Trip rates and how TRICS data is used in an assessment
A trip rate is only as good as the sites it came from. The filters applied to the database decide whether a highway authority accepts the transport assessment built on it.
A trip rate is the number of trips a development generates per unit of that development in a stated time period: per dwelling, per 100 m2 of gross floor area, per bedroom, per pitch. The trip rate TRICS produces comes from surveys of real sites already in operation, and the whole credibility of a transport assessment rests on whether the sites chosen to produce that rate look anything like the one being proposed.
Highway authorities know this. Which is why the argument about a transport assessment is almost never about the junction modelling and almost always about the site selection behind the rate.
What the rate expresses
Take a residential scheme of 60 houses with a derived vehicle trip rate of 0.52 two-way vehicle trips per dwelling in the weekday morning peak hour. Multiply and you get roughly 31 vehicle movements between 08:00 and 09:00, split into arrivals and departures by the directional ratio from the same surveys.
That figure then drives everything downstream: junction capacity testing, the need for access improvements, the scale of any contribution and whether the scheme is treated as having a severe residual impact.
Rates come in several flavours and the assessment must say which it is using. Vehicle trips count cars and light goods vehicles. Total person trips count everybody by every mode. Multi-modal surveys break person trips down into car driver, car passenger, bus, rail, cycle and walk. Using a vehicle rate where a person rate is needed, or the reverse, is a straightforward way to lose an argument at scoping.
Selecting comparable sites from the database
The database holds surveys by land use category, and within each category the analyst applies filters to build a subset. The filters that matter most are these.
- Land use category and sub-category, matched to the proposed use rather than to the nearest convenient one
- Region and whether Greater London sites are included, since London travel behaviour distorts a national rate
- Location type, from town centre through edge of town centre, suburban area, neighbourhood centre, edge of town and free-standing
- Population within 1 mile and within 5 miles, as a proxy for the catchment and accessibility of the surveyed site
- Site size range, set around the proposed development rather than left wide open
- Parking provision, which on residential sites is often the single strongest determinant of car trip rates
- Survey date, with older surveys excluded because travel patterns have moved
The output is a rate calculation report listing every selected site, every filter applied and the resulting rate with its range. Submit that report. An assessment quoting a rate without the selection report attached invites the highway authority to assume the selection was tuned to the answer.
How many sites is enough
There is no rule, and this is one of the honest uncertainties in the method. A rate built from two or three surveyed sites is fragile, because one atypical site moves the average a long way. A rate built from twenty sites is more stable but may have been achieved by loosening the filters until the subset looks nothing like the proposal.
The defensible position is a subset large enough to smooth outliers while keeping the filters tight on the characteristics that actually drive travel behaviour: location type, parking and catchment population. Where the tight selection leaves too few sites, say so openly, present both the tight and the wider selection, and explain which you have relied on and why.
Peak hours and the periods assessed
Most assessments report the weekday morning and evening network peak hours, conventionally 08:00 to 09:00 and 17:00 to 18:00, because those are the periods when the highway network is under most strain.
But the development peak and the network peak are not always the same hour. A primary school peaks at 08:30 and again at 15:15. A food store peaks on Saturday. A gym peaks in the early evening and again at 06:30. Where the development peak falls outside the network peak, assess both and explain which combination produces the worst case for the junctions concerned.
Saturday peaks matter for retail, leisure and places of worship, and a highway authority will ask for them if the use is one where weekend activity dominates. Agree the periods in the scoping note rather than discovering the gap at consultation. Our guide to agreeing a transport assessment scoping note covers what to settle before any modelling starts.
Multi-modal trips and internalisation
Person trip rates matter as soon as the scheme has a travel plan, a sustainable transport argument or a mixed-use element.
Internalisation is the reduction applied where trips stay within a mixed-use site: a resident walking to the on-site shop generates no highway trip. It is legitimate and frequently overstated. Apply it only where the uses actually serve one another and are physically connected, quantify it explicitly, and expect the highway authority to test the assumption.
Pass-by and diverted trips are the retail equivalent. A roadside food store draws a share of its custom from vehicles already on the road, so those movements are not new trips on the network even though they are new trips at the access. The share assumed has to be evidenced from the database or from published survey work, and it is the number most often challenged.
On a change of use, the calculation is a net one: the trips generated by the proposal minus the trips the existing lawful use is entitled to generate. We have set out how to build that comparison in a note on net trip generation for a change of use.
Agreeing the selection with the highway authority
Do this before running the assessment, not after. The scoping stage should settle the land use category, the filter set, the regional inclusion, the survey date cut-off, the assessment periods and the assessment years.
Where the strategic road network is involved, National Highways becomes a statutory consultee and applies its own expectations to the trip generation evidence. Whether that applies to your site is worth establishing early, and our note on checking whether the strategic road network is engaged explains how.
Send the highway authority the proposed filter set as a short table with the resulting site count. It costs an email and it removes the most common reason for a transport chapter being rejected.
Where a rate is successfully challenged
Four patterns come up repeatedly.
| Challenge | What it looks like |
|---|---|
| Unrepresentative location type | Suburban sites used to justify a rate for an edge of settlement scheme with no bus service |
| Parking mismatch | Surveyed sites with restricted parking used for a scheme with two spaces per dwelling |
| Stale surveys | A subset dominated by surveys carried out long before current travel patterns |
| Overstated internalisation | A mixed-use reduction applied without evidence the uses serve each other |
None of these are fatal at scoping. All of them are awkward at committee, and an objector's transport consultant will find them.
The rate is an input, not a conclusion. A Transport Assessment is judged on whether the residual cumulative impacts on the road network would be severe, and a well-evidenced rate simply means the argument happens on the right ground. Where the scheme also needs measures to reduce car use, the Travel Plan should reference the same person trip data rather than inventing a separate baseline.
PlanScope drafts transport documents to this structure and marks every rate and count as still to be confirmed until survey data is supplied. Councils expect a suitably qualified transport planner to author a transport assessment and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
What exactly is a trip rate?
It is the number of trips generated per unit of development in a stated period, such as vehicle trips per dwelling in the weekday morning peak hour or person trips per 100 m2 of floorspace. Multiplying the rate by the scheme size gives the predicted trip generation used in junction capacity testing.
Which filters matter most when selecting sites?
Land use category, location type, population within 1 mile and 5 miles, parking provision, site size range and survey date. Parking provision is usually the strongest single determinant of residential car trip rates, and regional inclusion matters because London travel behaviour distorts a national average.
How many surveyed sites should a rate be based on?
There is no fixed rule. Two or three sites make the rate fragile because one outlier moves the average. A very large subset may indicate the filters were loosened too far. Present the selection report, and where a tight selection leaves few sites, show both the tight and wider subsets and explain which you relied on.
Should the assessment use vehicle or person trip rates?
Both, usually. Vehicle rates drive junction capacity work. Person trip rates from multi-modal surveys support the travel plan, the sustainable transport case and any mixed-use internalisation argument. The assessment must state clearly which rate is being used in each calculation, because mixing them undermines the conclusions.
Can existing use trips be deducted?
Yes, on a change of use or redevelopment. The assessment compares trips generated by the proposal against those the existing lawful use is entitled to generate, and reports the net figure. The lawfulness and the survey basis of the existing use both need evidencing, because an assumed baseline that overstates existing trips will be challenged.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.