Using the trip rate database for a small scheme

Trip rate data for a small scheme is a selection problem, not a modelling problem. The national TRICS database holds surveyed trip counts at real development sites, and the number you end up quoting depends almost entirely on which of those surveys you decide are comparable to yours. Get the selection right and a highway officer can check your arithmetic in ten minutes. Get it wrong, or hide the filters you applied, and the whole transport case gets sent back.

For a scheme of five or ten dwellings, the trips in question are a handful of movements in the morning peak. The effort should go into showing that the handful is credible.

What the database contains

TRICS holds multi-modal counts taken at individual development sites across the United Kingdom and Ireland, organised by land use category. Each survey record carries the development's size (dwellings, bedrooms, gross floor area, pitches, seats or whatever unit fits the land use), the site's location type, parking provision, survey date, day of week, and counts in and out at fifteen-minute or hourly intervals across the survey day.

It also carries the contextual variables that make selection defensible: population within one mile and five miles, car ownership from the census, and whether the site sits in a town centre, edge of town centre, suburban area, neighbourhood centre or free-standing location. Those variables, not the headline land use, are what a reviewer will attack.

Counts are usually available for vehicles, taxis, pedestrians, cyclists, public transport users and total people, which is what lets you present person trips rather than cars alone.

Selecting comparable survey sites

Work outwards in a fixed order and write the order down.

1. Choose the land use category that matches the proposal, and resist stretching it. Houses privately owned, flats privately owned and mixed private housing behave differently and the database separates them.
2. Set a size band around your scheme rather than an exact figure. A band of roughly half to double the proposal size usually retains enough surveys.
3. Filter location type to match your site honestly. A suburban site should not be compared against town centre surveys with no parking.
4. Exclude surveys older than a sensible cut-off. Travel behaviour has shifted enough since the pandemic that very old surveys need a comment if you keep them.
5. Exclude surveys on days affected by school holidays, strikes, severe weather or roadworks, and say which you removed.
6. Only then look at the resulting rates.

That last point matters. If you look at the rates first and then adjust filters, you are selecting an answer rather than a sample, and an experienced reviewer will spot the pattern in your appendix.

Our note on measuring site area and floorspace for a threshold test covers the related question of which figure to put into the size filter, because gross, net internal and sales areas are not interchangeable.

Sample size and why it matters

A trip rate is an average, and an average of two surveys is not an average. Small samples produce wide ranges, and a highway authority can usually find a selection that yields a higher rate than yours if the sample is thin.

Practical guidance that holds up in discussion: aim for at least five comparable surveys, report the number you used, and report the range as well as the mean. If widening the geography is the only way to reach five, widen it and explain the trade-off. A rate drawn from eight surveys across three regions is more defensible than one drawn from two surveys in the same county.

Where the sample stays small, present the highest rate in the sample as a sensitivity test. Showing that the scheme works on the worst case in your own sample removes the obvious objection.

Weekday and weekend peak selection

For residential schemes the assessed periods are normally the weekday morning and evening network peaks, usually taken as 08:00 to 09:00 and 17:00 to 18:00, with a twelve-hour total from 07:00 to 19:00 for context. Check the local highway authority's own requirement, because some specify 07:00 to 10:00 and 16:00 to 19:00 and take the highest hour.

Retail, leisure and food uses often peak on a Saturday, and a weekday-only assessment of a shop will be challenged. Where the development peak and the network peak do not coincide, show both and say which governs. A nursery peaks before the network peak and a gym peaks after it, which is frequently the scheme's best argument.

Presenting rates so they can be checked

Put the arithmetic on the page. A reviewer should be able to multiply your rate by your quantum and arrive at your trip figure without opening a spreadsheet.

Those figures are illustrative and must not be reused: they exist to show the format. Alongside the table, include the survey site list with reference numbers, dates and locations, and the filter log. Rates should be stated per dwelling for housing and per 100 m² of gross floor area for commercial uses, with the unit spelled out.

Where the scheme falls below the local thresholds for a full assessment, this level of presentation fits comfortably inside a Transport Statement and does not need the junction modelling that a Transport Assessment would carry.

Where local survey data is better

Database rates describe development elsewhere. Sometimes the better evidence is on your doorstep.

Three situations favour a local survey. First, where a comparable development of similar size and tenure exists within a mile or two, a two-day count at its access costs little and is very hard to argue with. Second, where the objection is about on-street parking stress rather than junction capacity, a Parking Survey following the established accumulation methodology answers the question directly and trip rates do not. Third, where the existing use is unusual and the net assessment depends on it, count the existing use rather than estimating it.

Local data also helps on rural sites, where database samples for small housing schemes in free-standing locations are thin.

Common objections from highway authorities

The recurring ones, in rough order of frequency: the sample is too small, the selected sites have better public transport accessibility than the application site, the filter log is missing so the selection cannot be reproduced, person trips have been used where vehicle capacity is the issue or the reverse, the existing use deduction is not evidenced, the survey dates fall in school holidays, and the peak hours do not match the authority's published requirement.

Every one of those is avoidable at the writing stage. Most of them are avoided by one appendix.

This is general guidance rather than legal advice. Confirm the assessment scope with the local highway authority before you commission surveys, because published requirements differ between authorities.

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