TRICS parameters and trip rate selection
Every TRICS filter moves the trip rate. A practitioner's guide to choosing the land use category, setting the filters and writing a selection the highway authority can rebuild.
Trip rate selection in TRICS is a chain of filtering decisions, and every link in that chain moves the number. Land use category, region, survey date range, site size band, parking provision: change any one of them and the vehicle trip rate you hand to the highway authority can shift by half again on the same proposal. That is why a transport chapter which reprints a query output without explanation tends to draw a holding objection from the highway engineer. The selection needs a reason attached to it.
How the database is structured
TRICS is a survey library rather than a table of answers. Each record is a real site, counted across a day or a week, holding observed arrivals and departures by mode and time period alongside the site's own attributes: gross floor area, dwelling count, bed spaces, parking spaces, location type and the date of the count.
Rates are calculated at query time, by dividing observed trips by the independent variable for that land use. For housing that is normally the dwelling. For a food store it is usually 100 m² of gross floor area, and for a hotel it is bedrooms.
That arithmetic is the reason category choice carries so much weight. A rate per dwelling and a rate per 100 m² describe different things, and a scheme measured in the wrong unit produces a figure nobody can check.
Choosing the land use category
The land use hierarchy is the first judgement and the one most often fudged. Residential alone splits into privately owned houses, privately owned flats, affordable and local authority housing, retirement living and student accommodation, and the rates differ in ways a case officer will notice when she reads your table against a neighbouring application.
Pick the category the development will operate as, not the one that flatters the parking strategy. A block of one and two bedroom flats for private sale belongs in privately owned flats. A build to rent block with a concierge and no on-site parking is still flats, and its character is expressed through the parking and location filters rather than by moving it to another category.
Mixed schemes need more than one query. Run each component on its own, then set out how you have treated internal linked trips, because a single blended rate cannot be reproduced by anyone reviewing it.
And where the use is unusual, a crematorium or a climbing centre, the honest position is that the library is thin. Say so, then offer a first principles calculation or a comparable site survey instead of stretching a category that does not fit. The same discipline applies when you are arguing about whether intensification amounts to a material change of use, where the comparison is between two operating patterns rather than two floor areas.
Filters that change the answer
| Filter | What it does | Why it is contested |
|---|---|---|
| Region and area type | Limits the sample to comparable settlement types | Rural and inner urban samples carry very different car driver rates |
| Population within 1 and 5 miles | Proxies the catchment around each surveyed site | A wide band lets high car dependency sites into a city centre case |
| Date range | Excludes older or atypical surveys | Pre-2020 and post-2020 travel patterns are not the same |
| Size range for the variable | Keeps surveyed sites near your scheme's scale | Small sites show higher rates per unit than large ones |
| Parking spaces per unit | Separates constrained sites from generous ones | A car-free scheme compared against a two-space-per-house sample |
| Days and survey type | Weekday, weekend, manual or automatic counts | Weekend peaks matter for retail and leisure, not for offices |
Set each filter deliberately and write down the value you used. Where a filter has no justification beyond raising or lowering the answer, expect it to be stripped out and the query rerun by the authority's consultant.
Sample size and date range
A rate drawn from three surveyed sites is a weak rate, whatever its decimal places. Report the number of sites and the total survey days behind every figure, and give the standard deviation or range where the spread is wide, because a mean of 0.5 trips per dwelling from sites varying between 0.2 and 0.9 tells the reader something a bare mean hides.
Date range deserves its own paragraph in the methodology. The library holds counts taken before the 2020 restrictions, during them and after, and the three groups behave differently for office, retail and education uses. Say which years you admitted and why, then test the sensitivity of your conclusion if the range is widened.
Two queries, one narrow and one broad, with a short note on the difference, is far more persuasive than one query presented as the answer.
Multi-modal and person trip rates
Where a surveyed site was counted multi-modally, TRICS gives total person trips split by mode. That matters for any scheme whose case rests on mode shift, because you can then present person trips and apply a mode share drawn from census or local evidence rather than asserting a reduction against a car driver rate.
Be careful about double counting. If you reduce the car driver rate for a sustainable location and then claim a further reduction through the measures in your Travel Plan, you have discounted the same behaviour twice. State the baseline, state the adjustment, and keep them separate in the tables.
Documenting the selection
A transport chapter stands or falls on whether a stranger can rebuild your query. The appendix should carry, in this order:
- The land use category and sub-category, with the reason for the choice.
- Every filter applied, with its value.
- The number of surveyed sites and total survey days returned.
- The site list with survey dates and locations.
- The independent variable and the scheme quantum applied to it.
- The adopted rates for each assessed peak hour and the daily total.
- Any adjustment made after the query, with its evidence.
That appendix belongs in a Transport Assessment and, in shorter form, in a Transport Statement where the scheme is below the authority's assessment threshold.
Where the authority will challenge it
Three challenges come up again and again. The first is category drift, where a scheme has been queried as a use with lower rates than it will operate as. The second is a size band so wide that the sample bears no relation to the proposal. The third is an unexplained date cut that happens to exclude the higher rate surveys.
A fourth is quieter and harder to answer. If the surveyed sites sit in locations with better bus frequencies or tighter parking controls than the application site, the sample is flattering the scheme even though every individual filter looks reasonable. Read the site list before you adopt the rates, not after the objection arrives, and strike out any site you would not be willing to defend by name at an inquiry.
Where the rates are agreed, the argument moves on to what you do with them: distribution, assignment, junction capacity and the residual effect on the network. Those are separate judgements, and a clean trip generation section buys you the credibility to make them.
Scope the query with the highway authority before you run it, ideally at pre-application stage, and ask them to confirm the categories, the assessed periods and the acceptable date range in writing. It costs one email. It saves a round of consultation responses, and it removes the argument that your evidence base was selected to suit the answer.
If you want to see how the trip generation evidence sits alongside the rest of the submission, enter your postcode on PlanScope and work through the guided questions to see which reports your site is likely to need.
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.
Frequently asked questions
What is a TRICS trip rate?
It is the number of vehicle or person trips observed at surveyed sites, divided by an independent variable such as dwellings or 100 m2 of gross floor area. The rate is calculated when you run a query, not stored, so the figure depends entirely on which surveyed sites your filters let through and which variable you divide by.
Which date range should a TRICS query use?
There is no fixed rule, so state your choice and defend it. Travel patterns before 2020, during the restrictions and afterwards differ for office, retail and education uses. Most practitioners run a narrow recent range and a wider range, then show whether the conclusion changes. Agree the range with the highway authority at pre-application stage where you can.
How many surveyed sites make a reliable rate?
More than a handful, and enough that the spread is visible. Always report the number of sites and the total survey days, plus the range or standard deviation where sites vary widely. A mean of 0.5 trips per dwelling drawn from sites between 0.2 and 0.9 needs that spread shown, otherwise the reader cannot judge the figure.
Can one TRICS query cover a mixed use scheme?
No. Query each component against its own land use category and independent variable, then set out how you have treated linked internal trips between the uses. A single blended rate cannot be reproduced by a reviewer, which is usually enough on its own to attract a request for further information from the highway authority.
What if no TRICS category fits the proposed use?
Say so in the methodology rather than forcing the scheme into a category that does not match. The accepted alternatives are a first principles calculation built from staff numbers, visitor numbers and operating hours, or a survey of a closely comparable operating site agreed in advance with the highway authority.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.