TRICS site selection and the filters an officer checks

TRICS site selection is where most transport assessments are won or lost, and a highway officer reviewing one looks at the filters before looking at the rate. If the selected sites are the wrong location type, the wrong vintage or the wrong size band, the rate that comes out of them will not survive review.

The database holds multi-modal traffic surveys of real developments across the United Kingdom and Ireland, grouped by land use. Your job is to build a subset of those surveys that resembles your site closely enough that the average trip rate is a fair prediction, and then to show your working so the authority can reproduce it.

What the database holds and how surveys are banded

Surveys are organised by land use category, covering residential, retail, employment, education, health, leisure, hotel and food and drink, and mixed use, each with sub-categories. A residential survey is banded as houses privately owned, flats privately owned, affordable or local authority housing, mixed private and affordable, retirement living and so on. Picking the sub-category is the first decision and it does most of the work.

Each survey record carries the site's location, survey date, day of week, gross floor area or unit count, site area, parking provision, population within one mile and five miles, and the counted arrivals and departures by mode in each time period. Multi-modal surveys give pedestrian, cycle, public transport and vehicle counts, which is what allows a person trip rate to be built rather than only a vehicle one.

Rates are expressed per unit of development: per dwelling for housing, per 100 square metres of gross floor area for most commercial uses, per bed for hotels and care accommodation. Mixing bases within one assessment is a common error, and it makes the totals impossible to check.

Filtering by sub-category and survey date

Start narrow and widen only if the sample is too small.

1. Select the land use sub-category that matches the proposal, not the one that gives the answer you want.
2. Set a survey date range. Practitioners generally exclude surveys older than around ten years, and staleness is only part of the reason. Travel behaviour, online shopping and home working have all moved, so an older survey tends to overstate vehicle trips for retail and understate them for residential daytime periods.
3. Exclude surveys carried out in school holidays, on public holidays, or during obviously atypical conditions where the database flags them.
4. Decide on weekday against weekend before filtering, and keep the two sets separate rather than averaging across them.

The survey date question cuts both ways, and it is worth being honest about it in the assessment. A consultant selecting only recent surveys for a residential scheme will often produce a lower vehicle rate, and an officer is entitled to ask whether the selection was made for that reason. Explain the cut-off with a stated rule rather than a case-by-case judgement.

Geography, settlement size and parking filters

Greater London surveys and Irish surveys can be included or excluded. For a scheme outside London, including London surveys typically pulls the vehicle rate down and the public transport share up, and it will be challenged unless the accessibility of your site is comparable. Say what you did with them either way.

Parking provision deserves special attention. Where your scheme offers one space per dwelling and the selected surveys average two, your vehicle rate is being drawn from sites with more generous parking than yours, which overstates the prediction. Where the reverse applies, the assessment understates it, and that is the direction officers object to.

Excluding surveys that distort the rate

Look at the rank order list as well as the average. A dataset where one site produces a rate three times the next highest needs a decision, and that decision has to be recorded.

Exclude a survey where the site has an unusual characteristic that explains the outlier: a site sharing an access with a supermarket, a retirement scheme with an on-site day centre, an employment site occupied by a single shift-working operator. Do not exclude a survey because its rate is inconvenient. The test an officer applies is whether the reason given would have been applied equally to a low outlier.

Sample size matters and there is no fixed rule. Practitioners usually aim for a handful of sites at minimum, and a dataset of two surveys will not carry an assessment for a major application. Where the sub-category is thin, widen the geography before you widen the date range, and say so.

Multi-modal rates and person trips

For a scheme relying on a sustainable location, the person trip rate is the more useful output. Build the total person trips from the multi-modal surveys, then apply a mode share derived either from the same dataset or from census journey-to-work data for the local area, and explain which you used.

Do not mix the two methods within one calculation. A person trip rate from TRICS combined with a mode share from census data for a different geography produces a vehicle figure nobody can audit. Where the local authority has its own preferred mode share source, use it and note the difference.

Existing use credits belong in this part of the assessment too. Where the site is occupied, the net trip change is what matters, and the credit has to be evidenced by survey of the existing use rather than derived from the database, unless the existing use has ceased and a database rate is the only available proxy.

Presenting the filtered set

The appendices should let the reviewer repeat your work without asking questions. Include the list of selected sites with their survey dates and characteristics, the rank order list showing what was excluded and why, the calculated trip rates by time period, and a short note on each filter with the value applied.

A table in the main text summarising the filters, the resulting sample size and the adopted rates is worth more than three paragraphs of description. Then sense-check the output against something real: a traffic count on the local network, an existing comparable development in the same town, or the authority's own monitoring data. A rate that survives that comparison is defensible. One that does not needs explaining before an officer does it for you.

The rates then flow through into the Transport Assessment or, on smaller schemes, the Transport Statement, and the same numbers should appear in the parking case and in any Travel Plan target.

Where a highway officer will push back

The recurring objections are predictable, which makes them avoidable. Sample too small. Location type mismatched to the site. Parking provision in the dataset unlike the scheme. London surveys included without justification. Outliers removed in one direction only. Mode share borrowed from a different source than the trip rate. No sense-check against local counts. And the assessment quoting a single peak hour rate when the concern is a school-run conflict at a time the standard peak misses.

One honest uncertainty: different authorities take different views on how old a survey can be, and some publish a preference while most do not. Ask at pre-application, and record the answer in the assessment. Where the parking case also rests on observed demand, pair the trip rate work with a proper parking stress survey rather than relying on database averages alone, and see the wider discussion of justifying trip rates from a national database.

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