What is passing sight distance and how is it determined?

Passing sight distance (PSD) is the distance required for a vehicle to safely pass a slower vehicle on a two-lane highway. PSD = d1 + d2 + d3 + d4, where d1 = distance during perception-reaction and acceleration, d2 = distance while occupying opposing lane, d3 = clearance distance from opposing vehicle, d4 = distance traveled by opposing vehicle during passing. AASHTO PSD ranges from 200m at 50 km/h to 800m at 120 km/h. PSD determines where passing zones (broken yellow lines) are marked.