The illustration itself is food for thought. Walker describes it:
The bottomless void, in this metaphor, represents the essential unpredictability of the reckless or distracted motorist (there only needs to be one) combined with the destructive potential of their machine. The sidewalk is a narrow ledge on the edge of extreme danger. Crossing the street, even with a crosswalk, works when it works, but the rickety bridge perfectly captures the inherent risk...
Walker notes that if this were in a national park, they would have put up safety railings. "We tolerate this level of danger only for well-warned hikers in deep wilderness, and for almost everyone who ventures into the city without a car."
It's true. The idea of crossing the street on the rickety bridge is a good analogy; the only thing missing is a Balrog chasing you from behind.
Walker credits the image to Claes Tingvall, the Director of Traffic Safety at the Swedish National Road Administration; in fact it was drawn by Swedish artist Karl Jilg, commissioned by Tingvall's organization.
Tingvall is one of the creators of the concept of Vision Zero, now the vogue in New York City and an anathema to the War on the Car people, because Tingvall describes Vision Zero this way:
Some thought that perhaps it was a strange idea. Many didn’t understand it, because they saw it as a figure. Yes, the goal is zero, but that’s impossible. We said, it’s a mindset not a figure. In essence, what you’re saying is that you go from a situation where safety is a trade-off with mobility, to a situation where you say that life and health are paramount in the road transport system.
God forbid that the life and health of cyclists and pedestrians matter more than how fast people get to work.