Selecting and defending trip rates in a transport assessment

The trip rate is the number everything else in a transport assessment is built on. Junction capacity, parking demand, servicing, air quality inputs and the travel plan targets all descend from it, so a rate that the highway authority will not accept invalidates several hundred pages of work behind it. Selecting and defending trip rates is therefore a scoping exercise, not a reporting one, and it belongs at the start of the commission.

The defensible position is rarely the lowest rate. It is the one with a filter log the authority can reproduce.

Why the rate drives everything downstream

A trip rate expresses trips per unit of development, per dwelling, per 100 square metres of floorspace, per bed space, in a stated time period. Multiply it by the quantum of development and you have the trip generation, which is then distributed across the network and assigned to junctions.

Because the multiplication happens at the start, an error propagates. A rate that is 20 per cent high produces junction flows 20 per cent high, which can turn a junction that operates within capacity into one that does not, and can generate mitigation the scheme does not need.

The opposite error is worse for the applicant. A rate the authority rejects means the modelling is redone, usually after submission, usually while the determination period runs, and usually with the case officer already minded to refuse on highway grounds.

Choosing comparable sites and the filters you apply

Trip rates for most land uses come from surveyed sites held in the national trip rate database maintained by TRICS. The database is only as good as the selection made from it, and the selection is where the argument happens.

The filters that matter most in practice:

1. Land use category and subcategory, chosen to match the proposal rather than the nearest convenient heading
2. Geographic region, usually excluding Greater London for sites outside it and excluding Ireland, with a reasoned position on which regions are comparable
3. Survey date range, balancing recency against sample size, with an explicit position on surveys carried out during periods of unusual travel behaviour
4. Development size range, set as a band around the proposal rather than as an open range
5. Location type, meaning town centre, edge of town centre, suburban area, edge of town, neighbourhood centre or free standing, matched to the site
6. Population within one mile and within five miles, which is the crudest but often the most influential accessibility proxy
7. Parking provision on the surveyed sites, since a site with unconstrained parking generates a different vehicle rate from one without
8. Survey days, matched to the assessment days

State each filter, the value applied and the reason. An assessment that presents the rate without the filter log invites the authority to run its own selection, and its selection will not be yours.

Sample size and the single site problem

One surveyed site is an anecdote. Two are a coincidence. Local highway authorities commonly set a minimum number of sites in their own transport assessment guidance, often around five, and will say so in the scoping response if you ask.

Sample size interacts with filtering in an awkward way. Tighten the filters until the sites are properly comparable and the sample shrinks. Loosen them to keep the sample up and the comparability weakens. There is no clean answer, which is why the reasoning has to be visible.

Look at the spread as well as the mean. If eight sites produce rates ranging over an order of magnitude, the mean is not describing anything real, and the right response is to interrogate the outliers rather than to report the average with a straight face. Sometimes a single anomalous site is doing all the work, and removing it with a stated reason is more defensible than leaving it in unexamined.

Multi-modal rates and mode share

Vehicle trip rates answer only part of the question. Multi-modal rates, covering car driver, car passenger, public transport, cycling and walking, are what a travel plan and a sustainable transport case need.

Two approaches are common, and they should not be mixed without saying so. Either take multi-modal rates from the database for sites with comparable accessibility, or take total person trips and apply a mode share derived from census travel to work data for the relevant output area, adjusted for the characteristics of the proposal.

The census route needs care. Travel to work data describes commuting by residents of an area, not all trips to a development in that area, and it is collected at a point in time. Where you use it, say what you have used, from which geography, and what adjustment you have applied. Unadjusted census mode share applied to a retail development is a standard reason for a consultee objection.

Person trips against vehicle trips

Keep the two clearly separated throughout the assessment, and label every table.

The net trip position is where applicants gain or lose most ground. Where a site has an existing lawful use capable of being reoccupied without permission, its trip generation is a legitimate deduction, and the authority will test whether the use remains extant and lawful rather than long abandoned. Evidence of the last occupied date, rates records and the planning history support that deduction. Pass-by and diverted trips are a related adjustment for retail and roadside uses, and both need the calculation shown rather than asserted.

Agreeing the selection before you model

Send the proposed filter set to the highway authority at scoping and ask them to confirm it. Put the request in a short note with the filters in a table, the resulting site list and the rank order, and a sentence on why each filter was set where it was.

Three things follow from doing this. The authority owns part of the decision, which makes later objection to the rate harder. Disagreements surface before the modelling rather than after. And the scoping correspondence becomes an appendix that an inspector can read.

Where the strategic road network is affected, National Highways will want its own position on the rates, and how access to the strategic network is assessed sets out a separate consultation path with its own expectations. Getting both authorities to the same rate before submission is worth the delay.

Presenting the audit trail

The section on trip rates should let a reader reproduce the selection without contacting you. Include the full database output as an appendix, the filter table in the body, the list of selected sites with their key characteristics, the rank order report, the rates adopted with the peak periods stated, and a short paragraph on each site excluded and why.

Then state the assessment periods explicitly. Network peak and development peak are frequently different, and a food store assessed only at the weekday network peak has not been assessed at its busiest.

Whether a full assessment is needed at all is a separate question that turns on thresholds, and the threshold question and what triggers a full assessment is worth settling before the rate work starts. Where the answer is a lighter document, the structure of a Transport Assessment still shapes what the authority expects to see, and the targets in the Travel Plan should be derived from the same multi-modal rates rather than invented separately.

Enter your postcode on PlanScope to see which transport documents 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. For specialist technical reports, qualified professional input may be required.