At Home - Bill Bryson [168]
Let’s look at a fall in slow motion. Descending a staircase is in a sense a controlled fall. You are propelling your body outward and downward in a manner that would clearly be dangerous if you weren’t fully on top of things. The problem for the brain is distinguishing the moment when a descent stops being controlled and starts being a kind of unhappy mayhem. The human brain responds very quickly to danger and disarray, but it still takes a fraction of a moment—190 milliseconds to be precise—for the reflexes to kick in and for the mind to assimilate that something is going wrong (that you have just stepped on a skate, say) and to clear the decks for a tricky landing. During this exceedingly brief interval the body will descend, on average, seven more inches—too far, generally, for a graceful landing. If this event happens on the bottom step you come down with an unpleasant jolt—more of an affront to your dignity than anything else. But if it happens higher up, your feet simply won’t be able to make a stylish recovery, and you had better hope that you can catch the handrail—or indeed that there is a handrail. One study in 1958 found that in three-quarters of all stair falls no handrail was available at the point of the fall’s origin.
The two times to take particular care on staircases are at the beginning and end. As many as one-third of all stair accidents occur on the first or last step, and two-thirds occur on the first or last three steps. The most dangerous circumstance of all is having a single step in an unexpected place. Nearly as dangerous are stairs with four or fewer risers. They seem to inspire overconfidence.
Not surprisingly, going downstairs is much more dangerous than going up. Over 90 percent of injuries occur during descent. The chances of having a “severe” fall are 57 percent on straight flights of stairs, but only 37 percent on stairs with a dogleg. Landings, too, need to be of a certain size—the width of a step plus the width of a stride is considered about right—if they are not to break the rhythm of the stair user. A broken rhythm is a prelude to a fall.
For a long time it was recognized that people going up and down steps appreciate being able to do so with a certain rhythm, and that this instinct could most readily be satisfied by having broad treads on short climbs and narrower treads on steeper climbs. Classical writers on architecture had surprisingly little to say on the design of stairs, however. Vitruvius merely suggested that stairs should be well lighted. His concern was not to reduce the risk of falls but to keep people moving in opposite directions from colliding (another reminder of just how dark it could be in the pre-electric world). It wasn’t until the late seventeenth century that a Frenchman named François Blondel devised a formula that mathematically fixed the relationship between riser and tread. Specifically, he suggested that for every unit of increased height the depth of tread should be decreased by two units. The formula was widely adopted and even now, more than three hundred years later, remains enshrined in many building codes even though it doesn’t actually work very well—or indeed at all—on stairs that are either unusually high or unusually low.
In modern times, the person who took the design of stairs most seriously was, surprisingly, Frederick Law Olmsted. Although almost nothing in his work required it of him, Olmsted measured risers and treads fastidiously—sometimes obsessively—for nine years in an attempt to arrive at a formula that ensured staircase comfort