What best describes capacity drop on a congested freeway?

• A. It is an increase in capacity during congestion.
• B. It is the observed reduction in capacity when traffic is congested, compared to free-flow conditions.
• C. It refers to the capacity of detour routes only.
• D. It is the capacity of emergency vehicles.

Answer: (B)

Capacity drop describes how the freeway’s throughput actually falls when congestion occurs, compared with the rate you can sustain in free-flow conditions. When traffic becomes dense, speeds drop and drivers need more spacing, which means fewer vehicles can pass a point per hour. The congested region often discharges fewer vehicles downstream than were flowing into it, so the observed capacity during congestion is lower than the free-flow capacity. This isn’t about detours or emergency vehicle use; it’s about the fundamental reduction in throughput caused by congestion itself.