Worked example: trip rates for a small convenience store

Trip rate selection example: a 250 m² convenience store, an edge of town parade, and an evening peak figure that can swing by a factor of three depending on which comparable sites you keep. The rate itself is arithmetic. The defensible part is the filtering, and that is where highway officers spend their time.

This walkthrough uses invented rates so the sums are visible. Every number labelled illustrative is an assumption made for the example, not a database output, and a real submission must use the rates the filtered set actually returns.

Why the site selection decides the answer

Trip generation for small food retail is unusually sensitive to context. A store on a busy main road with a lay-by picks up drivers already passing. The same floorspace on a residential estate with no through traffic serves walk-in trade. Both are convenience stores of the same size, and their vehicle trip rates are not comparable.

The trip rate databases used in this country, principally TRICS, hold surveyed counts from real sites with attributes recorded against each one: gross floor area, parking provision, region, area type, day of week and date of survey. Selecting comparables means choosing which of those attributes must match your site and which can vary. Every filter narrows the sample, and a sample of two sites is not evidence, so the exercise is a trade between similarity and sample size.

State the trade-off in the report. An officer who can see you made the choice deliberately is much easier to deal with than one who suspects you filtered until the answer got small.

Filtering the comparable sites

For this example the filters are set as follows.

1. Land use: the retail category, then the sub-category covering small food stores rather than superstores or retail warehouses.
2. Floorspace band: 150 to 400 m² gross floor area, bracketing the proposed 250 m² without being so tight that nothing survives.
3. Region: England outside Greater London, because London sites carry much lower car mode shares and would understate a provincial rate.
4. Area type: edge of town and suburban, excluding town centre and rural sites.
5. Parking: sites with some customer parking, matching the proposed six spaces in broad terms.
6. Survey date: the last ten years, so that changes in shopping patterns are reflected.

That set leaves, for the sake of the example, seven surveyed sites. Seven is workable. Below about five, the sensible move is to relax one filter and say which, rather than proceeding on a sample that a consultee can dismiss in a sentence.

Two filters are worth resisting. Do not exclude sites solely because their trip rate looks high, and do not restrict the region to the county the site sits in unless there is a reason beyond convenience. Both look like selection to a result.

Days, time periods and survey age

Retail peaks rarely coincide with the network peak, and that mismatch is the point of the exercise. A convenience store is busiest in the late afternoon and early evening on weekdays and through the middle of Saturday. The highway network is busiest between 8am and 9am and between 5pm and 6pm on weekdays.

Select Monday to Friday counts for the weekday analysis and exclude surveys carried out in August or in the weeks either side of Christmas, because neither month is neutral. If a survey falls in a school holiday, say so and consider whether to keep it.

The illustrative rates

Assume the filtered set of seven sites returns the following. These are illustrative figures chosen for this example.

• Weekday 08:00 to 09:00: 4.0 two-way vehicle trips per 100 m² gross floor area.
• Weekday 17:00 to 18:00: 12.0 two-way vehicle trips per 100 m².
• Saturday 12:00 to 13:00: 14.0 two-way vehicle trips per 100 m².

Report the mean rate, and also report the range across the sample, because the range tells the reader how much confidence the mean deserves. If the evening peak rates across seven sites run from 6 to 21 trips per 100 m², the mean of 12.0 is a weak central estimate and the report should acknowledge that rather than presenting a single tidy figure.

Also record whether the rates are vehicle trips or person trips. Mixing the two is the most common arithmetical error in short transport statements.

Applying the rates to the proposed floorspace

The proposal is 250 m² gross floor area, so the multiplier is 2.5 hundred square metres.

Weekday evening peak: 2.5 multiplied by 12.0 equals 30 two-way vehicle trips in the hour. Split evenly, that is 15 arrivals and 15 departures, or one vehicle every two minutes in each direction. Weekday morning peak: 2.5 multiplied by 4.0 equals 10 two-way trips. Saturday midday: 2.5 multiplied by 14.0 equals 35 two-way trips.

Thirty movements in the evening peak hour is the number that decides whether the application needs a Transport Statement or a fuller Transport Assessment. On most networks a figure of this order supports a statement, because it is unlikely to change junction performance materially. On a constrained junction already operating near capacity, the same figure can justify modelling.

One point of vocabulary that matters for a shop of this size. A shop no larger than 280 m² selling mostly essential goods, including food, and at least a kilometre from another similar shop, sits in Class F2 rather than Class E under the Use Classes Order 1987 as amended in 2020. That classification changes what future changes of use are possible, and it is worth confirming before the description of development is settled.

Pass-by and diverted trips

Not every vehicle arriving at a convenience store is new traffic on the network. Some drivers were passing anyway and stop on their way home, which is a pass-by trip. Some drivers change route slightly to call in, which is a diverted trip. Only the remainder are new.

For the example, assume a pass-by proportion of 40 per cent is argued and accepted. The net new evening peak trips become 30 multiplied by 0.6, so 18 two-way movements, and that is the figure used in any junction assessment.

Be careful here. Pass-by percentages are the most heavily challenged element of a small retail transport statement, because they reduce the assessed impact and the applicant chooses them. The discount needs a basis: the classified traffic flow on the frontage road, the store's catchment, and ideally a pass-by proportion recorded at a comparable surveyed site rather than a figure asserted in a paragraph. If you cannot evidence it, present the gross figure and argue acceptability on that.

Presenting the selection so it can be audited

Give the reader everything needed to repeat the query. That means the database and version used, the date of the query, every filter applied with its values, the count of sites returned, a list of the retained sites by reference, and the rates with their ranges. Put the full output in an appendix. A transport statement that reports a rate without the filter set is not checkable, and an uncheckable rate gets replaced by the highway authority's own.

Sensitivity testing earns credit for very little effort. Run the same query with the floorspace band widened, and with the region filter removed, then report how much the evening peak rate moves. If the answer barely changes, you have shown the result is not an artefact of the filtering.

Where a highway officer will push back

Four challenges come up repeatedly: that the sample is too small, that the area type does not match, that the pass-by discount is unevidenced, and that parking demand has been assessed from trip rates rather than from observed accumulation. The last one is worth separating out, because peak parking accumulation and peak trip generation are different quantities measured differently, which is the reason a Parking Survey exists as its own piece of evidence. The method statement for that survey deserves the same transparency as the trip rate query, as set out in the guide to writing a parking survey method statement.

Servicing is the quiet one. A small store typically takes several deliveries a week, and a highway officer will ask where the vehicle stands, how long for, and whether it can enter and leave in forward gear. Answer it with a swept path drawing rather than a sentence.

Enter your postcode on PlanScope to see whether your scheme needs a transport statement, a parking survey or both.

This is general guidance, not legal advice; check the position with your local highway authority or a qualified transport planner before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.