How Do We Decide Speed Limits?

I can drive FIFTY FIIIIIIIVE!! Because it's more efficient. And safer! Right? Hey grandma drivers,  what's your 20? Old cars and highways often have a speed limit of 55 because of efficiency. It all started in 1973, when the U.S. was under an oil embargo. To conserve resources, Congress tied the state speed limits to federal highway funding; basically forcing states to adhere to 55 or else they'd lose millions in highway money. They picked 55 because the Argonne National Laboratory found that was the speed most cars would get the best miles-per-gallon. Once the embargo ended, Congress was reluctant to lift the limit, until 1995. Afterwards, States went on a tizzy trying to pick new speed limits. Most chose 70, but some use 65 or 80 (yes, Texas, I know, you got a lil' bit of 85, you get a cookie)! But how did they decide on those numbers? Well, firstly, exceeding the speed limit or driving too fast for the conditions is the primary cause of auto crashes globally. Which is why we have speed limits in the first place. But it's important to know "too fast for conditions" is a big deal. Because speed limits are determined by data, but the data is from three main things: nature, engineering, and humans! For example: when highway and traffic researchers decide a speed limit they take into account weather, visibility, terrain, location and so on -- nature. They also take into account the purpose of the road, crash risks, traffic and truck flow monitors (those two rubber tubes you sometimes run over on the road), what the road is made of, the amount of shoulder next to the road, maintenance schedules -- that's engineering. And of course how humans actually USE the road which we'll come back to in a sec. They then take that data and use a different computer and cost-based model to specifically pick the speed for that stretch of road! The human component has to do with safety, but ALSO our ability to actually DRIVE on that road at that speed. The operating speed model is often used, sort of crowdsources the speed limit using something called the "85th percentile rule." Basically, researchers observe the average speed people naturally want to use on a stretch of road, and lob off the speediest 15 percent! Research has shown, that the 85th percentile is a speed most people would be comfortable driving. Which is super important, because be it 55 or 85 the researchers and lawmakers want a speed limit that is "accepted as reasonable by a majority of road users." So, nature, engineering and humans together factor into our speed limits. And that all kind of makes sense. Faster speeds tend toward stretches of road in Montana or Texas, where roads are open, sparsely populated and straight, while dense, high-traffic, urban areas with more weather considerations tend toward slower speeds. Duh, right? Speed limits are human things, but they're based on physics plus biology. Faster speeds mean less time for our animal brains to react when something goes wrong. According to the European Road Safety Observatory: for every 1 km/h increase in speed there's a 3 percent increase in crashes. It's an old joke that it's not the crash that kills you, but the sudden stop at the end. The faster you are going, the data says, the more kinetic energy is in the vehicle, and the worse the crash is going to be. This is why some regions likely have lower speed limits for their conditions, roads and drivers. Once the speeds are picked, researchers are constantly going back and adding new data and testing the road at various speeds, and in various conditions. Look, everyone who drives a car wants to get where they're going as efficiently, quickly, and safely as possible, even traffic researchers! Every road, and every type of road is different! The reason that speed limit is on THAT road, is a mix of lots of data, and lots of human effort. But what about self-driving cars? Should they have speed limits too? Are they even safer? What do you think? Should speed limits be higher? Lower? Are you just waiting for the self-driving revolution?

As found on Youtube

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