About two miles (three kilometers) per hour. That's the conclusion of a group of researchers at the University of Sidney, who found that the walking speed of adults correlated inversely with their risk of death.
The Grim Reaper’s preferred walking speed is 0.82 m/s (2 miles (about 3 km) per hour) under working conditions. As none of the men in the study with walking speeds of 1.36 m/s (3 miles (about 5 km) per hour) or greater had contact with Death, this seems to be the Grim Reaper’s most likely maximum speed; for those wishing to avoid their allotted fate, this would be the advised walking speed.
Details of the methodology and analysis of the results are published in the British Medical Journal. The authors note also that "the preferred walking speed of the Grim Reaper while collecting souls is relatively constant irrespective of people’s geographical location, sex, or ethnic background."
Link. Image credit Belle Mellor.