Worked trip generation for a nine dwelling cul-de-sac

A trip generation small residential worked example usually comes about because a highway officer has asked what nine houses will do to the junction, and the honest answer is very little. The value of doing the calculation properly is that it converts an argument into a number, and the number for a scheme of this size is normally a handful of vehicle movements in the busiest hour. The method is the same one used on a major application: select comparable survey sites from a trip rate database, filter them until the set resembles your site closely, derive a rate per dwelling, and apply it.

The numbers used below are illustrative placeholders chosen to show the arithmetic. They are not database outputs and they must not be copied into a submission.

Whether a nine dwelling scheme needs trip generation at all

National guidance expects the scale of transport assessment work to be proportionate to the development. A Transport Assessment is the document for larger schemes, and the thresholds commonly applied sit at around 50 or more dwellings or 2,500 m² of floorspace. A nine dwelling cul-de-sac falls well below that.

What a scheme of this size usually needs is either a short Transport Statement or a paragraph in the planning statement, plus an access drawing with visibility splays and a turning area that works for a refuse vehicle. Trip generation appears because the highway authority asks for it, or because a neighbour has objected about traffic and the council wants something to answer with.

Ask the highway authority what it wants before commissioning anything. On a minor scheme the answer is often a single table, and paying for a full assessment is money spent on the wrong problem.

Choosing comparable survey sites

The trip rate database used in England for this purpose is TRICS, which holds surveys of actual developments with counts of vehicle and person movements by time period. Access is by subscription, which is one reason a transport consultant is usually involved even on a small scheme.

The selection process is a series of filters, and each one has to be justified in writing because the highway authority will check them.

Start with the land use category. Privately owned houses is a different category from mixed private and non-private housing, and from flats, so pick the one that matches your scheme rather than the one with the most surveys in it.

Then geography. Include the regions that resemble your area and exclude those that do not. Surveys from Greater London are rarely comparable with a village in the West Country, and surveys from Ireland are usually excluded on English applications.

Then location type. The database distinguishes town centre, edge of town centre, suburban area, edge of town, neighbourhood centre and free standing sites. A cul-de-sac off an existing residential road in a suburb is a suburban area site, and including edge of town surveys will inflate the rates because those sites are more car dependent.

The filters that matter most

Four filters change the answer more than the rest.

Size range. Set a range around your scheme rather than accepting the default. A set dominated by 200 dwelling estates will not represent nine houses, because larger schemes have different internal trip patterns and different accessibility.

Parking provision. The database records parking spaces per dwelling, and trip rates correlate with it. Filter to a range that matches what you are proposing. A scheme with two spaces per dwelling should not be justified with rates from sites with 0.8.

Survey date range. Older surveys reflect older travel behaviour, and the surveys carried out during the periods of national restriction in 2020 and 2021 are not representative of normal conditions. State the date range you used and why.

Survey day and period. Weekday surveys for a residential scheme, with the peak hours identified from the data rather than assumed. The residential morning peak commonly falls between 08:00 and 09:00 and the evening peak between 17:00 and 18:00, and both should be read off the profile rather than taken on trust.

Record the number of surveys remaining after filtering. A set of fewer than about five sites is thin, and a highway officer is entitled to say so. If filtering leaves you with two sites, relax the least important filter rather than presenting a rate derived from almost nothing.

Deriving a rate per dwelling

The output is a trip rate expressed per dwelling, separately for arrivals and departures, for each time period.

Keep arrivals and departures separate throughout. Residential traffic is directional, with departures dominating the morning peak and arrivals dominating the evening, and a two-way total hides the effect on a priority junction where the turn movement matters.

Decide whether you are reporting vehicle trips or person trips and label it. Person trips include walking, cycling and public transport and are larger. Highway capacity work needs vehicle trips. Sustainability arguments use person trips and modal split. Mixing them is a common error.

Use the mean rate from the filtered set. Where the set shows a wide spread, report the range as well, and consider whether the upper end is the more defensible basis on a site with high car ownership.

An illustrative calculation for nine dwellings

The following is illustrative only. The rates are placeholders used to show the method.

Assume the filtered set produces these vehicle trip rates per dwelling.

Applying those illustrative rates to nine dwellings gives:

1. Morning peak arrivals: 9 multiplied by 0.15 equals 1.35, so one to two vehicles.
2. Morning peak departures: 9 multiplied by 0.35 equals 3.15, so three vehicles.
3. Morning peak two-way total: approximately 5 vehicle movements.
4. Evening peak two-way total: approximately 5 vehicle movements.
5. Daily two-way total: 9 multiplied by 5.4 equals approximately 49 movements across the twelve hour period.

Five movements in the busiest hour is one movement every twelve minutes. That is the sentence which answers most objections about traffic on a scheme of this size, and it is worth stating plainly in the submission.

Do not round down at every step. Round the final figures and show the unrounded arithmetic, because a highway officer recalculating your table wants to arrive at the same numbers.

Netting off the existing use

The relevant figure is the net change in movements, not the gross figure for the new houses.

Where the site currently holds a dwelling, deduct its trips using the same rates. Where it holds a commercial or agricultural use with a lawful right to operate, deduct that use's trips using the appropriate database category, and state the lawful use you are relying on. A yard with a lawful haulage use can generate more peak hour movements than nine houses, which turns the transport case from neutral into positive.

Be careful with the baseline. You can only net off a use that is lawful and capable of resuming, which means evidence of the lawful use rather than an assertion about what used to happen. An abandoned use cannot be counted.

Where the existing use is a single house, the deduction is small and the honest presentation is to show it rather than to ignore it.

Sense checking against the street itself

Numbers derived from a database should be tested against the road in front of the site, and for a small scheme this takes one morning.

Stand at the junction between 08:00 and 09:00 with a tally counter and record movements in and out of the existing street. Repeat between 17:00 and 18:00. Note the parking situation, whether vehicles have to wait to pass, how many pedestrians and cyclists use the footway, and whether the visibility from the access is actually as the drawing shows.

Then compare. If your calculated five additional movements sit against an observed 120 movements an hour on the receiving road, the proportional increase is small and easy to state. If the existing street carries twelve movements an hour, your five is a noticeable proportional change and the argument shifts to absolute capacity rather than proportion.

Where on-street parking is the real concern, a Parking Survey carried out to the recognised overnight and weekend methodology gives the evidence, and it is a different exercise from trip generation. The two are often confused by objectors and should be kept apart in the submission.

Our note on a worked greenfield run-off rate for a small site follows the same principle for drainage: do the public calculation, show the working, label the assumptions.

Presenting the output and its caveats

A single page does it. State the database used and the date of the extract. List every filter applied and the number of surveys remaining. Give the rates in a table with arrivals and departures separated. Give the applied figures for your dwelling count. Give the net change after any deduction. Then the observed counts and the comparison.

Then state the caveats honestly rather than hiding them:

• The rates are averages from comparator sites and no individual development matches them exactly.
• The comparator set contains a stated number of sites, and a small set carries more uncertainty.
• Travel behaviour has changed since some surveys were carried out, particularly in relation to working from home and delivery traffic.
• Vehicle trip rates exclude walking, cycling and public transport movements.

A short, clearly caveated table is more persuasive to a highway officer than a long document that overstates its precision. And where the conclusion is that the effect is not material, say exactly that, then carry the same wording into the Planning Statement so the two documents agree.

Councils usually expect transport work to be produced by a suitably qualified author, and professional review is strongly advised before submission. This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional before relying on it.

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