Air quality assessment for a drive-through unit

A drive through air quality assessment differs from an ordinary development air quality assessment in one respect that drives everything else: the source is a line of stationary vehicles with engines running, not a stream of traffic on a link. Standard dispersion modelling of road traffic uses emission factors that vary with speed, and those curves are built for moving vehicles. Drop the speed to zero and the model is being asked a question it was not designed to answer.

That is the technical core. The planning argument around it is usually about queue length, whether the queue reaches the public highway, and which neighbours are close enough to matter.

Why a drive-through attracts the objection

Three things converge. Idling engines concentrated in one place, a queue that people can see and photograph, and a use that generates strong opinions before anyone reads the assessment.

Objections typically bundle air quality with noise from the order point speaker, odour from the kitchen extract, litter and late-night traffic. Those are four separate assessments with four separate methodologies, and answering them as one produces a document that satisfies none of the consultees. Keep them apart, and cross-reference.

The odour and kitchen extraction case is a distinct piece of work. Extract discharge height, termination, filtration and odour control belong in a Ventilation and Extraction Statement rather than in the air quality chapter, and environmental health will read them separately.

Deciding whether an assessment is needed at all

Not every drive-through needs full dispersion modelling. The screening step comes first, and it has two limbs.

The first limb is exposure. Is the site within or close to an air quality management area, and are there sensitive receptors within a meaningful distance of the queue, the car park and the access?

The second limb is traffic change. The planning guidance published jointly by the air quality institute and the environmental protection body sets out indicative criteria for when a development's traffic change warrants assessment, expressed as changes in annual average daily traffic and differentiated between light and heavy vehicles, with lower thresholds inside an air quality management area than outside it. Take the current figures from the guidance itself rather than from a previous report, because they have been revised.

The screening exercise still needs writing up. A short screening note that states the criteria, the traffic figures from the transport work and the conclusion is often all an environmental health officer needs, and it costs a fraction of a modelled assessment. Where the site sits inside an air quality management area, assume modelling will be required.

Modelling a queue rather than a road

There is no single accepted method, and an assessment that pretends otherwise is asking for a consultee objection. What a defensible assessment does is state the approach, justify it and test its sensitivity.

The practical options:

• Represent the queue as a series of short, very low speed road links, taking emission factors from the national emissions factors toolkit at the lowest speed it supports, and acknowledging that this extrapolates the curve.
• Apply an idling emission rate per vehicle and distribute it over the queue geometry as an area or volume source.
• Model the queue as a line source with a residence time derived from the transport assessment's throughput figures.

Whatever the approach, the inputs that matter are queue length, dwell time per vehicle at each position, and the peak period profile. Those come from the transport work, not from the air quality consultant, so the two disciplines have to exchange figures before either writes anything. A Travel Plan and the trip generation behind it are part of the same evidence chain.

Run a sensitivity case on dwell time. If the conclusion holds at double the assumed queue duration, say so. That single sentence disposes of most of the objections about optimistic assumptions.

Which receptors actually count

This is where assessments are won and lost, and where objectors and applicants talk past each other.

The air quality objectives apply where there is relevant exposure, judged against the averaging period of the objective. An annual mean objective applies at locations where members of the public are likely to be regularly present over that period, which means residential facades, gardens in regular use, schools, nurseries and care homes. It does not apply in the middle of a retail park car park, at a petrol forecourt, or on a pavement where people pass through.

So the receptor schedule needs to be reasoned rather than generous. List each receptor, its use, its distance from the queue and the access, its height, and the objective that applies to it. Where a receptor is included or excluded, say why in one line.

Expect to be challenged on two categories. Upper-floor flats above neighbouring units, which are often missed. And the outdoor seating area of the unit itself, which is not a relevant receptor for the annual mean but is a reasonable point for an officer to raise on amenity grounds.

Baseline matters as much as the modelled increment. Use local authority monitoring data where it exists, including diffusion tube results from the nearest sites, rather than relying only on national background maps. An assessment that ignores the council's own monitoring invites the obvious question.

Layout changes that cut idling time

Mitigation in the layout beats mitigation in the report. The design moves that actually reduce emissions:

1. Provide enough stacking capacity on site that the queue never reaches the public highway, sized on the peak period rather than the average.
2. Split ordering from collection, with separate windows, so the slowest transaction does not hold the whole queue.
3. Use two parallel order lanes merging after the order point, which shortens the time each vehicle spends stationary.
4. Provide marked bays for orders that take longer, so a waiting vehicle can pull out of the queue.
5. Orient the queue away from the nearest sensitive facade and put the building or a solid structure between them.
6. Design the exit so departing vehicles are not held at an internal give-way.

Queue length is a highway matter too, and the highway authority will be looking at whether stacking spills onto the access road. The same drawing usually answers both consultees, which is a good reason to resolve it early.

No-idling signage is worth including and worth being honest about. It changes behaviour at the margin. It is not mitigation you can quantify, and presenting it as though it were undermines the rest of the assessment.

Charging provision and what it does for the case

Electric vehicle charge points are required for new non-residential buildings with parking under the Building Regulations, with the number of points and cable routes set by the number of spaces. Meeting that requirement is a baseline rather than a mitigation measure, so do not claim credit for compliance.

Where provision goes beyond the requirement, it can be presented as a benefit, though the emissions effect on the site itself is modest because charging vehicles are not the ones queueing. The stronger argument is about the future fleet: a queue of vehicles in 2035 is a different source term from a queue today, and an assessment that presents current fleet emissions as the permanent condition overstates the effect across the development's life.

Say that explicitly and cite the emission factor projections rather than leaving it as an assertion.

Conditions, monitoring and what happens after opening

Expect some combination of a queue management plan, restricted opening hours, controlled delivery hours, an approved lighting and signage scheme and a travel plan secured by condition or obligation. Occasionally a condition requiring post-opening monitoring, most often diffusion tubes at agreed locations for a defined period, with a mechanism for what happens if results exceed a trigger.

Delivery movements are a separate and often underestimated part of the picture, particularly where the unit takes early morning deliveries next to housing. The issues are set out in night deliveries and servicing noise, and the delivery schedule you agree for noise should be the one used in the air quality traffic figures.

A permission does not settle complaints for good. The relationship between a consent and later enforcement is explained in statutory nuisance after planning permission is granted, and an operator who relies on a condition as a defence is often surprised.

Councils usually expect an Air Quality Assessment to be prepared by a suitably qualified practitioner, and professional review before submission is strongly advised. 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.