Linked trips and internalisation in mixed use schemes

An internalisation claim says that some of the trips generated by one part of a mixed use scheme are already counted in another part, so the external network sees fewer vehicles than the sum of the gross trip rates. Applied properly, linked trip and internalisation discounts in a mixed use transport assessment are legitimate and sometimes large. Applied as a flat percentage with no evidence, they are the first thing a highway authority officer deletes, and the junction results then look very different from the ones in your submitted report.

The principle is sound. A resident who buys milk on the way home has made one journey, not two. The dispute is always about how many of them do it, and how you know.

What internalisation claims

Gross trip rates come from surveys of single-use sites. A convenience store rate is derived from stores, a flat rate from blocks of flats, and neither survey knew about the other land use sitting next door.

Internalisation claims that a share of journeys begin and end inside the development. A resident walking to the on-site gym, an office worker buying lunch in the ground floor unit, a hotel guest eating in the restaurant: each is a trip that the single-use rate would put on the network and that in reality never leaves the site.

Linked trips are a related but distinct idea. Here the journey does reach the network, and serves two purposes on the way. A commuter stopping at the on-site nursery before continuing to work generates one arrival rather than two.

Both reduce net new trips. Both need evidence, and the reason is simple: the size of the discount decides whether the critical junction works, so the applicant has an obvious interest in the number being large.

Evidence required to support a discount

Four evidence sources carry weight, in roughly descending order.

Surveys at comparable mixed use sites are the strongest. If you can survey a scheme of similar composition, scale and location and measure the proportion of trips that stayed internal, the argument is close to unanswerable. It is also expensive, and the survey site has to be a fair comparator on all three counts.

Published trip rate database outputs with the appropriate survey selection come next. The national database used for trip generation in England allows sites to be filtered and includes guidance on multi-modal surveys and on the treatment of pass-by movements, which is more defensible than an assumed figure.

Interview surveys at the proposed development's catchment, asking existing customers at similar facilities about journey purpose, support pass-by and linked trip claims specifically.

And last, professional judgement with a stated and modest percentage, tested through sensitivity analysis. That is acceptable where the discount is small and the junction works without it. It is not acceptable as the basis for a 30 per cent reduction.

Pass-by and diverted trips

These two terms cause more confusion than internalisation does, and the distinction matters to the junction.

A pass-by trip is one already travelling on the road the site fronts, which turns in and rejoins. It adds no vehicle to the link flow. It does add two turning movements at the access, so it affects the access junction capacity and sometimes the capacity of the junction beyond it.

A diverted trip leaves its original route to reach the site, adding flow to the roads between the original route and the access while removing it elsewhere. It has to be reassigned rather than discounted.

The error to avoid is treating a pass-by trip as though it disappears entirely. It does not. It disappears from the link and appears at the junction, which is usually the thing under scrutiny.

Risk of double counting

Double counting happens in three ways, and officers look for all of them.

The first is applying the discount to both land uses. If 10 per cent of residential trips are internalised to the retail unit, those same movements cannot also be deducted from the retail trip generation. Pick one side and say which.

The second is using survey sites that were themselves mixed use to derive the gross rates, then applying an internalisation discount on top. The surveyed rate already reflects internal capture, so the discount is taken twice.

The third is combining an internalisation discount with a mode share assumption that already assumes walking and cycling for short internal journeys. The travel plan and the trip generation chapter have to be consistent with one another, which is one reason the content of a Travel Plan should be drafted alongside the assessment rather than afterwards.

How highway authorities test the claim

Officers apply a predictable set of tests, so anticipate them.

They check whether the discount is stated as a percentage of gross trips and whether the arithmetic is shown. They look for the source of the percentage and reject a figure attributed to professional judgement alone where the junction depends on it. They test whether the uses are complementary in composition and opening hours, because an office and a nightclub internalise nothing. They examine whether the internal walking routes exist and are attractive, since a discount premised on internal pedestrian movement across a service yard will not be accepted. And they rerun the critical junction without the discount.

That last test is the one to pre-empt. Report the junction results both with and without the claimed discount. If the junction works either way, the discount stops being contentious. If it only works with the discount, you have identified your main risk before the authority does.

Presenting net trip generation

Set the calculation out as a transparent sequence rather than a single net figure.

1. Gross trip generation for each land use, with the database selection criteria and the survey sites listed.
2. Internalisation deduction, with the side of the transaction it is taken from and its evidence source.
3. Pass-by and diverted adjustments, shown separately, with the turning movements retained at the access.
4. Net new trips on each link, and total movements at each assessed junction.
5. Sensitivity case with the discount removed.

A reviewer should be able to rebuild that table from your inputs. Where it cannot be rebuilt, the chapter gets treated as an assertion, and the officer substitutes their own assumption. The same transparency expectation runs through the whole of a Transport Assessment, and the scoping discussion is where the method should be agreed.

Where the argument fails

Three situations defeat an internalisation claim almost regardless of how well it is argued.

Where the uses have mismatched peak hours, the discount evaporates at the hour being assessed. An office and a primary school both peak in the morning, and the internal capture between them is minimal because the journeys are not substitutable.

Where the retail or leisure element is sized for a catchment much wider than the residential element, the internal population cannot support it, so most of its trips are external by definition. A supermarket beneath 40 flats is a supermarket with 40 flats above it.

And where the site layout separates the uses with a road, a level change or a service area, pedestrians will not make the internal trip. The discount then depends on people behaving in a way the design discourages.

The Planning Practice Guidance on transport assessments expects the scope and assumptions to be agreed with the local highway authority, so put the proposed discount and its evidence in the scoping note. Where public transport contributions are also in play, the net trip figures feed that negotiation directly, as set out in bus infrastructure contributions and their triggers. The relationship between site-wide travel planning and occupier-level measures is covered in residential and workplace travel plans compared.

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