Minimum Stair Width for Evacuation, Overtaking Movement and Counterflow

Traditional lane models such as the 560 mm (22-in.) unit of exit width are examined as historical artifacts and, when studied empirically, as flawed bases for minimum stair width determination. Criticisms of this lane model were presented separately by the authors as early as about 1970 and improved bases for minimum width determination were also presented. Currently, even the improved bases for minimum stair width—based on the authors ’ early work—need to be updated for stair user demographics and other factors that have changed in recent decades. Three types of crowd flow are considered; coherent flow, overtaking movement, and counterflow. All of these occurred in the evacuations of the World Trade Center in 1993 and 2001. Partly as a result of the latter incident, counterflow has recently received particular attention in some US standards and building code-change deliberations that led to a minor increase—from 1120 mm to 1422 mm (44 in. to 56 in.) in minimum, nominal exit stair width requirements for certai