Hey grandma drivers, this is big rig sciencepappy Trace for DNews, 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. Afterward, 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 lilttle bit of 85)！ 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 different

computer and cost-based models 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 crowd-sources 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

means 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.

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But what about self-driving cars? Should they have speed limits too? Are they even safer?

Trace looks int-- wait a minute, I'm TRACE! It's a good video, check it out. :D

What do you think? Should speed limits be higher? Lower? Are you just waiting for the

self-driving revolution? Tell me about it!