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What Are Those Dark-Green Mailboxes That Don't Accept Mail?

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You know a mailbox when you see one. (They’re those blue hunks of metal bolted to the sidewalk with the creaky flaps that go reeeeaaaaaallllk when you pull them open.) But what about the dark-green boxes that don’t have any slots to accept mail?

Called postal relay boxes, these work as storage containers for mail carriers as they make their rounds. Carriers can replenish their bags on the go, removing the need to constantly return to the distribution center (or carry everything at once). They are most prevalent in cities where USPS workers make deliveries on foot, and the boxes are either filled by the carriers themselves or postal workers in trucks who make larger delivery runs.

Ideally, these relay boxes are put at the most convenient possible locations along carriers’ routes. A 1992 study in the American Journal of Mathematical and Management Sciences titled “Locating Postal Relay Boxes Using a Set Covering Algorithm” [PDF] details the number-crunching that goes into this. Using data from Canadian mail routes, the researchers took into account things like maximum mailbag weight (35 pounds), average mail volume (depending on day), and the number of mail carriers who can use each relay box at once. The algorithm resulted in a lower number of needed relay boxes, which cut down on cost.

You may have noticed that a green relay box that was present on your corner, say, ten years ago may no longer be there. As the Internet further reduces the need for paper mail, carrier loads have been getting lighter, accounting for fewer relay boxes. When Gothamist asked a USPS representative about the disappearing boxes, they confirmed that they were being removed “if they were no longer needed.” The rep was also tight-lipped about the very nature of the relay boxes, telling Gothamist’s Jen Carlson, “[They] are for official postal use only. Any further information regarding them is proprietary."

This reticence was likely due to security concerns. According to the Postal Inspection Service’s law enforcement guide, “relay boxes can contain large quantities of mail in gray sacks that thieves cart off looking for checks and credit cards.” It goes without saying, but please leave the relay boxes alone; they're just trying to help.

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Big Questions
What Causes Sinkholes?
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Mark Ralston/AFP/Getty Images

This week, a sinkhole opened up on the White House lawn—likely the result of excess rainfall on the "legitimate swamp" surrounding the storied building, a geologist told The New York Times. While the event had some suggesting we call for Buffy's help, sinkholes are pretty common. In the past few days alone, cavernous maws in the earth have appeared in Maryland, North Carolina, Tennessee, and of course Florida, home to more sinkholes than any other state.

Sinkholes have gulped down suburban homes, cars, and entire fields in the past. How does the ground just open up like that?

Sinkholes are a simple matter of cause and effect. Urban sinkholes may be directly traced to underground water main breaks or collapsed sewer pipelines, into which city sidewalks crumple in the absence of any structural support. In more rural areas, such catastrophes might be attributed to abandoned mine shafts or salt caverns that can't take the weight anymore. These types of sinkholes are heavily influenced by human action, but most sinkholes are unpredictable, inevitable natural occurrences.

Florida is so prone to sinkholes because it has the misfortune of being built upon a foundation of limestone—solid rock, but the kind that is easily dissolved by acidic rain or groundwater. The karst process, in which the mildly acidic water wears away at fractures in the limestone, leaves empty space where there used to be stone, and even the residue is washed away. Any loose soil, grass, or—for example—luxury condominiums perched atop the hole in the ground aren't left with much support. Just as a house built on a weak foundation is more likely to collapse, the same is true of the ground itself. Gravity eventually takes its toll, aided by natural erosion, and so the hole begins to sink.

About 10 percent of the world's landscape is composed of karst regions. Despite being common, sinkholes' unforeseeable nature serves as proof that the ground beneath our feet may not be as solid as we think.

A version of this story originally ran in 2014.

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Big Questions
How Are Speed Limits Set?
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When driving down a road where speed limits are oppressively low, or high enough to let drivers get away with reckless behavior, it's easy to blame the government for getting it wrong. But you and your fellow drivers play a bigger a role in determining speed limits than you might think.

Before cities can come up with speed limit figures, they first need to look at how fast motorists drive down certain roads when there are no limitations. According to The Sacramento Bee, officials conduct speed surveys on two types of roads: arterial roads (typically four-lane highways) and collector streets (two-lane roads connecting residential areas to arterials). Once the data has been collected, they toss out the fastest 15 percent of drivers. The thinking is that this group is probably going faster than what's safe and isn't representative of the average driver. The sweet spot, according to the state, is the 85th percentile: Drivers in this group are thought to occupy the Goldilocks zone of safety and efficiency.

Officials use whatever speed falls in the 85th percentile to set limits for that street, but they do have some wiggle room. If the average speed is 33 mph, for example, they’d normally round up to 35 or down to 30 to reach the nearest 5-mph increment. Whether they decide to make the number higher or lower depends on other information they know about that area. If there’s a risky turn, they might decide to round down and keep drivers on the slow side.

A road’s crash rate also comes into play: If the number of collisions per million miles traveled for that stretch of road is higher than average, officials might lower the speed limit regardless of the 85th percentile rule. Roads that have a history of accidents might also warrant a special signal or sign to reinforce the new speed limit.

For other types of roads, setting speed limits is more of a cut-and-dry process. Streets that run through school zones, business districts, and residential areas are all assigned standard speed limits that are much lower than what drivers might hit if given free rein.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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