A 60 unit site crossing the transport assessment threshold

This transport assessment threshold example is illustrative rather than a real scheme. A developer promotes 60 dwellings on an allocated edge-of-settlement site with a single point of access onto a B road. At 48 units the application would have needed a Transport Statement. At 60 it needed a full Transport Assessment, because the authority's validation list sets the trigger at 50 dwellings, and those extra 12 houses added junction modelling, a walking and cycling audit and several months to the programme.

That arithmetic is worth doing before the layout is fixed, not after.

Why 60 units changed the evidence

The jump is not about traffic volume. It is about what the highway authority is entitled to ask for, and the answer changes once you cross the local threshold.

A Transport Statement describes the site, the access, the trip generation and the sustainable travel options. A Transport Assessment models the network. That means junction capacity modelling at agreed locations, an assessment year beyond opening, committed development added to the baseline, a walking and cycling audit, and mitigation designed to a standard the authority will adopt.

The thresholds PlanScope uses to flag a Transport Assessment are 50 or more dwellings or 2,500 m² or more of floorspace. Councils' own validation lists and the national policy position take precedence, and they differ, so read the list for the authority you are applying to. Some set the figure at 80 dwellings, some at 50, some express it as a trip-based trigger.

At 60 units the scheme was also major development, so the determination period was 13 weeks rather than eight, and that changed how the consultation sequencing worked. Active Travel England was not a statutory consultee at this scale, because its thresholds sit higher.

Scoping it with the highway authority first

The single most useful thing the project team did was send a two-page scoping note before any survey was commissioned, and get a written reply.

The note asked for agreement on seven things.

1. The study area and the specific junctions to be modelled.
2. The survey dates and times, with neutral months proposed and August and December excluded.
3. The assessment years: opening year and opening plus five.
4. The growth factors and the source for them.
5. The trip rate selection approach and the database sites to be used.
6. The committed developments to be included in the future baseline.
7. The mode share assumptions and whether a sustainable travel discount would be accepted.

The highway authority agreed six and refused one. It would not accept a mode share discount without a travel plan secured by obligation, which is a reasonable position and one it was always going to take. Knowing that at the start meant the travel plan was drafted alongside the assessment rather than bolted on at committee stage.

Scoping agreement is not binding on the authority. It is still the difference between one round of comments and three. The same lesson came out of our walkthrough of a school expansion and the transport evidence.

Trip generation and the database behind it

Trip rates came from the TRICS database, filtered to privately owned houses on edge-of-settlement sites in comparable authorities, excluding town centre and London sites. The filter criteria were listed in an appendix with the site references, which is the part that makes a trip rate defensible.

Two points decided the figure.

The first was the exclusion of sites with unusually high public transport accessibility, which would have understated vehicle trips. The second was the decision to present both arrivals and departures for the morning and evening peak hours separately rather than as a two-way total, because the junction model needed turning movements and a combined figure hides the directional split that actually loads the critical arm.

The assessment reported trip generation as a range with a central case, and used the upper end for the capacity modelling. An authority reviewing a Transport Assessment will test the inputs, and a model run on optimistic trip rates invites a request to run it again on different ones. Running the conservative case first removes that exchange.

Multi-modal trip generation was included because the walking and cycling case depends on it. Vehicle trips alone tell the officer nothing about whether the footway network can absorb the pedestrian movements the scheme creates.

Junction capacity and the mitigation offered

Three junctions were modelled: the site access, the nearest priority junction on the B road, and a mini-roundabout 600 metres away that was already congested.

The mini-roundabout is the interesting one, because it fails in the future baseline before the development is added. That is a common and awkward finding. The argument the assessment made was that the scheme's residual impact on an already constrained junction was marginal, and that the national policy test requires an unacceptable impact on highway safety or severe residual cumulative impacts before permission is refused on highway grounds.

Mitigation offered was proportionate rather than wholesale: a right turn lane and visibility improvement at the B road junction, a widened footway link towards the settlement, and a contribution towards a signal upgrade scheme the authority already had on its programme for the mini-roundabout. Offering a contribution to a scheme that exists is far easier to secure than designing a new one.

What the team did not do was propose a junction improvement the authority had not asked for. Unsolicited mitigation invites a request to design it properly, at your cost, and then to be criticised for the design.

The walking and cycling audit

The audit walked the routes rather than measuring them on a map, and that distinction produced the findings that mattered. A 900 metre route to the primary school looked acceptable at desk level and included a 40 metre section with no footway on either side, a dropped kerb with a 60 mm upstand and an uncontrolled crossing of the B road on a bend.

Audits written from aerial imagery miss all three. The audit was structured against the design criteria in LTN 1/20 for cycling and Manual for Streets principles for walking, and it scored each link for coherence, directness, safety, comfort and attractiveness.

Two of the three defects were fixed by the scheme's own works. The crossing was not, and the assessment said so plainly, with a costed option for a refuge island offered as a contribution. Identifying a defect you are not fixing and explaining why is more persuasive than an audit that finds nothing.

The travel plan and how it was monitored

A full Travel Plan was secured by section 106 obligation rather than condition, because the authority wanted the monitoring and the default payment to be enforceable against successors in title.

The plan committed to a travel plan coordinator, a welcome pack for each household, cycle parking to the local standard, a bus stop upgrade within walking distance, and household travel surveys at years one, three and five after first occupation. Targets were expressed as mode share shifts against the surveyed baseline, with a monitoring fee and a default contribution payable if targets were missed by a stated margin.

The default payment is the part developers resist and the part that makes the mode share discount acceptable to the highway authority. If you want credit for sustainable travel in the capacity modelling, something has to carry the risk that it does not materialise.

Conditions and the section 278 agreement

The highway works in the public highway were delivered under a section 278 agreement under the Highways Act 1980, with the estate roads offered for adoption under section 38. Both were negotiated in parallel with the planning application, which is the only way the dates work.

Conditions on the permission covered the access to be constructed before first occupation, visibility splays to be provided and kept clear, a construction traffic management plan to be approved before work starts, cycle and refuse storage details, and a restriction on occupation until the footway link was complete.

A Construction Traffic Management Plan was conditioned rather than submitted with the application, which is normal and sensible, because the contractor is not appointed at application stage. Draft the heads of terms for it anyway, so the condition wording matches what is deliverable.

The section 278 bond was the item that nearly delayed a start on site. Agree the bond value and the form of security early, because the highway authority's legal team works to its own timetable and the construction programme does not interest it.

Enter your postcode on PlanScope to see which reports a scheme of this scale is likely to need. This is general guidance, not legal advice, and the scheme above is illustrative. Check the position with your local planning authority or a qualified professional before relying on it.

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