When a planning application lands, one of the first questions a council asks is simple to say and harder to answer: how much extra traffic will this generate? The answer usually comes from two linked things, trip rates and a database called TRICS. Get them right and the transport case stands up. Get them wrong and the application stalls. Here is how the method works and why it matters.
What a trip rate actually is
A trip rate is the number of journeys a type of development produces for a given unit of size, over a set period. It might be expressed as trips per dwelling, per 100 square metres of floorspace, or per employee, usually measured in the busy morning and evening peak hours. Multiply the rate by the size of the scheme and you have an estimate of the trips it will generate. Crucially, a trip rate covers all modes, not just cars: walking, cycling, public transport and car journeys are all counted, which matters for schemes leaning on sustainable travel.
Where the numbers come from: TRICS
The rates are not invented. They come from TRICS, the national trip generation database used across the UK and Ireland. TRICS holds thousands of real surveys of actual developments, counting the trips to and from sites of every type, from supermarkets to housing to offices. A consultant selects comparable surveys, filters them for location, size and setting, and derives a defensible rate from real observed behaviour rather than guesswork. TPA is closely involved in the TRICS community, and our Rupert Lyons was confirmed as guest chair of the TRICS User Meeting.
Choosing the right comparison sites
This is where judgement comes in, and where assessments are won or lost. Pick survey sites that are too generous or too different from your scheme and the council will challenge them. Good practice is to be transparent: set out the filters used, the number of surveys behind the rate, and the reasoning, so the local highway authority can follow the logic. An edge-of-town retail unit and a city-centre one behave very differently, and the selection has to reflect that.
From trip rates to impact
Trip generation is only the first step. Once you know how many trips a site produces, those trips are distributed across the network and tested at junctions, which is where the wider modelling begins. We walk through that next stage in our piece on how modelling shapes development decisions. Trip rates also feed the transport assessment or statement, and the net change matters: for a site replacing an existing use, it is the difference in trips that counts, not the total.
Why councils scrutinise it so closely
The whole transport case rests on these numbers. Under the National Planning Policy Framework and its Planning Practice Guidance, an authority can only refuse on transport grounds where the residual impact is severe, so the trip figures decide whether mitigation is needed and how much. A solid, well-evidenced trip rate is your strongest footing in that conversation.
How we help
We prepare trip generation as part of our transport planning work, selecting comparable TRICS sites, deriving rates that hold up to scrutiny, and setting out the reasoning clearly in the planning application. If you have a scheme coming forward, get in touch.
