Discounting trips: internalisation, pass-by and diversion

Gross trip rates overstate the highway impact of almost every commercial scheme, and the adjustments that get you to a net figure are the most scrutinised pages in a transport assessment. Pass-by, diverted and internalised trips, plus the credit for the lawful existing use, are all legitimate. What makes them survive review is showing each one separately, with its own evidence, applied at the right point in the network.

The reviewer's instinct is that discounts are where optimism hides. Earn the benefit of the doubt by making the arithmetic visible.

Why the gross figure is the wrong number

A trip rate database gives you vehicle movements observed at comparable sites. Those observations are complete: they count every car that entered, whether the driver made a special journey or pulled in on the way home from work.

For a new destination in a new location, gross is close to the truth. For a convenience store on a distributor road, a drive-through on a retail park or a petrol station on a commuter corridor, it is not. Much of the traffic already exists and is already counted in the baseline flows, so adding the gross figure on top double counts it.

That is the whole basis of discounting. You are removing trips that the survey counted twice, not inventing a reduction.

Pass-by trips and how they are evidenced

A pass-by trip is one already travelling on the road the site fronts, which stops at the development without deviating from its route. The vehicle was in the baseline flow before, it is in the baseline flow after, and the only change is a turning movement at the access.

Evidence comes from surveyed data for a closely comparable use. Comparability here means the same broad land use, a similar road type and traffic composition, and a similar catchment. A rate observed at a trunk road service area tells you nothing about a shop on a village high street.

Two rules keep this honest. Apply the discount to network links and to junctions away from the site, never at the access, where a pass-by vehicle loads the turning movements in full. And state the assumed pass-by percentage in the text, with its source, rather than burying it in a spreadsheet cell.

Sensitivity testing helps. If the scheme still works with a pass-by rate ten percentage points lower than the one you have assumed, say so, and the argument largely goes away.

Diverted trips and the catchment argument

Diverted trips are already on the network but change route to reach the site. They are not new traffic in the study area, but they do move traffic from one link to another, which means the discount is a redistribution rather than a deletion.

Handling this properly requires a distribution assumption you can defend: where the trade is coming from, along which corridors, and what the trips were previously doing. Census travel to work data, catchment modelling from the operator, or an existing store's loyalty data can all support it, and the source should be named.

Officers challenge diverted trips more often than pass-by trips because the effect is spatially uneven. A junction that gains 40 movements is not comforted by a junction two miles away losing them.

Internalisation in mixed use schemes

Where a site combines housing, employment, a nursery and a convenience unit, some journeys stay inside the red line. A resident walking to the shop makes no external vehicle movement at all.

Internalisation is real but usually smaller than the promoter hopes. Its size depends on the balance of uses, the walking distances inside the scheme and whether the internal routes are pleasant enough to use. On a 40 hectare site with a supermarket at one end, the internal walk may be theoretical.

Set out the internalisation matrix explicitly, use by use, and keep the assumed rates modest. Claim a large internal capture and the reviewer will ask for the evidence at every junction rather than accepting a headline percentage.

Existing use credit and the lawful fallback

The credit for the existing use is the discount that produces the most argument, partly because the sums are large and partly because the assumptions are contestable.

Start from the lawful use, evidenced by the planning history or a certificate rather than by what happens to be on site. Then apply a realistic assumption about how that use would operate. A vacant unit is the difficult case: authorities commonly resist a full credit for a building empty for years, on the basis that the traffic is not currently on the network and may never return.

A fallback position is stronger where it is really available. A Class E building that could lawfully be occupied by another Class E use without any permission gives a real fallback. A speculative claim that a site could operate at a previous intensity, which the market has not supported for a decade, is weaker and should be presented as a sensitivity rather than as the base case.

Documenting the discounts so they survive review

Present gross and net figures side by side, with a short paragraph for each adjustment giving the value, the source and the reason. Show the arithmetic in an appendix table that a highway officer can rework in ten minutes.

The scheme that gets sent back is rarely the one with aggressive assumptions clearly stated. It is the one where the net figure appears without explanation and the reviewer has to reverse engineer it. Where the residual impact is small after discounting, a Transport Statement may be proportionate, and there is no need to build a full Transport Assessment around it.

Watch for double counting between adjustments. Taking full existing use credit and then applying a pass-by discount to the same movements counts the same vehicle twice, and it is the single most common error a reviewer finds.

Where residential elements are involved, the trip rate itself may need adjusting before any discount is applied, which is the issue that arises with student and age restricted housing. Parking follows the same discipline: the case for parking provision above or below the standard rests on observed demand, and a parking survey carried out to a recognised methodology carries far more weight than an assertion about accessibility.

The policy test in the current National Planning Policy Framework, published on 17 August 2026, is whether residual cumulative impacts on the road network would be severe, or whether there would be an unacceptable impact on highway safety. Residual means after mitigation and after the discounts. Get the net figure right and that test is usually where the argument ends.

Enter the site postcode on PlanScope to see which transport and highways documents your scheme is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Councils expect transport evidence to be prepared by a suitably qualified transport planner, and this article is general guidance rather than legal advice.