The correct answer is: D. origin and destination studies.
Desire lines are plotted in origin and destination studies to show the movement of people or vehicles between different locations. They are used to understand how people travel and to plan transportation systems.
Traffic volume studies are used to measure the number of vehicles that travel on a particular road or highway. They are used to determine the capacity of a road and to identify areas where congestion is likely to occur.
Speed studies are used to measure the average speed of vehicles on a particular road or highway. They are used to identify areas where the speed limit is not being obeyed and to determine the need for speed bumps or other traffic calming measures.
Accident studies are used to identify the causes of accidents and to recommend ways to prevent them. They are used to improve the safety of roads and highways.