Which speed describes the condition of traffic when density is zero?

When density is zero, it indicates that there are no vehicles present on the roadway. Under these conditions, the speed at which vehicles can travel is known as the free-flow speed. This speed represents the maximum speed at which vehicles can operate without any restrictions due to other road users, such as traffic congestion or obstacles.

In free flow conditions, the traffic flow is smooth and uninterrupted, and vehicles can maintain their speeds without having to slow down for other vehicles. This is the scenario where drivers can drive at the highest speeds safely permitted under ideal circumstances.

Other terms like optimal speed or target speed refer to desired conditions that may not reflect the absence of vehicles but rather the conditions promoting efficiency and safety in traffic flow when vehicles are present. Maximum speed often refers to legal limits that vehicles must adhere to, which may or may not be achieved depending on the conditions of the road and surrounding traffic. Thus, free-flow speed is the most accurate description of traffic conditions when density equals zero.