Trip rate reductions for a sustainable location

A trip rate reduction for a sustainable location is the most frequently challenged adjustment in a transport assessment, and it fails for a predictable reason: the applicant asserts that the site is accessible and applies a percentage without showing where the percentage came from. Highway authorities accept mode share adjustments. What they want is a named dataset, a stated geography, an explanation of why that geography is a fair proxy for the development, and a demonstration that the proposed measures will actually produce the behaviour claimed. Get those four things in and the adjustment usually survives. Leave any of them out and the officer reverts to unadjusted rates.

The underlying trip rates themselves normally come from the TRICS database, with sites selected by land use category, region, date range and site characteristics. The adjustment is a separate step applied to the mode split, and it should be presented as such rather than buried in the rate selection.

Why adjustments are challenged

Three concerns sit behind the challenge. The authority has to be satisfied the assessment is not understating impact, because a junction that fails at unadjusted rates and passes at adjusted rates is a decision resting entirely on the adjustment. It has seen adjustments justified by accessibility that never materialised after occupation. And it has no mechanism to recover the position if the reduction turns out to be optimistic.

That last point is the one applicants can answer. An adjustment linked to measures secured by condition or obligation, with monitoring and a remedy if targets are missed, addresses the authority's actual risk. An adjustment linked to nothing does not.

Using census travel to work data

Census travel to work data is the obvious source and it needs handling with care. The method of travel to workplace tables are published at small-area geographies, which lets you describe the behaviour of households in the immediate area rather than the district average. That is the right geography for a residential scheme.

The caveat has to be stated, not buried. The 2021 census was taken on 21 March 2021, during a period of widespread home working and restricted travel, so its travel to work distribution is not representative of normal conditions. The Office for National Statistics has published guidance on interpreting those results for exactly this reason. Using the 2021 figures without comment is the fastest way to have the whole adjustment dismissed.

The defensible approaches are to use the earlier census for the baseline mode split with a note on its age, to use the 2021 data alongside it and discuss the divergence, or to treat census data as supporting evidence behind a stronger primary source. Say which you have done and why.

Two further limits. Travel to work covers commuting only, so it says nothing about education, shopping or leisure trips, which are a majority of movements at a residential site. And it describes the existing population, who may not resemble the occupiers of a new scheme in tenure, age or car ownership.

Comparable site selection as an alternative

The stronger evidence is usually a survey of a comparable development. Pick two or three existing sites with similar characteristics, survey them properly, and present the observed rates against the TRICS rates for the same land use.

What makes a comparable site comparable in practice:

• similar accessibility, measured the same way at both sites
• similar tenure and dwelling mix, because car ownership tracks both
• similar parking provision, which is the strongest single determinant of car use
• fully occupied for long enough that travel patterns have settled
• surveyed on neutral days, avoiding school holidays, August and December

Multi-modal counts at a comparable site cost a fraction of a refused application, and they put observed numbers rather than inference in front of the officer. Agree the site selection with the highway authority at scoping so the choice is not challenged after the survey has been paid for.

Accessibility scoring and its limits

Accessibility indices are useful for describing a site and poor at predicting behaviour. In London, public transport accessibility levels are calculated on a published methodology and are the common currency of borough parking and trip rate discussions. Elsewhere authorities use their own accessibility indices or a simpler walk-time analysis to services.

Be straight about what these measure. They quantify the supply of public transport at a point, weighted by frequency and walk time. They say nothing about whether the services go where the occupiers want to go, about crowding, reliability, fares or the quality of the walking route to the stop. A site can score well on frequency and badly on destinations.

So use the score as context and the behavioural evidence as the justification. A high accessibility score plus observed mode shares from a comparable site is a strong pairing. A high score on its own is an assertion with a number attached.

Linking reductions to committed measures

This is where an adjustment becomes defensible. Tie each element of the reduction to something the authority can hold you to:

1. Restricted parking provision, with a parking ratio stated per unit and enforced by the layout rather than by hope.
2. A permit-free or car-capped agreement secured by planning obligation where the authority operates a controlled parking zone.
3. Cycle parking above the local standard, secure, covered and at ground level, with a stated ratio.
4. Car club provision, where the practical questions of securing car club bays and keeping them viable need settling before the obligation is drafted.
5. A Travel Plan with named targets, a coordinator, a survey programme and a remedial mechanism, secured by condition or obligation.
6. Physical improvements to the walking and cycling routes to the nearest stops and services, delivered through a highway agreement.

State the reduction attributable to each measure separately rather than presenting one combined figure. The officer can then accept some and query others instead of rejecting the lot.

Sensitivity testing and the fallback case

Run the assessment twice. Present the unadjusted case using the raw TRICS mode split as a sensitivity test alongside the adjusted case, and show what happens at the critical junctions in both.

Where the unadjusted case still works, the adjustment stops being contentious because nothing depends on it. Where it does not, the sensitivity test tells the officer exactly what the adjustment is carrying, and the discussion moves to the mitigation and monitoring rather than to whether the percentage is defensible.

Include a reverse sensitivity too. If the travel plan targets are missed by a stated margin at the first monitoring survey, what is the remedy. A pre-agreed remedial package, funded by a bond or a contribution released on failure, is a far better answer than an argument two years after occupation.

Presenting the adjustment transparently

Give the full audit trail in the document rather than in an appendix nobody reads. State the TRICS land use category and the filters applied, including region, date range, site size and any excluded sites with reasons. State the unadjusted rates and mode split. State the adjustment, its source and its arithmetic. State the resulting rates. Then show both cases in the junction assessment.

Where a parking stress case is also being run, keep the two consistent, because a Parking Survey showing low on-street availability sits awkwardly with a claim that occupiers will not own cars. A Transport Assessment that reconciles them explicitly is the one that gets agreed. PlanScope drafts the policy framework, structure and narrative of that assessment from your scheme inputs, flagging every figure the modeller has to supply. Councils usually expect a suitably qualified transport planner, so professional review is strongly advised, reports should be reviewed before submission, and PlanScope does 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.