Choosing and defending TRICS trip rates

A TRICS trip rate is only as defensible as the site selection behind it, and highway officers know exactly where to look. They open the appendix, count how many survey sites you used, read the filters and ask why the sites that would have raised the rate are missing. Get the selection right and the rest of the transport case follows. Get it wrong and every downstream number, from junction capacity to parking demand, is contaminated.

The habit that prevents most of this is unglamorous. Agree the methodology in writing before you run the query.

What the highway officer is actually checking

Four things, in this order.

Whether the land use category matches the proposal. Whether the filters produce a comparator set that resembles the application site. Whether the sample is large enough to mean anything. And whether the exclusions were made for stated reasons rather than to move the answer.

That last point is the one that sinks reports at appeal. An inspector reading a transport case can see when four sites were dropped without explanation, and the natural inference is not flattering.

Getting the land use category right

TRICS land use categories are more specific than most proposals. Privately owned houses and privately owned flats generate different rates, and mixing them into one query to get a bigger sample defeats the purpose. Retail categories separate food superstores from discount food stores from retail parks. Employment categories separate offices from business parks from industrial units.

Two errors recur. The first is choosing a category by the name of the operator rather than by the activity. The second is treating a mixed scheme as one land use, when the honest approach is to run each element separately and sum them, with an explicit and stated allowance for linked trips where the evidence supports one.

Where the proposal has no good match, say so. A transport case that explains why the closest category is imperfect, and then tests sensitivity around it, reads far better than one that pretends the fit is exact.

The filters that change the answer

Every filter is an argument you are making about comparability, so make each one deliberately and record the reasoning.

• Region and area type, which is where most disputes start. A suburban site in the north of England and an edge of town site in the south east are not interchangeable, and the highway authority will have a view.
• Location type, meaning town centre, edge of town centre, suburban area, neighbourhood centre, edge of town or free standing. This filter usually moves the rate more than any other.
• Survey date range, since travel behaviour shifted after 2020 and older surveys attract challenge on that basis. State your cut-off and defend it.
• Days of the week surveyed, separating weekday from Saturday counts rather than averaging across them.
• The size range of the independent variable, whether that is dwellings, gross floor area or bed spaces. Selecting sites of broadly similar size to the proposal is more persuasive than a wide range with a convenient average.
• Population within one mile and five miles, and car ownership levels, both of which let you match the catchment rather than only the site.
• Parking provision at the survey sites, which matters enormously for retail and employment uses.

Record the filter settings in a table in the appendix, not in prose. Officers check them line by line, and a table makes their job faster, which makes your consultation response faster too.

Averages, regressions and the site you cannot include

TRICS offers arithmetic average rates and, where the data supports it, trip rate regression. Regression is stronger when the relationship between size and trips is not linear, which is common in residential development where larger schemes generate proportionally fewer trips per dwelling. Where you use regression, show the fit and the sample, and be honest when the fit is weak.

The rank order graph is the tool to reach for when a highway authority alleges cherry-picking. It shows every site in the selected set in order of trip rate, so you can point to where your chosen sites sit within the distribution rather than asserting they are representative.

Excluding a site is legitimate when there is a stated reason: a survey affected by roadworks, a site with an unusual on-site constraint, a count that includes a shared access serving something else. Excluding a site because it is the highest is not. Write the reason for every exclusion in the report, and accept that a reason nobody would have accepted verbally will not survive in writing either.

Sample size deserves a sentence of its own. A rate derived from two or three sites is fragile, and a highway authority is entitled to treat it as such.

Existing use credit and the extant lawful use

Netting off the trips generated by the existing use is normal practice, and it is also where transport cases are most often overstated.

Three questions decide how much credit is reasonable. Is the existing use lawful, which usually means an extant permission or an established use. Is it currently operating, or has the building been vacant for years, in which case the highway authority may argue the trips are not really being removed from the network. And could the existing use lawfully resume at the level you are claiming, at the intensity you have assumed?

Where a building has been vacant, survey data from a comparable operating site, or the operator's own historic records, carries more weight than a TRICS derived theoretical figure. Set out the basis plainly and expect it to be tested.

Illustrative example: a scheme of 60 flats on a former builders merchant claims a credit for the merchant's vehicle movements including heavy goods vehicles. The highway authority accepts the car element, questions the goods vehicle element because the yard had been closed for four years, and asks for a sensitivity test with no credit at all. Running that test at the outset, and showing the junction still works, would have saved a round of consultation.

Making the numbers survive scrutiny

Scope first. Write to the highway authority with your proposed land use categories, filters, survey date range, assessment years, peak hours and the junctions to be modelled. Ask them to confirm. A short agreed scoping note is worth more than twenty pages of justification later, and it is the difference between a transport assessment that gets read and one that gets argued with.

Then publish your working. Include the full TRICS output, the site list with addresses and dates, the filter table, the rank order graph and the calculation sheet. Where the scheme falls below the threshold for a full assessment, a transport statement still benefits from the same transparency at a smaller scale.

Finally, connect the trip rates to the mitigation. If the rates support a lower parking provision, back that with a parking survey using a recognised methodology rather than assertion, and secure the behavioural element through a travel plan with monitoring and named targets. Numbers without a mechanism are just numbers.

A transport assessment normally needs a suitably qualified transport planner, and councils expect one. An AI-assisted draft can carry the policy narrative, the site description and the structure, and it flags where site data is missing rather than inventing it, but the trip generation work and the modelling belong to the specialist. For specialist technical reports, qualified professional input may be required.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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