The Critical Ten - a 10 MPH difference make a huge safety difference | streets.mn

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The Critical Difference Between 30 and 20

I’m talking about traffic speed. If you look at the average speed of traffic on a urban commercial streets, there are a lot of things that begin to change when you slow down cars from the 30 to 35 mile per hour range into the 20 to 25 mile per hour range. Most importantly, perception, reaction time, and crash outcomes are far better at 20 than at 30 mph, while traffic flow doesn’t seem to change very much.

The perception angle is perhaps the most interesting. [See above chart.] Driving speed has a dramatic effect on the driver’s “cone of vision.” You can see a lot more detail at 20: people on the sidewalk, a bicyclist in the periphery, or the ‘open’ sign on a storefront. At 30 mph, the window shrinks dramatically.

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The Critical Ten - a 10 MPH difference make a huge safety difference | streets.mn

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