Worked parking stress example using the Lambeth method

A parking stress calculation example is worth more than a description of the method, because the arguments with a highway officer are always about the inputs. Capacity is where the disagreement starts. Two surveyors measuring the same street can produce counts twenty per cent apart depending on how they treat dropped kerbs, junction protection and the awkward gap outside the chip shop.

Everything below is illustrative. The lengths and counts are invented for the arithmetic, and a real survey needs measured lengths and counted vehicles on the actual streets.

Defining the survey area

The convention derived from the Lambeth survey methodology is to survey every street within a reasonable walking distance of the site, commonly taken as 200 metres measured along walkable routes rather than as a radius.

Walking distance matters. A street 150 metres away as the crow flies but 400 metres on foot because a railway sits between them is not part of the same parking catchment, and including it dilutes the result in your favour, which a highway officer will notice.

Draw the area on a plan and list the streets with the extent surveyed, from junction to junction. Where you exclude a street inside the nominal distance, say why. Where a controlled parking zone boundary crosses the area, show it, because restricted and unrestricted kerb behave differently overnight.

Counting capacity and occupancy

Capacity is the length of kerb where a car can lawfully park overnight, converted to spaces.

Measure the total kerb length on both sides of each street, then deduct:

• Vehicle crossovers and dropped kerbs serving driveways.
• Junction protection, conventionally taken as 10 metres either side of a junction.
• Bus stops, taxi ranks and loading bays.
• Single and double yellow lines that apply at the survey time.
• Permit bays and pay and display bays, counted separately where the development would not have access to them.
• Any length obstructed by street furniture, build outs or pedestrian crossings.

Convert the remaining unrestricted length to spaces using a standard car length. Five metres per vehicle is the figure most often applied, and bays already marked out are counted as marked rather than recalculated.

Occupancy is a straight count of parked vehicles within the surveyed extent at the survey time, including vehicles parked illegally, because they occupy kerb whether or not they should.

Survey nights and time periods

Residential parking stress is measured when residents are at home and their cars are on the street, so the survey window runs in the small hours, conventionally between 00:30 and 05:30.

Two surveys is the minimum: one weekday night and one weekend night. Avoid school holidays, public holidays, the fortnight around Christmas, and any week with unusual local conditions such as a street closure or a major event. Record the weather, because heavy rain changes where people park.

State the surveyor, the dates, the start and finish times for each street, and the direction of travel. A highway officer reading a survey with no times cannot judge whether the count captured the overnight peak.

Setting out the illustrative calculation

Take an illustrative survey area of six street sections with a combined kerb length of 2,100 metres across both sides.

Deductions come to 860 metres: 420 metres of crossovers, 200 metres of junction protection across ten junction arms, 90 metres of bus stop and loading bay, and 150 metres of yellow line in force overnight.

Unrestricted kerb is therefore 1,240 metres. At five metres per vehicle that gives a capacity of 248 spaces.

Counted occupancy on the weekday night is 196 vehicles, and on the weekend night 210.

Stress is occupancy divided by capacity, expressed as a percentage. So the illustrative area sits at 79 and 85 per cent, and the weekend figure is the one that will be quoted against you.

Interpreting the stress percentage

The working threshold used by most highway authorities is 85 per cent. Below it, the street network is treated as having spare capacity. At or above it, parking is regarded as effectively full, because the remaining spaces are scattered and a driver cannot rely on finding one near their home.

The threshold is a convention rather than a national standard, so check whether the authority applies a different figure or a different interpretation. Some apply it street by street rather than across the area, which produces a harsher result where one street is saturated and its neighbours are not.

Report both survey nights and both interpretations. An assessment that reports only the area average, when the street outside the site is at 96 per cent, invites the officer to redo the arithmetic and distrust the rest.

Adding the development's demand

The next step is to add the demand the development would generate and test the result against the threshold again.

Take an illustrative conversion of eight flats with a predicted car ownership rate producing five vehicles. Adding five to the weekend occupancy of 210 gives 215 against a capacity of 248, which is 87 per cent. The scheme has taken an area at the threshold and pushed it over.

Derive the ownership rate from census based data for the area at the smallest geography available, or from a trip and ownership database where the authority accepts it, and state the source. A rate borrowed from a different ward with different public transport accessibility will be challenged first.

Then consider the mitigation the authority will expect. Permit free agreements secured by a legal agreement, cycle parking above the minimum standard, a car club bay, or in some cases a reduction in unit numbers. Where a permit free restriction is proposed, confirm that the controlled parking zone can actually deliver it, because the restriction relies on the permit scheme existing.

Presenting results to a highway officer

Submit the raw material as well as the conclusion.

1. A plan of the survey area with each street section labelled and the extent surveyed marked.
2. A capacity schedule per street section showing gross length, each deduction and the resulting space count.
3. The completed count sheets for each survey night, by street and time.
4. Photographs of any contested length, such as an unmarked crossover or a disputed junction protection zone.
5. A summary table of capacity, occupancy and stress by street and for the area.
6. The development demand calculation with its source.
7. A short method statement covering dates, times, weather, surveyor and any departure from the standard method.

The method statement is the document officers read first, and the structure in our note on writing a parking survey method statement applies directly. Submit the whole thing as a Parking Survey and summarise the conclusion in the Transport Statement, rather than duplicating the figures in both, because inconsistent numbers between two documents undermine both.

Where the survey must be repeated

Surveys age. Expect to repeat the work where any of these apply.

The survey is more than about a year old at the point of determination. A controlled parking zone has been introduced, extended or changed its hours. Kerbside has been altered by a highway scheme, new crossovers or cycle hangars. A large development nearby has been occupied since the survey. Or the survey was carried out during a period when travel patterns were not representative.

One honest limitation to state in the report. A stress figure describes a few nights in a year, and a street at 84 per cent on two survey nights may be at 90 per cent in February. Where the result is close to the threshold, a third survey night costs little and removes the argument that the sample was too small. The Planning Statement should carry the conclusion and the limitation together rather than only the favourable number.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and confirm the survey method the highway authority expects before going out.

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