Here's How Speed Limits Are Set

Speed limits are something you might just take for granted. I know I did, until I taught my kids how to drive. We gradually went from low-speed city roads to the countryside, the bypass, and then the interstate, while I pointed out why conditions are different for each. The posted speed limit is affected by how curvy, crowded, and wide the road is, how many turnoffs and intersections there are, visibility, and a few other factors. The procedure for setting speed limits also includes engineers studying the "prevailing speed."    

Our hypothetical engineer figures out how many drivers are on the road and how fast they drive. Once that data’s collected, it’s plotted out to help determine how fast the majority of drivers are traveling. In particular, the engineer wants to determine the “85th percentile speed”—meaning how fast 85 percent of the cars travel.

The 85th percentile speed, a blogger at engineer company SEH put it, is called the “prevailing speed,” because it’s considered the safest speed to travel. Interesting, right?

There are other factors that go into setting speed limits. And some places throw all the data out the window anyway, like speed traps where the limit is ridiculously low and long highways out west where the limit is pretty high. Read about how speed limit decisions are made at Jalopnik.

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Apart from a few very congested areas and schools etc. speed limits are set so they can be cash cows in the UK. Nothing to do with road safety, just yet another extra tax. Why else would you have a 70Mph dual carriageway suddeny become 40 Mph the other side of a bridge that obscures the sign until you are right on top of it? That sneaky trick and more than a few others rake in many hundreds of millions in "fines".
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In some places they are set as to be speed traps. A couple places in Florida are infamous for that. Other issues happens when an old country road becomes a busy area of commerce and the speed limit isn't reduced but should be. My favorite is when we were going to a wedding in Toronto. After crossing the border we got on to the QEW. I see a sign that says "SPEED LIMIT 100", wow. I think why bother with a speed limit at all, then I realize they must have changed to metric. Sure enough, the next sign I see is KILOMETERS. My brother in law told me the change over was done by sticking paste on signs over the old MPH signs until they were replaced with permanent signs. I'm glad we . never did that here.
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Let me qualify that first statement: speed limits were at the top of my consciousness when, not long after I got my license, they suddenly reduced the speed limit on the interstate from 75 to 55. It was really hard to slow down that much on a highway engineered for high speeds.
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