The Data Behind the Wheels Versus Doors Debate

In case you haven’t been following the latest debates on social media, here’s a brief refresher. On March 4th, 2022, Twitter user @NewYorkNixon put out a poll that asked people to vote whether they think there are more doors or wheels in the world. While this question reminds me of *hits blunt* memes with a consulting-like brain teaser twist, the Internet collectively decided to make the poll go viral.

Surprisingly, the results were pretty balanced, with a slight majority in favor of wheels over doors. Still, over 100k Twitter users thought that there were more doors than wheels.

Intrigued, I decided to test my personal intuition by gathering some data and approximating the true wheel and door population numbers.

Disclaimer: This analysis is not to be taken too seriously.

Definitions

It is crucial to unambiguously define both terms to avoid falling prey to philosophical debates about what constitutes a wheel or door. The Oxford Languages dictionary defines both terms as follows:

Wheel (/(h)wēl/)

a circular object that revolves on an axle and is fixed below a vehicle or other object to enable it to move easily over the ground.

Note that, using this definition, objects like gears do not qualify as wheels. Any wheel also has to be fixed below the object it is moving over the ground.

Door (/dôr/)

a hinged, sliding, or revolving barrier at the entrance to a building, room, or vehicle, or in the framework of a cupboard.

Party in the U.S.A.

Trying to find data for each and every country would probably prove to be an arduous (and fruitless) undertaking. So instead, this little investigation is going to focus on the U.S. only since there’s plenty of publicly available data (and I’d also like to avoid being a complete buzzkill). The goal of this analysis is to determine approximations for the following KPIs:

Using these, we settle the debate once and for all (ish).

Abstracting, Slow and Fast

Part of the reason this poll gained such popularity, in my opinion, is that human brains tend to oversimplify very abstract questions to save resources. Like our computers, humans have limited computational resources and constantly need to prioritize how to best put them to use. Thus, I would be tempted to bet that a significant portion of the participating users based their decision on the fact that a car usually has four wheels (but also four doors). To avoid oversimplifying, let’s try to approach this question by industry.

While this list is definitely not exhaustive, it should be sufficient to get a rough idea.

Transportation 🚘

Transportation is probably the first industry most people think of when debating whether there are more wheels or doors. Luckily, the Bureau of Transportation Statistics publishes an annual report on the number of vehicles in the United States. [1] However, since they did not have the foresight to add the average number of doors and wheels per vehicle, I imputed estimates using popular vehicles in each category as a proxy. For example, to determine the average number of wheels and doors for passenger planes, I used data for the Boeing 737–800.

In addition to the data above, I looked into three other transportation-related categories:

Bicycles

According to an annual survey conducted by Ipsos, a little over 52 million people participated in bicycling in the U.S. in 2020. [3] Ignoring the fact that smaller parts of certain bikes might also be classified as wheels and assuming that 80% of participants actually own a bike and don’t rent it, we arrive at an additional 84.4 million wheels.

Shared E-Scooters

Electric scooters are a relatively new contributor to the number of wheels in the United States. The Bureau of Transportation Statistics’ “Bikeshare (Docked and Dockless) and E-scooter System” report allows for a rough estimation of the total number of scooters available. [2] Assuming that all providers stock their stations with 20 scooters, on average, there are a little over 110k scooters in the United States. Therefore, we can add another 220k wheels to the total.

Wheelchairs

Based on Grand View Research’s “North America Wheelchair Market Size, Share & Trends Analysis” report, there were around 3.3 million wheelchair users in the U.S. in 2016. [4] So, again, assuming that about 80% of wheelchair users personally own one, we arrive at an additional 5.28 million wheels.

Real Estate 🏢

If doors are indeed the winner, then what better place to find them than in buildings. While exact data is hard to come by, we can try to approximate the number of doors by breaking this category down into residential and commercial buildings to limit the complexity of this analysis.

Residential buildings

As many Twitter users noted, buildings come with all sorts of doors. Front doors, backdoors, closet doors, and many more. According to the Census Bureau, there were around 140 million housing units in 2020. [6] Combined with the fact that the total U.S. home square footage in 2020 was 250 billion [6], we get an average housing unit size of around 1770 square feet. Assuming that these units have 4 doors (front door + bathroom door + bedroom door + closet door), on average, there are approximately 563 million doors in residential units.

Commercial buildings

Since this analysis does not distinguish between, for example, hotels and warehouses, let’s just assume that commercial buildings have 15% of the number of doors that residential units have. Based on the U.S. Energy Information Administration’s “Commercial Buildings Energy Consumption Survey,” there were just shy of 100 billion square feet of commercial real estate in the U.S. in 2018. [7] Applying the assumption above, that’d mean that there are around 34 million doors in commercial buildings across the United States.

Manufacturing 🔩

Based on the definitions outlined at the beginning of this article, most wheel candidates in manufacturing do not qualify. Pulleys, gears, or even water wheels could be considered wheels under different circumstances. However, they are excluded from this analysis as they are not necessarily installed below an object and usually remain in place and do not move.

Still, manufacturing remains an important industry in this comparison, primarily due to the abundance of pallet jacks and forklifts used in the nation’s many fulfillment and distribution centers. Based on Allied Market Research’s “Forklift Truck Market by Power Source” report from March 2021, there are around 850,000 forklifts in the United States. [8] One forklift usually has four wheels and zero doors, so that’s an additional 3.4 million wheels In addition, assuming that the U.S. makes up a conservative 20% of the global pallet jack market of $38.2 bln [9] and an average price of $500 apiece (as some are manual and others electric), that’d mean an additional 61 million wheels.

Furniture and Appliances 🗄️

Applying the definition of doors from Oxford Languages, kitchen cabinet doors (and, therefore, fridge doors, too) qualify as doors for the purposes of this analysis. Assuming 90% of residential units and 5% of the equivalent commercial units have at least one cupboard with two doors and a fridge with one, that adds an additional 389 million doors.

Entertainment 🎢

While roller coasters are more fun than shopping carts, there are only around 760 of them in the United States [10]— shopping carts, on the other hand, are everywhere. According to a Nielsen report, there were a little over 38k supermarkets in the U.S. in 2018. [11] However, it’s essential to recognize that these vary wildly in size.

Assuming an average of 600 shopping carts across all categories (loosely based on this Reddit thread), we arrive at an additional 92 million wheels. The true number of shopping cart wheels is probably even higher when considering all types of stores that offer shopping carts to customers.

The Verdict

Putting it all together, we get the following wheel and door distributions by industry:

Finally, let’s look at the wheels- and doors-per-capita ratios and determine whether there are more wheels or doors per person in the United States:

Aaaaand there it is: doors win. Of course, as mentioned earlier, these numbers are to be taken with a grain (or two) of salt. Different definitions will lead to vastly different results, and adding more industries will likely also lead to a significant shift in wheel-to-door dynamics. For example, adding the U.S. military would be interesting since their large stock of vehicles and other objects is more or less publicly available.

If you’d like to extend this analysis, perform it for a different country, or modify the existing assumptions, feel free to duplicate my Deepnote notebook. I’ve also uploaded the data used for this analysis.

Sources:

[1] “Number of U.S. Aircraft, Vehicles, Vessels, and Other Conveyance.” Bureau of Transportation Statistics. Accessed March 10, 2022. https://www.bts.gov/content/number-us-aircraft-vehicles-vessels-and-other-conveyances.

[2] “Bikeshare (Docked and Dockless) and E-scooter Systems by Year and City Served.” Bureau of Transportation Statistics. Accessed March 10, 2022. https://data.bts.gov/Bicycles-and-Pedestrians/Bikeshare-Docked-and-Dockless-and-E-scooter-System/cqdc-cm7d.

[3] “2021 Outdoor Participation Report.” Outdoor Foundation. Accessed March 10, 2022. https://outdoorindustry.org/wp-content/uploads/2015/03/2021-Outdoor-Participation-Trends-Report.pdf.

[4] “North America Wheelchair Market Size, Share & Trends Analysis Report.” Grand View Research. Accessed March 10, 2022. https://www.grandviewresearch.com/industry-analysis/wheelchair-market.

[5] “Connected Home Roadmap (2019).” Continental Automated Buildings Association (CABA). Accessed March 10, 2022. https://www.grandviewresearch.com/industry-analysis/wheelchair-market.

[6] “Number of U.S. housing units 1975–2020.” US Census Bureau. Accessed March 10, 2022. https://www.statista.com/statistics/240267/number-of-housing-units-in-the-united-states.

[7] “2018 Commercial Buildings Energy Consumption Survey.” U.S. Energy Information Administration. Accessed March 11, 2022. https://www.eia.gov/consumption/commercial.

[8] “Forklift Truck Market by Power Source.” Allied Market Research. Accessed March 12, 2022. https://www.alliedmarketresearch.com/forklift-truck-market.

[9] “Pallet Trucks Market — Analysis, Outlook, Growth, Trends, Forecasts.” Future Market Insights. Accessed March 12, 2022. https://www.futuremarketinsights.com/reports/pallet-trucks-market.

[10] “12 U.S. Roller Coasters You Have to Ride.” tripsavvy. Accessed March 12, 2022. https://www.tripsavvy.com/must-ride-roller-coasters-in-the-united-states-4685000.

[11] “Progressive Grocer Magazine April 2019.” Nielsen. Accessed March 12, 2022. https://www.statista.com/statistics/240892/number-of-us-supermarket-stores-by-format.