Using TRICS data properly in a transport statement

TRICS trip rate selection is the part of a transport submission most likely to be picked apart, and the part most often done on autopilot. A highway officer who disagrees with your trip rates disagrees with every number downstream of them: junction capacity, parking demand, severance, air quality inputs and the case for or against mitigation.

The database is not the problem. Poor site selection and an unexplained audit trail are. Two consultants can pull rates for the same 60 dwelling scheme and arrive at numbers that differ by a factor of two, both defensibly, depending on which surveyed sites they chose.

What the database holds

TRICS holds multi modal traffic surveys from sites across the United Kingdom and Ireland, recorded at individual development sites and classified by land use. Each survey counts arrivals and departures in five minute or fifteen minute periods across a full day, by vehicles and, at many sites, by pedestrians, cyclists and public transport users.

Each site record carries a description of its context: the location type, the surrounding population, the parking provision, the floor area or number of units, and the survey date. Those attributes are what you filter on, and they are also what a reviewer will check your filtering against.

Two points about the data itself. Surveys are usually carried out in neutral months, avoiding school holidays and the weeks either side of Christmas, so a rate derived from them assumes a normal week. And the counts are of the site as surveyed, which means a site with constrained parking produces lower vehicle trip rates for reasons that have nothing to do with the land use.

Choosing comparable survey sites

The selection is a judgement, and it should be recorded as one. Work down from land use to context rather than starting with a target number.

Land use category comes first, and picking the right one is not always obvious. Privately owned houses and privately owned flats behave differently. Mixed tenure schemes may justify a blended approach. A care home is not a residential institution in the same sense as student accommodation.

Then context. The attributes that most affect vehicle trip rates are location type, the population within a defined radius, parking provision per unit, and public transport accessibility. A suburban site with two parking spaces per dwelling is not comparable to an edge of town centre site with one.

Then sample size. A rate drawn from three surveys is fragile and will be challenged. More sites give a more stable mean and let you show the range rather than a single figure. Where the filtering leaves you with very few sites, that is information: it means your scheme is unusual and the rate needs supporting from elsewhere.

Date range is the last filter to apply, not the first. Older surveys are still useful for establishing the shape of the daily profile even where the absolute rates have moved.

Filters that matter and filters that mislead

Some filters sharpen comparability. Others quietly select for the answer you wanted.

The honest test is simple. If changing a filter in either direction moves your rate by more than a few per cent, that filter needs a justification in the text rather than a tick in an appendix.

Excluding individual sites is where credibility is won or lost. There are good reasons to exclude a survey, such as roadworks on the day or a site with an atypical operator. Write the reason down for each one. A selection that has dropped the three highest trip generating sites without explanation will be read as exactly what it looks like.

Presenting the selection so it can be checked

The appendix should let a reviewer reproduce your rates without contacting you. That means the filter settings in full, the list of sites included with their reference numbers, the list of sites excluded with reasons, and the calculated rates with the range and the standard deviation alongside the mean.

In the body of the Transport Statement or the fuller Transport Assessment, explain the logic in a paragraph. Which land use category and why. Which context filters and why. How many sites survived and whether that is enough. What the spread looks like and whether you have used the mean or something more conservative.

Councils usually expect a suitably qualified transport planner to prepare this work, and professional review is strongly advised before submission.

One practical habit: agree the selection with the highway authority at pre application stage and record the agreement. A rate signed off before the modelling starts almost never becomes an issue afterwards.

Adjusting for mode share

Raw vehicle trip rates from a set of surveyed sites describe those sites, not yours. Where your scheme sits in a more accessible location, an adjustment can be justified, but the method has to be visible.

The defensible approach works in person trips. Derive total person trips from the surveyed data, apply a mode share taken from a named local source such as census travel to work data for the relevant output areas, and convert back to vehicle trips using a stated occupancy assumption. State the source, the geography and the year for every input.

What does not survive scrutiny is an unexplained percentage discount applied to vehicle trips because the site is near a station. If the accessibility is real, it will show up in the mode share data, and the calculation will make the case for you.

Be careful with double counting. A discount for accessibility plus a further discount for travel plan measures plus internalisation within a mixed use scheme can quietly remove a third of the traffic. Each adjustment needs to be separately evidenced, and the Travel Plan should not be carrying credit for trips that the mode share adjustment has already removed.

Criticisms officers make

The recurring ones are predictable and almost all avoidable.

1. Too few surveyed sites, with no acknowledgement that the sample is thin.
2. Sites in locations less car dependent than the application site.
3. Parking provision at the surveyed sites materially different from the proposal.
4. Exclusions without stated reasons.
5. Use of the mean where the range is wide, with no sensitivity test at the upper end.
6. Peak hours taken from the database profile rather than from the local network peak, which can be an hour apart.
7. No multi modal data presented, in a submission that then claims a sustainable location.

The sixth is worth a specific check on rural and edge of town schemes, where the school run can produce a network peak well before the standard morning hour.

Where the trunk road network is in play, the assessment will be read by another party again, as set out in our note on whether the strategic road network is engaged.

When there is no comparable data

Some uses are not well represented, and pretending otherwise is worse than admitting it. Drive through units in unusual configurations, specialist care models, large logistics with unusual shift patterns and some leisure uses all fall into this group.

Three routes forward. Commission a bespoke survey of one or more operational sites that closely match the proposal, agreed with the highway authority in advance. Build a first principles estimate from staff numbers, shift patterns, delivery schedules and customer throughput supplied by the operator, and present the assumptions openly. Or use the closest available category and test the sensitivity of the conclusions to a rate 25 per cent higher, showing that the mitigation still works.

The third option is often the most useful, because it moves the argument from what the rate is to whether the answer changes if you are wrong. Where parking demand rather than traffic generation is the contested point, a Parking Survey using a recognised methodology provides the local evidence that trip rates alone cannot.

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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.