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If you ever find yourself in crippling Beijing traffic, you might want to direct your road rage toward a numerical superstition rather than your fellow drivers.

China’s capital has a vehicle plate restriction program that’s intended to decrease congestion and air pollution around the city center. On weekdays, private cars with license plates ending in two digits, zero to nine, can’t drive inside Beijing’s 5th Ring Road from 7 a.m. to 8 p.m. So if the pairing for a particular day is 4 and 9, for instance, then drivers with those tail numbers have to find another travel mode in the restricted zone.

That’s all well and good—except that the Chinese can get pretty superstitious about their numbers. Some numbers are lucky; eight is one of them, and eighth-floor apartments in China have been known to fetch a higher market price accordingly. But the number four is considered unlucky because it sounds a lot like the word for “death,” and as a result Chinese buildings often lack a fourth floor (just as American buildings sometimes skip the 13th).

Likewise, Chinese drivers avoid license plates ending in four. Whereas U.S. drivers don’t have much choice about their license numbers unless they get a vanity plate, the Chinese evidently get to choose from a bunch of options with randomly generated numbers. If possible, many pick a plate without any fours at all; if not, they’ll at least pick one that doesn’t end in the deadly digit.

Turns out that seemingly innocuous custom has a pretty big impact on Beijing traffic. In a new working paper, an international research group reports that six license plate tail numbers (0, 1, 2, 3, 5, and 7) each reflect their fair statistical share of vehicles, roughly 10 percent. Three lucky numbers (6, 8, and 9) are overrepresented on plate endings, with 12 or 13 percent each. License plates ending in 4, meanwhile, account for only 1 to 3 percent of all cars.

Here’s what that means for your Beijing commute: more cars on the days that ban tail numbers 4 and 9 from the restricted zone. On these days, only 14 percent of vehicles end up ineligible to drive through the 5th Ring Road; on all other days the restriction reaches 20 to 22 percent of all cars. That’s a huge relative spike in traffic; a 10 point jump in the share of cars allowed on the road increases travel delay 34 percent over open roads, according to the study.

With the rise in traffic comes a related dip in happiness, says paper coauthor Michael Anderson, an environmental economist at University of California, Berkeley. “[O]n days when #4 plates are restricted, traffic increases and happiness falls,” he tells CityLab via email. “And we infer that the former causes the latter (the reverse seems incredibly unlikely).” So it’s fair to say that on days when these plates are banned in Beijing, the city is a less happy place.

Then again, you might also say that becoming more comfortable with death can lead to a more peaceful life. Something to ponder in traffic.