Here's Jeff Speck to explain how speed kills -- and there's Team Gahan, ignoring him.

Jeff Speck was on Twitter, Christmas Eve. 

While many different factors influence the safety of humans in cities, none matters nearly so much as the speed at which vehicles are traveling. The relationship between vehicle speed and danger is, to put it mildly, exponential. (1/4)

The death risk to pedestrians from vehicles takes a dramatic upturn at 25 mph. (2/4)

The diagram above is one of many that can be found to communicate this relationship. Others show people falling out of buildings, with 20 mph equaling the second floor and 40 mph equaling the seventh. (3/4)

The basic message to remember is that you are about five times as likely to be killed by a car going 30 as a car going 20, and five times again as likely to be killed by a car going 40. (4/4)

Continued this morning.

This threshold zone of 20 to 40 mph, is basically where it all happens—the difference between bruises, broken bones, and death. And 20 to 40 is roughly the range of speeds that we find cars traveling on the best downtown streets. (1/5)

Keeping cars on the lower end of that range, therefore, must be the central objective of urban street design. (2/5)

The speed of the impact itself is not the only factor. As cars move faster, the likelihood of a crash also rises. Drivers and pedestrians alike have less time to respond to conflicts, stopping distances lengthen, and the driver’s cone of vision narrows. (3/5)

As drivers move more quickly, their cone of vision narrows, making crashes more likely. (4/5)

These factors multiply the impact of speed beyond those indicated in the above graph. It is safe to say that a car traveling 30 mph is probably at least three times as dangerous as one going 25. (5/5)