Selecting trip rates inside a Transport Assessment

Trip rates decide the outcome of a Transport Assessment before any junction is modelled. Choose a set of comparison sites that are slightly too urban and your scheme generates fewer vehicles than it will. Choose sites that are slightly too remote and you end up designing a junction nobody needs. Everything downstream, from the capacity results to the mitigation package to the section 278 cost, follows from a filtering exercise that takes an afternoon.

Highway officers know this, which is why experienced ones ask to see the trip rate selection before they look at anything else.

Why the selection governs the assessment

A Transport Assessment works forward from trip generation: how many trips the development produces, when they happen, how they split by mode, where they go, and what that does to the network. The first of those numbers multiplies through every later stage.

Small differences matter more than they look. A rate that is 15 per cent low on a scheme of 200 dwellings removes roughly thirty vehicle movements from the modelled peak hour, which can be the difference between a junction operating within capacity and one requiring signalisation.

The defence against argument later is not a more sophisticated model. It is a selection that was agreed in advance and documented well enough that somebody else could reproduce it.

Filtering the database

The trip rate database holds surveys of real sites, and the filtering decisions are where judgement enters.

• Land use category, chosen to match the proposal rather than the nearest convenient heading
• Region, usually excluding areas with structurally different travel behaviour from the site's context
• Location type, such as edge of town, suburban area or town centre, which is often the most influential filter of all
• Site size range, set as a band around the proposal rather than an open range
• Survey date range, agreed with the highway authority, because older surveys predate changes in retail and home working patterns
• Day of week and survey period
• Parking provision at the surveyed sites, where the proposal's parking ratio differs sharply from the sample

Record how many sites survive each filter. A selection that ends with three surveyed sites is thin, and an officer will say so. Where the filters leave too few, widen the least influential one first and state that you did.

Person trips against vehicle trips

This choice changes the whole method. Vehicle trip rates carry the mode share of the surveyed sites with them, which may reflect locations far more car-dependent than the application site, or far less.

Person trip rates, where the surveys are multi-modal, let you apply a mode share derived from local evidence and then convert to vehicle movements. That is the more transparent route, and it is the one that allows an accessibility argument to be made honestly.

The risk with person trips is double counting the benefit: applying an optimistic local mode share to a set of sites that were already well served by public transport. Check the accessibility characteristics of the surveyed sample before assuming your site is different.

For smaller schemes reported through a Transport Statement, vehicle rates from a tightly filtered sample are usually accepted without the extra step.

Choosing the assessment peak hours

Two peaks matter and they are not always the same. The network peak is when the highway is busiest, conventionally an hour in the morning and an hour in the late afternoon. The development peak is when the proposal generates most trips, which for a supermarket is Saturday lunchtime and for a school is quite different again.

Junction capacity work is normally carried out on the network peak, because that is when spare capacity is scarcest. Access design and internal layout may need the development peak as well.

Where the two coincide, say so. Where they do not, present both and let the officer see that the worse combination was tested rather than avoided.

Adjustments and the arguments they attract

Four adjustments come up repeatedly, and each needs justification rather than assertion.

Existing site trips: where a site is in lawful use, the trips it already generates can be credited against the proposal. Evidence has to show the use is extant and operating, not merely permitted.

Pass-by trips: some retail trips are drawn from vehicles already on the network rather than being newly generated. The proportion applied should come from published evidence, and it should not be applied to the junctions the traffic passes through anyway.

Internalisation: on mixed-use schemes some trips stay within the site. The claimed proportion needs to reflect the actual mix, and highway authorities are sceptical of high figures.

Mode share uplift from measures in a Travel Plan: acceptable as a target, rarely acceptable as an input to capacity modelling unless secured and monitored.

Where parking supply is the contested issue, a Parking Survey carried out to a recognised methodology carries more weight than any adjustment to trip rates.

Seasonality and survey conditions

Two smaller points worth checking in the surviving sample. Surveys carried out in August or late December reflect atypical conditions, and most authorities expect neutral months. Surveys taken during periods of disrupted travel behaviour need the same treatment as census data from the same period: usable with a caveat, misleading without one.

Where the sample is small enough that excluding those surveys would leave too few sites, keep them and report the effect on the resulting rate rather than quietly averaging it away.

Sensitivity testing an agreed rate

Agree the selection with the highway authority in writing before the modelling runs. Send the filters, the surviving site list and the resulting rates, and ask for confirmation. Twenty minutes of officer time at that stage saves a redesign.

Then test sensitivity. Run the model with rates 10 or 20 per cent above the selected figure and report the result. If the junction still works, the argument is over. If it does not, you have found the point at which the mitigation package changes, and it is far better to find it yourself than to have a consultee find it in week six of determination.

Sensitivity testing is also the honest answer to the uncertainty in this method. Nobody can say exactly what a scheme will generate. What you can show is that the conclusion holds across a plausible range.

Recording the selection so it can be audited

The appendix should contain the database output as generated, listing the surveyed sites used, with the filters applied shown in full. Add a short table in the main text stating the selected rates, the number of sites behind each, and the reason for each filter decision.

Name the database, its version and the date of extraction. The database is licensed, so reproduce the standard output rather than retyping figures into a table of your own.

An assessment that does this well is quicker to review, which means it is quicker to determine. That is worth more to a programme than any argument about a decimal place.

This is general guidance rather than legal advice, and trip generation methods are agreed with individual highway authorities, so check the position with the authority or a qualified transport planner before relying on it.

PlanScope drafts transport documents structured to UK planning policy, with survey data and agreed trip rates flagged where they are missing. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.