TRICS site selection parameters an officer will accept

Arguments about trip generation are almost never arguments about arithmetic. They are arguments about TRICS site selection parameters, settled on the filtering screens before a single rate is calculated, and a highway officer reading your Transport Assessment will turn to the selection parameters page before they look at the junction results. Get the filters agreed in a scoping note and the rest of the assessment proceeds. Pick them to suit the answer you want and the whole chapter gets reopened at consultation.

Planning Practice Guidance on travel plans, transport assessments and statements expects trip generation to be evidenced and the methodology to be agreed with the local highway authority. That is the hook for everything below.

Why the selection is contested before the output is

The database holds surveyed sites, each with a measured trip rate per unit of development. Your selection decides which of those sites acts as a proxy for yours. Change the sub-category and the rate moves. Widen the region and it moves again. Nothing in the process is dishonest, and every choice is arguable, which is exactly why officers treat the filters as the substance.

Two habits defuse most of the argument. Write the selection down before you run it, as a short methodology note sent to the highway authority with the site details and the proposed filters. And print the selection parameters page into the appendices so a reader can reproduce your extraction.

Land use category and the sub-category choice

Start with the category that matches the proposal as it will be occupied, not the use class on the application form. A build to rent block and a block of private flats are both flats, and they generate different travel patterns. Student accommodation, retirement living and care homes each have their own sub-categories, and borrowing a general residential rate for any of them produces a number nobody believes.

Where the development is mixed, build the assessment up from each component rather than looking for a single composite category, then make an explicit allowance for internalised and linked trips with the reasoning shown. Where no sub-category fits, say so plainly and explain the nearest proxy and the direction of the error. An officer will accept an honest proxy with a stated limitation more readily than a forced fit.

Check the unit of measurement as well. Rates are expressed per dwelling, per 100 m² gross floor area, per bedroom or per employee depending on the category, and converting between them is where quiet mistakes happen.

Geographic and population filters

The regional filter is the one most often set too wide or too narrow. A single region rarely gives enough sites, and the whole of England and Wales gives a sample with little relationship to your context. The usual compromise is to exclude Greater London for a provincial site and to exclude purely rural regions for an urban one, with the reasoning stated.

For a London site, exclude non-London sites and use the public transport accessibility filter. Elsewhere the population filters within one mile and five miles are the better control on context, because they capture urban density more reliably than a regional label does.

Location type and location sub-category matter as much. A town centre site compared against edge of town retail parks will understate car driver trips, and a suburban site compared against free standing sites will overstate them. Match the location type, state the match, and note where the surveyed sites differ.

Parking provision is the filter people forget. Surveyed sites with generous parking generate more car driver trips than your site will if you are proposing a low parking ratio with car club provision, and the supporting Parking Survey evidence needs to sit alongside the trip rates rather than contradicting them.

Survey dates and seasonality

Set a date range that is recent enough to reflect current travel behaviour and wide enough to produce a usable sample. Travel patterns shifted after 2020, particularly for employment uses, so a sample dominated by surveys from the 2010s needs a comment on whether it overstates or understates peak hour trips. For office and employment categories that is now the first question an officer asks.

Use neutral survey periods. Exclude school holidays, the weeks either side of Christmas and Easter, and the summer peak. Keep weekday surveys and weekend surveys separate, and assess the peak hours the authority is interested in rather than the ones that suit the result. The daily profile matters for schools, retail and food and drink uses, where the critical hour is not the highway network peak.

Excluding sites defensibly

Exclusions are legitimate. Unexplained exclusions are not.

• Give a reason tied to comparability: location type, parking ratio, an atypical catchment, a survey disturbed by roadworks or an unusual on-site operation.
• Never exclude a site because its rate is high, and never describe a high rate as an anomaly without a cause.
• List every excluded site with its reference and the reason, in a table.
• Show the mean with and without the exclusions so the effect is visible.
• Keep the number of exclusions small. Four exclusions from a sample of nine reads as a selected sample rather than a filtered one.

The same discipline applies to including sites. If a site survives your filters but plainly does not resemble your development, say why you have kept it anyway.

Presenting the rank order

The rank order output, listing each surveyed site and its individual rate, is the most useful page in the appendix and the one most often left out. Include it. It shows the spread, it shows the sample size, and it lets the reader see whether the mean sits in a tight cluster or between two groups.

Then present the calculation in a form an officer can check in five minutes:

A short paragraph in plain words explaining what the resulting vehicle movements mean on the ground, per hour and per direction, does more for a committee than three pages of tables.

Sensitivity testing before the officer does it

Assume your rates will be challenged and answer the challenge in the document. Run the assessment with a wider regional sample, with the exclusions reinstated, and with a higher trip rate, and report what happens to the critical junction. Where the conclusion holds across all three, say so, and the trip rate debate is effectively over.

Where the conclusion does not hold, you have learned something before the highway authority did, and you still have time to change the access design or the parking strategy. That is the whole point of running it early.

Mode share assumptions deserve the same treatment. Census travel to work data for the local area gives an independent check on the mode split in your Travel Plan, and if the plan's targets imply a mode share that local data does not support, the targets will be treated as aspiration rather than evidence.

A transport assessment is a document councils expect from a suitably qualified author, and professional review is strongly advised before submission. A draft can set the structure, the policy references and the checklist of what the chapter needs to cover, and it can hold the site description and the policy context while the technical work proceeds. The site selection judgements, the extraction itself and the junction modelling have to come from a transport planner with access to the database and the professional judgement to defend each filter.

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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.