Case Studies

The Toilets Down Under

McNickle, Donald (1998). "Queueing for Toilets," OR Insight, 11(2), 2-5.

The New Zealand Building Codes dictate the minimum number of "sanitary fixtures" in a building. This number is based on the number of people in the building and the type of facility. The code was extremely old and very inconsistent when the New Zealand Consultancy Services was contracted to use queueing theory to revise the tables in the building code.

For three weeks, data about wait times were collected from thirteen different types of buildings including office buildings, schools, theatres, swimming pools, and shopping plazas. Arrival times for the stalls were measured by cutting a pair of infrared beams, and occupancy times were measured by magnetic switches on doors or infrared beams. Gender ratios, total building occupancy, and average peak rates were considered in the model.

A spreadsheet simulation was used to develop new building codes to ensure that ninety percent of the people would wait one minute or less for a "sanitary fixture." Although the spreadsheet simulation was only intended to produce starting values for a more complex simulation model, the spreadsheet model was extended and modified because of its ease of use, reliability, and accuracy.

As expected, the old building codes provided more than enough "sanitary fixtures" for males to achieve the one minute wait goal, but not enough for females to achieve the same goal, especially in theatres, cinemas, and sports arenas. The new building codes corrected this and saved an estimated $82,000,000. These savings were largely due to a reduction in the amount of commercial office space allocated to "sanitary fixtures."

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