This basic truth — feeling safe kills — lies beneath many of the book’s insights. Americans think roundabouts are more dangerous than intersections with traffic lights. Roundabouts require you to adjust your speed, to merge, in short, to pay attention. At an intersection, we simply watch the light. And so we may not notice the red-light runner coming at us or the pedestrian stepping off the curb. A study that followed 24 intersections that had been converted from signals or stop signs to roundabouts showed an almost 90 percent drop in fatal crashes after the change.
For similar reasons, S.U.V.’s are more dangerous than cars. Not just because they’re slower to stop and harder to maneuver, but because — by conferring a sense of safety — they invite careless behavior. “The safer cars get,” Vanderbilt says, “the more risks drivers choose to take.” (S.U.V. drivers are more likely to not bother with their seat belts, to talk on cellphones, and to not wear seat belts while talking on cellphones.) So it goes for much of the driving universe. More people are killed while crossing in crosswalks than while jaywalking. Drivers pass bicyclists more closely on a road with bike lanes than on one without.