-work data carries a caveat that has to be stated: the 2021 census was taken during a period of unusual working patterns, so its travel-to-work responses do not describe a normal week in some areas. Where you use it, say what you have done about that, whether that is a sensitivity test or a cross-check against another source.

Assignment then routes the distributed trips onto the network, usually by shortest path or by observed turning proportions at the junctions being modelled. The output is a set of link and turning flows that feed junction capacity modelling.

Two things go wrong here more than anywhere else. Assignment that sends all traffic to the nearest main road, when local knowledge says a rat run exists. And distribution taken from a national average when the site sits next to a single dominant employer.

Mode share and multi-modal rates

TRICS holds multi-modal survey data, so trips can be reported by car, public transport, walking and cycling rather than as vehicle movements alone. Use it. A vehicle-only assessment invites a request to redo the work in accessible locations.

Mode share should reflect the site, not an aspiration. Taking a mode share from a city centre survey site and applying it to an edge-of-settlement scheme is the kind of adjustment officers notice. Where the applicant proposes measures that would shift mode share, such as a travel plan with incentives, the honest structure is to assess the unmitigated position first and present the shift as mitigation with monitoring attached.

Where mode share is used to justify reduced parking, it should be consistent with the parking evidence. An assessment claiming 30 per cent car mode share alongside a parking layout at one space per dwelling is arguing against itself.

Accessibility mapping supports the mode share case, and a travel plan is where the measures and monitoring targets belong rather than in the assessment itself.

Peak hour selection and reporting

The Planning Practice Guidance on travel plans, transport assessments and statements sets the framework: the scope should be agreed with the highway authority before the work is done. That single step removes most of the argument, because peak hours, assessment years, growth factors and committed developments are all agreed in advance.

Assessment years normally include the year of opening and a future year, often ten years after opening, with background growth applied from national road traffic forecasts and committed development added separately. Getting the committed development list from the highway authority rather than compiling it yourself avoids a predictable objection.

Where the scheme is below the threshold for a full assessment, a transport statement covering trip generation, access and parking is proportionate. Above it, a transport assessment with junction modelling is expected. National guidance points to indicative thresholds and the local highway authority applies its own, so agree the scope in writing.

Where highway officers challenge the numbers

The recurring challenges are worth listing because they are avoidable.

Category selection: choosing a TRICS land use category that produces a convenient rate rather than the one that describes the development. Filter transparency: presenting an average without the site list, the filters applied and the sample size. Sample size: a rate derived from three surveyed sites, two of which are twenty years old. Existing use credit: claiming trips for a use that cannot lawfully resume. Distribution: a pattern that conveniently avoids the constrained junction. And peak selection: assessing an hour that misses the development's own peak.

The defence against all six is the same. Show the working, name the source, state the filters, and agree the method before you run it. A transport chapter that a reviewer can reproduce is one that survives consultation.

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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, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.