Drafted transport statements and the trip rate question

Of all the documents in a planning pack, a transport statement is the one where the limits of AI drafting show most clearly. The structure, the policy framework and the accessibility narrative can be drafted well. The transport statement trip rates cannot, because they come from a licensed trip generation database and from a series of judgements about which surveyed sites resemble yours. No amount of drafting substitutes for that extraction, and a number that appears without it is worth nothing to a highway officer.

Knowing where the line falls is useful rather than disappointing. It tells you which parts of the document you can have in an afternoon and which parts need a transport planner and a fee.

What a transport statement has to evidence

Planning Practice Guidance distinguishes a transport statement, for development with smaller transport implications, from a fuller transport assessment. Local thresholds decide which applies, so settle that at scoping. Either way the content list is similar in kind and different in depth.

• The site, its existing accesses and the character of the surrounding network
• Accessibility by walking, cycling and public transport, with distances and journey times
• The personal injury collision record for the study area, usually five years of data from the highway authority
• Access design, including visibility splays, geometry and any departure from standards
• Parking provision against the adopted standards, including cycle parking and electric vehicle charging
• Trip generation, distribution and assignment
• The effect on junctions and links, where the scale warrants it
• Servicing, refuse collection and construction traffic
• Travel plan measures and targets

Our Transport Statement page sets out when the document is required and what it covers; the fuller Transport Assessment applies above the local thresholds, and PlanScope flags it for schemes of 50 or more units or 2,500 m² or more, with the caveat that the authority's own validation list takes precedence.

Trip rates come from a licensed database

Trip generation evidence in England is normally drawn from the national trip generation database, which holds surveyed sites with measured trip rates per dwelling, per 100 m² or per other unit. Access is licensed. A consultant with a licence runs an extraction and prints the selection parameters so the result can be reproduced.

That is the step a draft cannot take, and the reason is structural rather than technical. The rates are not published reference figures to be looked up. They are the output of a filtered query over survey data, and the query itself is the evidence.

Borrowing rates from another scheme's submitted report is the common shortcut and it fails for the same reason. A highway officer asks for the selection parameters, the sample size and the rank order of the surveyed sites, and a borrowed figure has none of those attached to your site. Where a rate appears in a document with no extraction behind it, the usual outcome is a holding objection rather than a debate.

Site selection is judgement, not retrieval

Even with a licence, the output depends on choices. Which land use sub-category matches the proposal as it will be occupied. Which regions to include. What population bands around the site. Which survey date range, given how travel patterns shifted after 2020. Whether to exclude a surveyed site, and why. Whether to use the mean or to test a higher rate.

Each of those is a planning or engineering judgement that has to be defended in writing. A competent transport planner can explain why a build to rent block in a town centre should not be assessed against private flats on an edge of town site, and will say so in a sentence an officer accepts. That reasoning is the professional content of the chapter.

This is the same boundary we describe in site judgement cannot be drafted for design statements. The pattern repeats across technical reports: policy and structure generalise, site-specific judgement does not.

Distribution and assignment assumptions

Once the number of trips is settled, two further assumptions decide whether any junction looks busy. Distribution is where the trips go. Assignment is the route they take to get there.

Common evidence sources are census travel to work data for the local area, the authority's own transport model where one exists, and observed turning counts at the junctions in question. Each has limits, and the honest approach is to state which was used, show the resulting percentages on a plan, and test an alternative distribution where the result is marginal.

A draft can set out the method and the sources to be used, and leave the figures to be inserted. What it must not do is produce percentages that look plausible. PlanScope's drafts insert a to be confirmed marker wherever a site-specific fact is missing, which is deliberate: a flagged gap gets filled, while an invented number gets submitted.

Where a draft earns its place

On a transport statement, the drafted content is the half of the document that is the same on every scheme and still takes a day to write.

The policy framework, with the national transport policies in the current Framework, the relevant practice guidance, the adopted local plan transport and parking policies, and the design references such as Manual for Streets and the cycle infrastructure design guidance. The site and surroundings description. The accessibility narrative, once distances and services are supplied. The travel plan framework, with measures, a coordinator role, monitoring and targets. The appendix list and the structure the authority expects.

That is real time saved, and it is also a checklist: a draft that lists the sections an officer expects makes it obvious which ones have no evidence behind them yet. The honest summary of what to expect is in right about structure and wrong about evidence.

Junction capacity and the modelling behind it

Capacity work sits entirely outside drafting. Priority junctions and roundabouts are assessed with established capacity software, signals with signal modelling packages, and complex networks with microsimulation. The outputs are a ratio of flow to capacity, a degree of saturation, queue lengths and delay, usually for the morning and evening peak hours in a future assessment year.

The benchmark most officers work to is a ratio of flow to capacity of around 0.85, treated as practical capacity. Above it, queues lengthen quickly and the results become sensitive to small changes in the inputs, which is why the trip rate argument and the capacity argument are really the same argument.

Models need calibration against observed counts, a stated assessment year, committed development included and a sensitivity run. None of that can be produced from a description of the site, and presenting modelled outputs that were not modelled would be a serious matter rather than a shortcut. PlanScope drafts reports: it does not run capacity models, and it does not hold survey data for your site.

What the highway authority checks first

The order of reading is predictable, and it is worth writing the document to match.

1. The access: geometry, visibility splays against the design speed, and whether a departure is sought.
2. The trip rate source: the selection parameters page and the sample.
3. The collision record, and whether any pattern is addressed.
4. Parking against the adopted standards, including cycle parking.
5. Servicing and refuse, with swept path drawings where needed.
6. The capacity results, if the scale warrants them.
7. The travel plan, and whether its targets are credible.

Items one to five decide most applications. Developers often assume the capacity modelling is the centre of the document, and on a scheme of twenty houses it frequently is not.

Councils usually expect a suitably qualified author for transport evidence, and professional review before submission is strongly advised. The practical pattern that works is to draft the structure and policy, commission the extraction and any modelling, then send the whole document to a transport planner who can review and sign off the work before it goes to the authority.

Enter your postcode on PlanScope to see which transport documents your scheme is likely to need.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional 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.