TRICS: where trip rates in transport reports come from

TRICS is the database of traffic and multi-modal surveys that UK transport planners use to predict how many trips a new development will generate. When a Transport Assessment says a scheme of 60 homes will add 34 vehicle movements in the morning peak, that figure almost always comes from a TRICS trip rate multiplied by the size of the proposal. The name stands for Trip Rate Information Computer System. The database is run by TRICS Consortium Limited, a company owned by a group of county councils in southern England, and access is by annual licence.

Trip rates matter because they drive everything downstream in a transport report: junction capacity tests, the case for or against a right-turn lane, the parking demand argument, and the highway authority's view on whether the local network can cope. A rate that is too low invites a refusal on highway safety grounds. A rate that is too high buys mitigation nobody needs. So it is worth knowing how the number is built.

What the TRICS database holds

Each TRICS record is a survey of a real, operating development: a housing estate, a supermarket, an industrial unit, a GP surgery. The surveyors count movements in and out of the site access across the day, usually in 15-minute slots, and record the site's characteristics alongside the counts. Those characteristics include the land use category, the floor area or number of dwellings, the location type (town centre, edge of centre, suburban, edge of town, free-standing), the region, the survey date and the day of the week.

Many surveys are multi-modal. As well as cars and light goods vehicles they record pedestrians, cyclists, bus and rail passengers, taxis and heavy goods vehicles. That is why a modern Transport Assessment can quote a walking or cycling trip rate rather than treating every arrival as a car.

The database is organised by land use category and sub-category. Residential is split by tenure and house type, retail by format, employment by office or industrial, and so on. The land use codes are the first thing a reviewer looks at, because choosing the wrong sub-category is the easiest way to produce a misleading rate.

Trip rates are expressed per unit of development. For housing the unit is normally the dwelling. For most other uses it is 100 square metres of gross floor area, although some categories use bedrooms, pupils or pitches. The rate is arrivals, departures and total movements per unit for each time period, and the software multiplies it by the size of the proposal to give the predicted trips.

How a practitioner selects comparable sites

TRICS does not hand over a single number. The practitioner filters the database down to a set of surveys that resemble the proposal, and the software averages them. The TRICS Consortium publishes a Good Practice Guide that sets out how that filtering should be done, and highway authorities expect the Transport Assessment to show that the guide was followed.

The usual filters are, in order:

1. Land use category and sub-category matching the proposal.
2. Size range, so that a 12-unit scheme is not compared with a 900-unit urban extension.
3. Location type, matching the proposal's setting.
4. Region, typically excluding Greater London and Ireland for a site elsewhere in England, and sometimes excluding the region where the proposal sits if a local survey would skew the sample.
5. Survey age, dropping older counts that no longer reflect current travel habits.
6. Day of week, keeping neutral weekdays and removing weekend, Monday and Friday surveys unless the use is weekend-focused.

The output of that process is a list of selected sites, and that list should be printed in the appendix of the report. A reader can then see whether the ten surveys behind a residential rate were mid-sized suburban estates in comparable towns or a mix of a rural hamlet and a city-centre block.

What a good selection looks like depends on judgement. Too few sites and the average swings on one unusual survey. Too many and the sample includes places that share nothing with the proposal except a land use code. The Good Practice Guide sets expectations on sample size, and a report that departs from them should say why.

Peak hour and daily trip rates explained

Most highway authorities test the network in the weekday morning and evening peak hours, because that is when junctions are closest to capacity. The TRICS output therefore usually reports three things: the AM peak hour rate, the PM peak hour rate, and a daily total.

Each of those is split into arrivals and departures. For housing the pattern is predictable: departures dominate in the morning as people leave for work and school, arrivals dominate in the evening. For a food store the split is more even and the busiest hour is often Saturday late morning rather than a weekday peak, which is why a retail assessment will report a weekend peak as well.

The peak hour itself is not fixed at 08:00 to 09:00. TRICS identifies the busiest 60-minute window in the survey data, and the assessment should say which hour was used and whether it matches the network peak the highway authority cares about. A site whose own peak falls at 07:15 can look quieter than it is if the report only quotes the 08:00 hour.

A further choice is whether to use the mean of the selected surveys or a higher percentile. The default TRICS output is an average. Some highway authorities ask for the 85th percentile rate as a cautious test, particularly where a junction is already near capacity. If a report uses the mean, expect the question.

Net versus gross trips on a change of use

Where a site already generates traffic, the relevant figure is the change, not the total. A vacant warehouse being converted to flats, or a car showroom becoming a nursery, has an existing trip generation that the new use replaces.

There are two ways to establish the existing trips. The first is to survey the site as it currently operates, which is the better evidence if the use is still active. The second is to take a TRICS rate for the existing use, which is what practitioners do when the building is empty. The Transport Assessment then presents gross trips for the proposal, existing trips for the current use, and the net change.

Highway authorities scrutinise the existing-use figure closely. A use that has been dormant for years may carry little weight, and there is a planning question about whether the lawful use has been abandoned; the glossary entry on abandonment of use covers that test. Where the existing use is real and recent, a net assessment can turn an apparently significant scheme into one with no material impact, which is exactly why the evidence behind it needs to be solid.

For a smaller change of use the council may only expect a Transport Statement, where the net trip comparison is often the whole argument.

Common criticisms of TRICS outputs

TRICS is the accepted tool, but it is not beyond challenge. The recurring criticisms are these.

Sample age. Travel behaviour changes. Surveys from before smartphones, home delivery and hybrid working describe a different world, and a residential rate built on old suburban surveys can overstate car use in a town-centre scheme. The counter-argument is that dropping old surveys shrinks the sample, so the report should show the trade-off.

Geography. A rate assembled from surveys in one part of the country may not transfer to another. Highway authorities in areas with poor public transport are wary of samples dominated by well-connected towns, and the reverse applies in London, which is why Transport for London and the London boroughs often prefer local evidence.

Location type mismatches. Labelling a site edge of centre when it sits beside a bypass with no footway is a classic way to lower the car trip rate. The location type should be defended with facts about walking distances and bus frequencies, not asserted.

Survey conditions. Counts taken in school holidays, in bad weather, during roadworks or during periods of unusual travel restriction distort the average. Practitioners are expected to exclude them, and a reviewer can ask for the survey dates.

Multi-modal double counting. Where a report quotes a multi-modal total and then applies a mode share from elsewhere, it is possible to count the same person twice. The method should be explicit about which figure feeds the junction model.

None of these is a reason to reject TRICS. They are reasons to read the selection appendix rather than the summary table.

Questions to ask about a trip rate table

Whether you are commissioning a Transport Assessment, reviewing one for a client, or objecting to an application, the same short list separates a defensible trip rate from a convenient one.

Which land use sub-category was used, and does it match the proposal as described in the planning statement? Which sites were selected, how many, and are their sizes, regions and location types shown? What survey date range was applied, and were weekends and holiday periods excluded? Was the mean or a percentile used, and did the highway authority ask for something different at pre-application stage? Which hour is the peak, and is it the site peak or the network peak? For a change of use, is the existing trip generation surveyed or assumed, and is the existing use still lawful and active?

A report that answers all six in its own text, without the reader having to reverse-engineer the appendix, will usually survive the highway authority's review. One that quotes a rate with no site list should be sent back before it is submitted.

TRICS also feeds other documents. The trip totals inform a Travel Plan's mode share targets, and the peak-hour vehicle figures are often the starting point for a Parking Survey or a parking demand calculation. Get the trip rate right and those documents inherit the credibility; get it wrong and every one of them is open to the same challenge.

This is general guidance, not legal advice. Check the current TRICS Good Practice Guide and your local highway authority's validation requirements before relying on any trip rate method.