Trip rate evidence for a transport statement

Trip rate evidence for a transport statement stands or falls on site selection, and site selection should be agreed with the highway authority before anybody runs a query. The rates themselves are the easy part. What gets challenged at consultation is the choice of comparison sites, the filtering applied to them, and whether the existing lawful use has been credited properly against the proposal.

Get the scoping note agreed in week one and the transport chapter becomes an arithmetic exercise. Skip it and you will be redoing the analysis in month four, after the highways officer has written a holding objection.

Where trip rate data comes from

Almost every Transport Statement and Transport Assessment in England uses TRICS, the subscription database of development trip surveys covering the UK and Ireland. It holds counts taken at real sites, by land use category, with site characteristics recorded alongside: floorspace, dwelling numbers, parking provision, location type and surrounding population.

Three other sources do work that TRICS cannot. Census journey to work data gives mode share for the small area your site sits in, which matters more than any national average. The National Travel Survey gives trip frequency and purpose at a national level, useful for sense-checking. And local authorities often hold their own survey banks, particularly for schools, town centre car parks and residential parking.

Each has a different function. TRICS gives you generation. The Census gives you mode split. Local data gives you the baseline network conditions. Using one to do another's job is the fastest way to lose credibility with a reviewer.

Selecting comparable sites and filtering them

The database will return dozens of surveys for a land use. Most of them will be wrong for your site, and the filtering is where the judgement lies.

Work through the filters in a fixed order and record each decision:

1. Land use category, chosen to match the proposal rather than the nearest convenient category.
2. Region and location type, so an edge-of-town site is not compared with a city centre one.
3. Survey date, excluding anything old enough that travel patterns have changed materially, and treating 2020 and 2021 surveys with care.
4. Days of week surveyed, matching the days you intend to assess.
5. Site size range, usually set as a band around the proposal rather than an exact match.
6. Parking provision and public transport accessibility, where these differ sharply from your site.
7. Population within a defined radius, where the database records it.

Then look at what survives. A selection of three sites is thin and invites a demand for sensitivity testing. A selection of twenty across four regions with a spread of sizes will produce a stable average, which is what you want. Where the sample is small, say so in the text and run the analysis at both the mean and the upper end so the authority can see the range you are working within.

Do not filter until you reach the answer you wanted. Somebody will rerun it.

Agreeing the approach with the highway authority first

A scoping note is a short document, often three or four pages, sent before the assessment work starts. It sets out the proposal, the assessment years, the committed developments to be included in background growth, the junctions to be modelled, the survey data to be used, the land use categories and filters proposed for trip rates, and the mode share assumptions.

Ask for written agreement or written comments. An email from the highways officer saying the approach is acceptable is worth more at committee than any amount of methodological explanation afterwards.

Some authorities publish their own transport assessment guidance with preferred categories, growth factors and a list of junctions they always want assessed. Check whether yours does before you write the scoping note, because ignoring a published local requirement is a poor start. Where the scheme is large enough for Active Travel England to be a statutory consultee, factor its expectations on walking and cycling provision into the same note.

Person trips against vehicle trips

National policy expects development to be assessed on the basis of all trips rather than car movements alone, and multi-modal rates change the conversation about capacity.

TRICS holds multi-modal surveys for many categories, and where they are available, use them. Where they are not, derive person trips from vehicle trips using a car occupancy assumption and a mode share drawn from Census data for the local area, adjusted for the accessibility of the site. State both numbers in the report. A scheme that generates 240 person trips in the morning peak of which 90 are by car reads very differently from one presented as 90 vehicle trips with no context.

The mode share assumption is also what a Travel Plan is later monitored against, so an optimistic split agreed at application stage becomes a target somebody has to hit. Set it where you can defend it.

Choosing the peak hours and the assessment period

Two peaks matter and they are not always the same. The network peak is when the surrounding roads are busiest, typically an hour within 08:00 to 09:00 and another within 17:00 to 18:00 on a weekday. The development peak is when your land use generates most trips, which for a nursery is the drop-off window, for a food store is Friday evening and Saturday, and for a crematorium is late morning.

Assess both, and say which is which. Where the two coincide, the case is simpler and the modelling is harder.

Surveys and counts should be taken in neutral months. Avoid August, late December and the fortnight around Easter, and avoid school holidays for anything education related or residential. If existing traffic counts are more than a few years old, expect to be asked to refresh them or to apply an agreed growth factor from national road traffic forecasts.

Presenting rates so they can be audited

The reviewing officer wants to reproduce your numbers without ringing you. That means the appendix carries the full database output, including the list of selected sites with reference numbers, the filters applied, the rank order report and the individual site rates behind the average.

In the body of the report, give the calculation as a table: land use, unit of measurement, trip rate per unit, quantum proposed, resulting arrivals and departures for each assessed period. Then show the existing use credit on the same basis, because the net change is what the highway authority is actually assessing. A former builders' merchant generating 40 two-way vehicle movements in the morning peak is a real deduction, provided the use is lawful and has not been abandoned.

Where parking is contested, a parking survey carried out to the Lambeth methodology, on midweek and weekend nights outside holiday periods, settles arguments about stress levels that trip rates cannot touch.

When a bespoke survey is the answer

The database is thin for some uses. Specialist older persons' accommodation, drive-through units, self-storage, places of worship, crematoria and film studios all tend to return small or poorly matched samples. So do proposals with no real precedent.

In those cases, survey two or three operating sites yourself, with the locations agreed with the highway authority in advance and the survey method described in the report. Classified counts, occupancy checks and interview surveys each answer different questions. The cost is real, and on a major application it is small next to the delay caused by an unresolved highways objection.

A Transport Assessment is a report councils expect from a suitably qualified transport planner, and professional review is strongly advised even where the arithmetic looks straightforward. An AI-assisted draft can assemble the policy context, the site description and the structure, and flag every place a site-specific figure is still missing, leaving the transport planner to supply the judgement.

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. You can browse the transport and highways report types to see what a full evidence base looks like.