Number of users who finish a route divided by number of users who start.

Number of users that finish crossing the road divided by the number of users that start crossing.

The joke is there, but it's not actually quite that macabre -- if a route feels unsafe, more users will abandom it midway through.

The Triborough bridge had a pedestrian walkway; I've never made it across because the rail is below my center of mass, and nope.