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Part 2 of 7 · Parking permit issuer series ~5 min read

How many permits for how many spaces

The arithmetic supports over-issuing and the arithmetic is not the whole story, because the two failure modes are not equally bad.

Key takeaways

  • Measure occupancy before deciding anything; counting is enough.
  • Peak day matters more than average day.
  • Being turned away with a valid permit is far worse than not having one.
  • Start conservative and increase the ratio on evidence.
  • Re-derive annually; working patterns move.

Measuring occupancy

How car park occupancy is measured before setting a permit ratioA vertical chain of five steps entered by a box labelled Before deciding anything, measure. Step one counts cars at ten in the morning every working day, with a side box saying it takes a person, a phone and thirty seconds. Step two continues for a month, four weeks minimum. Step three computes average and peak, both of which matter. Step four looks for any pattern such as Tuesdays or month end. Step five produces a ratio, conservative at first. A note says thirty seconds a day for a month is the entire data collection exercise.AWS ACCOUNTBefore deciding anythingmeasureCount cars10:00, every working dayHowa person, a phone,thirty secondsFor a monthfour weeks minimumAverage and peakboth matterAny pattern?Tuesdays, month endA ratioconservative at firstThirty seconds a day for a month is the entire data collection exercise.
Fig 1. How the ratio is established. The measurement is deliberately crude because a crude measurement is one that actually gets done.
  • Compute
  • App integration
  • Management
  • Analytics
  • People

Peak, not average

The same argument as the capacity forecaster in Day 121. A car park that averages seventy-one per cent occupancy and hits ninety-four per cent on Tuesdays is full on Tuesdays, and the ratio has to be set against the day people actually come in.

Hybrid working has made this sharper. A building where most people come in on Tuesday and Wednesday has a peak that is very different from its average, and a ratio derived from the average produces a car park that overflows twice a week.

The asymmetry

The relative cost of three parking outcomesA bar chart with three bars showing relative annoyance. No permit: three. Permit with a space free: zero. Permit but turned away: nine. A note says somebody who drove in on the strength of a permit has no alternative at twenty to nine.05101520~3No permit~0Permit, space free~9Permit, turned awayRelative annoyance, roughlySomebody who drove in on the strength of a permit has no alternative at 08:40.
Fig 2. The three outcomes and their relative cost. The asymmetry between not having a permit and being turned away with one is the argument for a conservative ratio.

Somebody without a permit plans around it: public transport, a different arrival time, a paid car park. Somebody with a permit who arrives to a full car park has made a decision based on it and is now looking for street parking while late for something.

Which is why the ratio should be conservative even where the occupancy data supports more. A car park that is occasionally ninety-five per cent full is working; one that turns people away weekly has broken the promise the permit represented.

Starting conservative

Days the car park was full at three different permit ratiosA stacked bar chart with three bars showing days per quarter. Two series: days full in red, and days with space in green. Year one at a ratio of one point three five: two days full and fifty-eight with space. Year two at one point five: five days full and fifty-five with space. Year three at one point six five: fourteen days full and forty-six with space. A note says year three is where it starts to hurt and year two was the right answer.020406080~60Year 1: 1.35~60Year 2: 1.50~60Year 3: 1.65Days full, per quarterDays with spaceYear three is where it starts to hurt. Year two was the right answer.
Fig 3. Three years of increasing the ratio on evidence. The point at which days-full rises sharply is the point to stop, and it is only findable by moving gradually.

Moving the ratio gradually and measuring the days-full count is a straightforward way to find the right level, and it has the useful property of being explicable: the ratio went up because there was space, and it will come back down if there is not.

It also means the ratio is a thing the organisation has evidence about, which converts an argument about fairness into a question about a number.

Re-deriving annually

Working patterns move, headcount moves, public transport changes, and a ratio set three years ago is describing a different building. Re-measuring for a month once a year is a small exercise and it prevents a ratio quietly becoming wrong.

What is not in the ratio

Accessible bays, visitor spaces, and any operational vehicles are outside the pool entirely. They are not permits, they are not allocated by the rule, and counting them in the ratio produces both a wrong ratio and an unpleasant conversation.

Next: who gets one.

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