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Drivers Curious How Speed Limits Are Determined

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Petr Prucha, Blake Wheeler

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Drivers Curious How Speed Limits Are DeterminedWBZ

They are silent reminders for the lead foot among us to slow down. We’re talking about speed limit signs.

Some seem high, others too low.

J.K. in Boston Declared his Curiosity:

“Who determines the speed limit for roads and how are they determined?”

David Wade found out, there is a lot more to it than you might think.

A bucket of black ink, some heavy metal and a printer – that’s how you make a speed limit sign. But from the sign shop, which sign goes where?

Rick Wilson doesn’t give out tickets. He works for Mass. Highway. Today, he’s charting the speed of 100 cars in Medford.

Knowing how fast you’re driving helps him figure out what the limit should be. “We want to bring the lower speeds up and the upper speeds down. The more folks we have driving the same speed, the less conflict you’ll have.”

Cities and towns can post their own speed limits.

If a road seems too fast or too slow, they ask the state to analyze it.

Using Rick’s chart, the speed limit is usually set using the “85 percentile rule.”

“So if 85 percent of the drivers are doing 40 (miles per hour) or less, they make up a 40 mph sign,” explains the state’s chief engineer, Frank Tramontozzi.

That’s unless Rick’s “ballbank indicator” says the road has too many curves. “This measures the combined effect of the super elevation of the curve, the body roll angle and the friction of the tires on the roadway.”

One of the reasons the state does not let cities put up their own signs is because they don’t want you driving down the road doing 40 mph and five seconds later, you’re suddenly in another city and the speed limit is 20 mph.

That would make for quite the speed trap.

So in many ways, Rick is making sure the speed limit is fair to you.

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