Short answer. A parking occupancy study measures how full a facility gets by counting occupied spaces at fixed intervals across the day, typically every 15 to 60 minutes. Adding licence-plate matching between sweeps produces duration and turnover. Together, occupancy, duration and turnover distinguish a pricing and enforcement problem from a genuine capacity shortage — the same peak occupancy figure can mean either.
The three measurements
Occupancy is how many spaces are in use at a moment in time, usually expressed as a percentage of inventory. Duration is how long each vehicle stays. Turnover is how many different vehicles a space serves over the study period. A study that measures only occupancy — which many do — cannot distinguish between the two most common parking problems, and those two problems have opposite solutions.
Consider a lot at 92% peak occupancy. If average duration is 90 minutes, the lot is turning over healthily and full because demand genuinely matches supply at the peak: a capacity or shared-parking conversation. If average duration is eight hours, the same lot is full because long-stay users have occupied it, and short-stay visitors cannot get in: a pricing, time-limit and enforcement conversation. Building more spaces in the second case is an expensive way to solve nothing.
Choosing a sweep interval
| Sweep interval | Suits | Trade-off |
|---|---|---|
| 15 minutes | High-turnover kerbside, drop-off zones, fast food | Highest effort; the only way to catch short stays |
| 30 minutes | Retail lots, mixed-use, town centre kerbside | Good general-purpose balance |
| 60 minutes | Commuter lots, employee parking, residential permit areas | Misses stays shorter than an hour entirely |
The rule of thumb: the interval must be shorter than the shortest stay you care about measuring. If ten-minute pick-ups matter to the question, hourly sweeps will report them as an empty space and you will conclude the kerb is underused when it is churning constantly.
Study period and days
Cover the full operating day plus a margin at both ends — for most urban sites that means roughly 07:00 to 19:00, extended into the evening where there is restaurant, entertainment or residential demand. Truncating the study at 17:00 in a mixed-use district misses a second evening peak that may be the binding constraint.
On days: one representative weekday plus one Saturday is the practical minimum. Add days where the site has strong weekly or seasonal variation — retail before a holiday, campuses in and out of term, stadiums on event and non-event days, places of worship on their peak day. Two well-chosen days beat five poorly chosen ones.
Measuring duration and turnover
Duration is derived by recording a partial licence plate for each occupied space on every sweep and matching between sweeps. A vehicle appearing in three consecutive 30-minute sweeps and absent on the fourth was present for somewhere between one and two hours; across a full day and a whole lot, this resolves into a reliable duration distribution.
Two practical points. Report duration as a distribution with bands, not as an average — an average of three hours could be a uniform three hours or a bimodal mix of 20-minute visitors and nine-hour commuters, and only the distribution tells you which. And keep the output anonymous: plate observations exist to match a vehicle between sweeps and should be reduced to duration bands in the deliverable, which matters when the study will be published in a public meeting packet.
The 85% rule
A parking facility is conventionally treated as practically full at about 85% occupancy. Above that threshold, drivers can no longer find a space by observation and start circulating — which adds traffic, adds emissions, and creates a perception of shortage while empty bays technically remain. The last 15% of spaces are, in effect, the search margin that makes the facility usable.
The practical consequence is that a facility peaking at 88% should be treated as at capacity, not as having 12% headroom. Reports that describe such a facility as underused are misreading their own data.
Reading the results
- Peak occupancy and the hour it occurs — the headline, but not the finding
- Duration distribution in bands, which identifies whether long-stay users are displacing short-stay demand
- Turnover per space per day, low values indicating long-stay dominance
- Occupancy by zone or block, since aggregate figures routinely hide a full front row and an empty far corner
- On-street against off-street split, where free kerbside is often saturated while a paid lot 100 metres away sits half empty
- Violation and misuse rates, particularly in accessible and loading bays
- Duration against the posted time limit, which measures whether enforcement is having any effect
Common errors
- Sweep interval longer than the stays being studied, which systematically erases short-stay demand
- Aggregating a whole district into one occupancy figure and concluding there is capacity, when the desirable blocks are full
- Surveying one weekday and generalising to a site with strong weekend or seasonal patterns
- Counting only occupancy and then recommending construction, with no duration data to justify it
- Ignoring illegal and double parking, which is unmet demand and belongs in the total
- Treating an 88% peak as spare capacity rather than as effectively full
- Inventory taken from the striping plan rather than verified on site, where bays have been lost to plant, skips or damage
Frequently asked questions
How often should parking occupancy be counted?
Every 15 to 30 minutes for high-turnover kerbside and retail, and hourly for commuter or employee lots. The interval must be shorter than the shortest stay you need to measure.
How is parking turnover calculated?
Divide the total number of distinct vehicles observed in a space over the study period by one — in practice, count unique plate observations per space per day. It is derived from the same plate matching used for duration.
What does 85% parking occupancy mean?
It is the conventional practical-capacity threshold. Beyond about 85%, drivers cannot find spaces by observation and begin circulating, so the facility behaves as full even though some bays are empty.
Do parking studies collect personal data?
They should not. Plate observations are used only to match a vehicle between sweeps and should be reduced to anonymous duration bands in the deliverable, with no plate-level data published.
Can a parking study justify reducing required parking?
Yes, and this is now a common use. Measured peak occupancy, duration and turnover at comparable sites is the standard evidence for a parking reduction or shared-parking agreement where a zoning minimum exceeds observed demand.
The service behind this guide
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Occupancy, duration and turnover by bay and by block, on-street against off-street, with the real peak occupancy hour rather than an assumed one.
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