Trip rates and how TRICS output is challenged
A trip rate drives the junction modelling, the mitigation and the highway contributions. Here is how TRICS rates are selected, where they go wrong, and how to challenge one credibly.
A trip rate is the number of movements a site generates per unit of development, and in trip rate assessment planning work that figure almost always comes out of the TRICS database rather than off your own site. Multiply it by the unit count or the floorspace and you have the traffic generation evidence that drives junction modelling, mitigation design and the highway contributions in a section 106 agreement. Which is exactly why highway authorities argue about it. Pick the wrong comparable sites and everything built on top of the rate wobbles.
What a trip rate actually counts
TRICS holds surveys of real development sites, each one counting vehicles, people or both across the working day. A rate is expressed per dwelling, per 100 m² of gross floor area, per bed space or per pitch, depending on the land use category. So a residential rate of 0.4 two-way vehicle movements per dwelling in the morning peak means 40 movements for 100 homes.
The rate is an average of the selected surveys, and the spread behind the average matters as much as the average itself. Two sites with identical rates can sit on very different standard deviations. A transport planner who quotes the mean without showing the range has hidden the part an officer most wants to see.
Rates are also directional. The morning peak at a housing site is heavily outbound, the evening peak inbound, and a junction model fed with an undirected total will understate the critical arm.
How comparable sites are picked
Selection is a sequence of narrowing choices, and each one is a judgement you can be asked to defend.
- Choose the land use category and sub-category, for example privately owned houses rather than mixed private and affordable flats.
- Set the region and whether Greater London and Northern Ireland sites are included or excluded.
- Set the site area, unit range and parking provision ranges.
- Exclude sites surveyed before a chosen year, because travel behaviour has shifted.
- Decide whether town centre, edge of centre, suburban or village locations are in scope.
- Review the remaining sites individually and drop any that are plainly unlike the application site.
That last step is the one most often skipped. TRICS gives you the survey details for each site, including the local population density and the number of parking spaces, and reading them is how you catch the retail park survey sitting in a supposedly residential set.
The filters that move the number
Filters are not neutral. Tightening the parking range on a residential set tends to push the vehicle rate up, because sites with generous parking generate more car trips. Including town centre sites pulls it down. A set of eight sites can produce a rate 50 per cent higher or lower than a set of 30 drawn from the same category, which is why the number of surveys in the selection is always worth stating.
Sample size is the honest weak point of most trip generation evidence. There is no rule that says six sites is too few, and local authorities differ on what they will accept. Say how many sites you used, name the filters, and attach the TRICS output report so the selection can be rerun. Hiding the parameters invites a request for the whole exercise to be redone.
Where the application site has no good comparator, a bespoke survey of a similar operating site is the defensible route, agreed with the highway authority before it is carried out.
Choosing the peak hour and the assessment period
Network peak and site peak are different things. Housing usually peaks with the network, around 08:00 to 09:00 and 17:00 to 18:00 on a weekday. A primary school peaks earlier and sharper. A food store peaks on Saturday lunchtime, a drive-through at Friday teatime, and a crematorium at mid-morning on weekdays.
The assessment period should cover the network peak hours when the development will add to existing congestion, plus the site peak if it falls elsewhere and creates its own problem. For retail and leisure schemes the Saturday peak is often the one that governs the junction design, and leaving it out is a standard reason for a holding objection.
| Land use | Period usually governing | Why |
|---|---|---|
| Housing | Weekday 08:00 to 09:00 and 17:00 to 18:00 | Coincides with network peak |
| Primary school | Weekday 08:00 to 09:00 | Short, intense arrival peak |
| Food retail | Saturday late morning to afternoon | Site peak exceeds weekday |
| Warehousing | Shift change times | Peaks may fall outside network peak |
Person trips, mode share and linked trips
Multi-modal rates count people rather than vehicles, split by mode. They are the right starting point for an urban scheme where the argument is about bus capacity and footway width as much as junction capacity. Taking a vehicle rate from a suburban site and applying it to a scheme beside a tube station overstates car use and buries the case for a lower parking ratio.
Two adjustments routinely reduce the net figure, and both need evidence rather than assertion. Internalisation covers trips that stay inside a mixed use site, such as a resident walking to the on-site nursery. Pass-by and diverted trips cover vehicles already on the network that call in on an existing journey, which matters for roadside retail and fuel stations.
Discount either without showing the basis and the highway authority will reinstate the full rate. A sensible submission presents the gross rate, the adjustment, the source for the adjustment percentage and the net figure, so the officer can accept part of it and reject the rest.
Challenging a rate so the challenge sticks
Objectors and consultees both tend to challenge rates badly, by asserting that the number feels low. That gets nowhere. The challenges that work are specific and reproducible.
- Rerun the TRICS selection with the stated filters and show that the applicant's set cannot be reproduced.
- Identify named survey sites in the set that are unlike the application site on parking, location type or survey year.
- Show that the sample size is small enough that one site dominates the mean.
- Point to a local survey of a comparable operating site that produces a materially different rate.
- Show that the peak period chosen misses the site peak.
Appeal inspectors have treated trip generation as a matter of evidence rather than principle, so the side with the reproducible selection and the stated assumptions is usually in the better position. Do not expect an inspector to be impressed by a rate dispute that does not change the junction results.
What the highway authority looks at first
Most highway officers go to three things before they read the narrative. The TRICS output report, to see the category, filters and site count. The comparison of gross and net trips, to see what has been discounted. And the distribution assumptions, because a credible rate assigned badly still produces a wrong answer at the junction.
Agreeing the rate and the assessment period at pre-application, in writing, removes the largest single source of delay in a Transport Assessment. On smaller schemes the same discipline applies to a Transport Statement, where there is less room to hide a weak assumption. Where the scheme leans on reduced car use, the mode share in the rate has to match the commitments in the Travel Plan, and a mismatch between the two documents is noticed. An intensification of an existing use raises a prior question about whether the change is development at all, which is covered in our note on intensification as a material change of use.
PlanScope can draft the transport documents around the trip rate figures you supply and mark any input it does not hold as still to be confirmed rather than inventing it. Enter your postcode on PlanScope to see which reports your site needs.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Is a TRICS trip rate the only acceptable evidence?
No. TRICS is the usual starting point because highway authorities recognise it, but a bespoke survey of a comparable operating site is accepted where no sensible comparator exists in the database. Agree the survey location, dates and counting method with the highway authority before carrying it out, otherwise the result is easy to dismiss once the application is submitted.
How many TRICS survey sites should a selection contain?
There is no fixed minimum in national guidance. What matters is that the sites are comparable and that no single survey dominates the mean. A small set is defensible if the sites are close matches and the spread is reported. Always state the count and the filters so the selection can be rerun by the officer.
What is the difference between a vehicle rate and a person trip rate?
A vehicle rate counts vehicle movements across the site access. A person trip rate counts people and splits them by mode, including walking, cycling and public transport. Urban schemes usually need the person trip version, because the planning argument is about bus and footway capacity as well as the road junction.
Can pass-by trips be deducted from the trip generation figure?
They can, where the use really does intercept traffic already on the network, such as roadside retail or a fuel station. The deduction needs a stated source and a clear calculation. Present the gross rate, the adjustment and the net figure separately so the highway authority can accept or reject the adjustment on its own.
Who decides which peak hours are assessed?
The highway authority, in practice, and it is worth settling at pre-application. The default is the weekday network peaks, but a site peak elsewhere must also be tested where it is larger. Food retail and leisure schemes often turn on the Saturday peak, and omitting it is a common reason for a holding objection.
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.