Worked example: parking stress survey arithmetic

A parking stress survey calculation has two halves: how many cars the kerb can hold, and how many are on it. Divide the second by the first, express it as a percentage, and you have the figure that decides whether an authority accepts a car-free conversion or refuses it on highway grounds. The arithmetic below is illustrative throughout, using invented but realistic figures to show the method rather than to describe any real street.

The survey convention most authorities recognise is the overnight methodology developed in London, which counts parked vehicles in the small hours when residents are at home.

Measuring usable kerbside length

Walk the survey area, which is normally every street within a stated walking distance of the site, often 200 metres, and measure the kerb on both sides. Do it with a measuring wheel and a plan, marking each deduction where it occurs rather than totalling them at the end, because the officer will want to see the working street by street.

Then take out everything a car cannot legally or physically park on.

• Dropped crossings serving private driveways, measured across the full width of the crossing.
• Junction protection, typically 10 metres back from the junction on each arm.
• Double yellow lines and any other waiting restriction that applies at the survey time.
• Bus stop clearways, taxi ranks and loading bays.
• Disabled bays, which are counted separately rather than removed, since they are capacity but restricted capacity.
• Vehicle crossings to commercial premises, and any length physically obstructed.

What remains is usable unrestricted kerb. Where a street has marked bays instead, count the bays and skip the conversion entirely.

Converting length to capacity

The convention is 5.5 metres per car on unrestricted kerb. That figure accounts for the gaps real drivers leave, and using a shorter module to flatter the result is the first thing a highway officer checks.

An illustrative calculation for one street:

241 divided by 5.5 gives 43.8, which rounds down to 43 spaces. Always round down. A second street in the same survey area has 28 marked bays, giving a study area capacity of 71 spaces.

Two streets of apparently similar length can differ by 15 spaces once the deductions are made, which is why the measurement cannot be estimated from an aerial image. Record the measurement method. A survey that states capacity without showing the deductions is unauditable, and an objector who walks the street with a measuring wheel will find the dropped crossings you left in.

Counting occupancy

The overnight methodology asks for two survey nights, one midweek and one at the weekend, each with two passes between roughly half past midnight and half past five in the morning. Count every parked vehicle in the study area on each pass, including vehicles parked illegally, because they occupy space whatever their legal status.

Record illegally parked vehicles separately as well as in the total. A street with 20 per cent of its occupancy on yellow lines is telling you something the headline percentage hides.

Avoid school holidays, the fortnight around Christmas, and any week with roadworks or an event in the area. A survey taken in late August on a residential street near a university will understate demand by a wide margin, and the officer will say so.

The stress calculation

Take the highest occupancy count from the passes on each night and divide by capacity. Continuing the illustrative example with a capacity of 71 spaces:

The threshold most authorities apply is 85 per cent, the level at which a street is treated as effectively full because finding a space requires circulating. The midweek figure above sits just over it.

Now add the development. For an illustrative conversion of a house into six flats with no off-street parking, applying a car ownership assumption of 0.6 vehicles per flat gives 3.6, rounded up to four vehicles. Midweek occupancy becomes 65 against a capacity of 71, or 92 per cent.

That is the number the case officer will quote. Everything else in the survey exists to make it credible.

Notice how small the increment is. Four vehicles moved the figure by six percentage points, because the capacity denominator is small. On short residential streets the result is sensitive to a single dropped crossing, and that sensitivity cuts both ways in an argument.

Interpreting the result honestly

A result above the threshold does not end the application, and a result below it does not win it.

Where stress is already high, the arguments that work are about whether the development actually generates the assumed demand. Evidence from census travel-to-work data for the area, the presence of a controlled parking zone with a permit cap, a car club bay within walking distance, and a commitment to a car-free obligation excluding future occupiers from permits are all material. The last of those only works where a controlled parking zone exists, which is why zone boundaries matter more than the survey sometimes does.

Where stress is low, say so plainly and resist the temptation to add pages. A clear table and a short conclusion carry more weight than a long narrative.

The car ownership assumption is worth attention, because it is the input most often taken from a figure that suits the applicant. Using an area-specific figure from published census data, and stating the output area used, is harder to argue with than a national average.

Timing, validity and presentation

Surveys age. Most authorities treat a survey older than about two years as needing repeating, and any survey taken before a controlled parking zone was introduced is worthless afterwards.

Present it so it can be checked without a site visit. That means a plan of the study area with the surveyed streets marked, a table of deductions per street, the raw counts by pass with date and time, the capacity calculation, the stress percentages, and a short statement of the methodology and its source.

A Parking Survey prepared this way sits alongside a Transport Statement on most small conversions, and the two should use the same trip and ownership assumptions. Where the authority asks for a commitment rather than a calculation, a Travel Plan with a car club membership and cycle parking is often what unlocks a borderline case.

Every figure above is illustrative and exists only to show the arithmetic. Use the authority's own adopted methodology where it has one, since local standards override any general convention.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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