Trip rate: what it measures and how it is derived

A trip rate is the number of trips a development generates per unit of that development, in a defined time period. Vehicle trips per dwelling in the morning peak hour. Person trips per 100 m² of gross floor area on a Saturday. The unit matters as much as the number, and a trip rate quoted without its unit, its period and its source tells you nothing at all.

Everything in a transport chapter flows from it. Junction capacity, parking demand, the need for a travel plan, the level of contribution a highway authority asks for, whether your scheme needs a full assessment or a short statement. Get the rate wrong at the start and every number downstream is wrong too.

What a trip rate measures

Three elements define any rate.

The unit of development, which is dwellings for residential, 100 m² of gross floor area for most commercial uses, and sometimes a bespoke unit such as beds, pitches or pupils. The time period, typically a peak hour but also daily totals and specific periods such as school arrival. And the direction, since arrivals and departures behave very differently and the total is rarely the useful figure.

A residential rate in the morning peak is dominated by departures. A primary school rate in the same hour is dominated by arrivals followed immediately by departures as parents leave. Presenting only two-way totals hides the thing a junction actually experiences.

Peak hour also needs definition. Network peak is when the road is busiest, usually 08:00 to 09:00 and 17:00 to 18:00 on a weekday. Development peak is when your site is busiest, which for a supermarket or a place of worship falls somewhere else entirely. Highway authorities generally want both.

Where the rates come from

Almost all trip rates used in English transport evidence come from surveyed sites held in the TRICS database, which records counts at real developments across a range of types and locations. A transport planner selects a set of comparable surveyed sites and derives an average rate from them.

That selection is the whole ballgame. The database holds hundreds of sites per land use category, with very different rates depending on where they are and what they are. The rate you get out depends on which sites you allow in.

Some authorities publish their own local rates, derived from local surveys, and a few require them in preference to the national database. Check the council's transport evidence requirements or its validation guidance before commissioning the work.

Filtering and its effect on the answer

The filters a transport planner applies are, in effect, arguments about comparability. Expect to justify each of them:

• land use category and subcategory, which is where a "private houses" selection differs sharply from "mixed private and affordable"
• geographic region, and whether Greater London sites are included or excluded
• location type, such as town centre, edge of town, suburban area or village
• parking provision at the surveyed sites, which correlates strongly with vehicle trip rates
• population within one mile and within five miles, as a proxy for accessibility
• survey date range, since older surveys reflect different travel behaviour
• development size range, to avoid comparing a 12 unit scheme with a 400 unit one

A selection that includes only low density suburban sites with generous parking produces a high vehicle rate. A selection weighted to town centre sites with restricted parking produces a low one. Both can be defended in isolation, which is why the highway officer will ask to see the selection itself, not just the output.

Include the full selection record in the appendices. A transport chapter that quotes a rate without the underlying site list invites a request for further information, and the resulting delay is entirely self-inflicted.

Person trips against vehicle trips

Vehicle trip rates answer the capacity question. Person trip rates answer the sustainability question, and increasingly the policy question.

Multi-modal survey data gives trips by car, on foot, by cycle and by public transport. That matters where a scheme relies on accessibility to justify low parking provision, where the council expects a modal shift target, or where an authority wants trips assessed in people rather than vehicles as a matter of policy.

Mode share assumptions imported from elsewhere are a weak point. Census travel to work data for the ward is a defensible starting point for a residential scheme. A mode share lifted from a comparable scheme in a different town is not, unless you can show the accessibility is comparable.

Netting off the existing lawful use

This is the most valuable arithmetic in the whole exercise, and it is routinely done badly.

The relevant figure for highway impact is the net change in trips, not the gross generation of the new use. Where a site has an existing lawful use, the trips that use generates can be deducted. A vacant office building still has a lawful use and still counts, as long as the use has not been abandoned.

The conditions are:

1. The existing use must be lawful. Evidence of that may be the planning history or a lawful development certificate.
2. The use must not have been abandoned, which turns on the length and circumstances of the vacancy, the physical condition of the building and any intervening use.
3. The deduction is normally based on the same survey source and period as the proposed use, not on an actual count taken while the building sat empty.

Highway authorities sometimes resist full netting off where a building has been vacant for years. The argument is that the network has adjusted and the trips are not there in practice. It is not a strong argument in planning terms, but it is common, and it is worth addressing directly rather than hoping nobody raises it.

Peak periods and assessment years

The assessment year is normally the year of opening, with a future year commonly five or ten years later for a larger scheme. Background growth is applied using national traffic growth data, and committed developments in the area are added as separate trip generation.

Two errors recur. Applying growth factors to a base year that already includes committed development double counts it. And using a base count taken in an atypical month, such as August or the fortnight before Christmas, understates the baseline. Neutral month counts, usually spring or autumn outside school holidays, are the expectation.

An illustrative netting off calculation

The following uses assumed rates purely to show the arithmetic. They are not database outputs and should not be used for any real assessment.

Take a 1,000 m² vacant office proposed for conversion to 14 flats. Assume the office generates 1.6 vehicle trips per 100 m² in the morning peak, giving 16 trips. Assume the flats generate 0.35 vehicle trips per dwelling in the same hour, giving 4.9, which rounds to 5. The net change is a reduction of 11 trips in the morning peak.

That result changes the character of the whole submission. A scheme with a net reduction rarely needs junction modelling, and the evidence can often be presented as a Transport Statement rather than the fuller Transport Assessment required above the usual thresholds. Where parking rather than capacity is the concern, a Parking Survey addressing on-street stress is generally the more useful document.

Whether the rate you have chosen is defensible depends in part on the parking standards and accessibility policies in force locally, so check what carries weight using our note on the development plan and what counts.

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