Finding trip rate data and using it defensibly

Trip rates in a transport statement almost always come from TRICS, the industry trip rate database, and a highway officer's first question is which sites you selected and why. The output is only as good as the selection, and a selection nobody can reproduce is evidence nobody has to accept. On a small scheme the honest answer is often that you do not need the database at all: a single dwelling or a pair of houses can be dealt with from first principles in a paragraph.

Where you do use it, the discipline is simple. Record the query, record what you excluded, and say why.

What the database contains and how access works

TRICS holds multi-modal traffic and travel survey data from sites across the United Kingdom and Ireland, organised by land use category. Each survey record gives counted vehicle, pedestrian, cycle and public transport movements in time bands across a survey day, with site details including size, location, parking provision, and the character of the surrounding area.

Access is by subscription, held by consultancies, highway authorities and some councils. A self-builder will not normally hold one, which gives you three options: appoint a transport consultant for a short piece of work, ask whether the local highway authority will provide or accept figures, or make the case from first principles where the scale allows it.

The consortium publishes a good practice guide setting out how the database should be interrogated and reported. Officers read it. An output that departs from it without explanation is the first thing challenged.

What the database does not do is tell you what trip rate is appropriate for your site. It gives you comparable surveys, and the judgement about comparability is yours and has to be defended.

Selecting comparable sites honestly

Comparability has four dimensions: land use, scale, location type and travel context.

Start with the land use category that matches the proposal rather than the one that produces a convenient answer. Privately owned houses, affordable or local authority housing, flats and mixed private housing are separate categories with different rates, and using the wrong one is the error most often spotted.

Then set a size range around your scheme. A trip rate per dwelling derived from surveys of 200-unit estates does not transfer well to a four-plot infill scheme, because small sites behave differently and because a rate per dwelling drawn from a large sample flattens the variation that matters at small scale. Keep the range plausible and say what it was.

Location type and area context come next: town centre, edge of town centre, suburban area, edge of town, neighbourhood centre, free standing. The classification affects car availability and mode share more than almost any other filter. Finally, consider whether the surveyed sites are in a travel context like yours, which in practice means similar public transport accessibility and similar parking provision.

Aim for a sample of several sites rather than one or two. A single survey is a data point, not a trip rate.

Filters, date ranges and regional selection

Each filter you apply narrows the sample and shifts the answer, so each one needs a reason.

Date range is the filter that attracts most argument. Older surveys may predate changes in travel behaviour, online retail delivery patterns and home working. Excluding everything before a particular year is defensible if you say so and if the remaining sample is adequate; silently excluding surveys that produce inconvenient numbers is not. Where surveys from the pandemic period fall within your range, consider whether they represent normal conditions and state your decision either way.

Regional selection is a trade-off. Restricting to the region containing your site improves contextual similarity and shrinks the sample. Opening it up gives a larger sample with weaker local relevance. Many assessments exclude Greater London from a non-London sample, which is usually right because car availability there is atypical.

Day of week and time period selection should follow the purpose. For a residential scheme the weekday morning and evening peaks matter; for retail or leisure the Saturday peak may govern. Count the hours you have selected and make sure the peak hour used in your junction assessment is the same one.

Parking provision filters are underused and worth applying. A surveyed site with ample free parking generates more car trips per dwelling than one with constrained parking, and if your scheme is parking-constrained the unfiltered sample overstates its trips.

Presenting the selection so it can be reproduced

The reporting standard is straightforward: anyone with access to the database should be able to run your query and get your numbers.

Include in an appendix the land use category, the geographical area or regions selected, the date range, the size range, the location and area type filters, the day and time period selection, the parking filters if used, the list of sites included with their reference numbers and sizes, and the list of sites excluded with a reason against each. Then give the calculated rates in arrivals, departures and two-way movements for each time band.

That last column, the reasons for exclusion, is what separates a reproducible assessment from an assertion. A site excluded because it is a retirement development, or because the survey day coincided with roadworks, is an exclusion an officer will accept. A site excluded with no reason given is an invitation to re-run the query without your filters.

Measurement of the development itself needs the same transparency, since the rate is applied to a quantum. Our note on measuring site area and floorspace for a threshold test covers the conventions, and getting gross and net floorspace confused will skew the result more than any filter choice.

Where a highway officer challenges the output

Five challenges come up repeatedly, and all five are answerable in advance.

The sample is too small, so present the number of sites and the spread of results rather than only the mean. The land use category is wrong, so state why yours fits. The area type is more urban or more rural than the site, so give the accessibility evidence that supports your classification. The surveys are too old, so justify the date range and show the effect of widening it. And the existing use has been over-credited in a net trip calculation, so evidence the lawful existing use and its actual intensity rather than assuming full occupancy.

Net trip generation deserves particular care. Where you are claiming credit for an existing use, the officer will ask what that use actually generated, not what its land use category would suggest. A unit vacant for three years cannot be credited at full theoretical intensity without an argument, and the authority will say so.

Offer a sensitivity test. Running the assessment at a higher rate and showing that the conclusion holds removes the argument entirely, and it costs an afternoon.

Alternatives where no comparable data exists

Some proposals have no useful analogue in the database: a specialist care scheme, an unusual sui generis use, a farm diversification with a mixed operation.

Three alternatives work. A first principles calculation built from the operational detail, with staff numbers and shift patterns, delivery and servicing schedules, visitor numbers and opening hours, each stated and sourced from the operator. A survey of a comparable existing operation, agreed with the highway authority in advance as to location, dates and method. Or census travel-to-work and car availability data for the relevant output area, used to derive mode share and applied to person trips rather than vehicle trips.

Whichever you use, agree the method with the highway officer before you do the work. An unagreed method produces a report that gets rejected on method rather than on substance, which wastes the whole exercise. Checking the physical context of comparable sites from imagery helps the argument, and reading aerial photography as planning evidence covers what can and cannot be shown that way.

Recording the query parameters

Keep a one-page record with the assessment: the date you ran the query, the database version, every filter value, the included and excluded site lists, and the resulting rates. File it with the submitted documents.

Two reasons. Appeals happen two years later, by which time the database has been updated and re-running the original query is impossible without the parameters. And where a scheme is amended and the transport work has to be revisited, the record lets the next person extend the analysis rather than start again.

Where the layout is still fluid, run the transport and environmental checks together before fixing it. Noise is the other constraint that reshapes residential layouts late in the day, and checking strategic noise maps before you fix a layout explains why.

Enter your postcode on PlanScope to see which transport documents your scheme is likely to need, then send the draft for professional review.

This is general guidance, not legal advice. Transport methodology should be agreed with the highway authority, so check the position with that 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. For specialist technical reports, qualified professional input may be required.