What is Passenger car equivalent?

Passenger car equivalent or passenger car unit is a metric used in transportation engineering, to assess traffic-flow rate on a highway.

A passenger car equivalent is essentially the impact that a mode of transport has on traffic variables compared to a single car. For example, typical values of PCE are:

Highway capacity is measured in PCE/hour daily

A common method used in the US is the density method. However, the PCU values derived from the density method are based on underlying homogeneous traffic concepts such as strict lane discipline, car following and a vehicle fleet that does not vary greatly in width.