
Passengers per hour per direction

Passengers per hour per direction or passengers per hour in peak direction (pphpd or p/h/d) is a measure of the route capacity of a rapid transit or public transport system.
Corridor capacity.
The Corridor Capacity in the passenger transport field refers to the maximum number of people, which can be safely and comfortably transported per unit of time over a certain way with a defined width. The corridor capacity does not measure thus the number of vehicles which can be transported over such way, since the nuclear objective of passenger mobility is to transport passengers as such, and not to transport vehicles. 
In the International System of Units the corridor capacity is measured in terms of formula_1 or formula_2, i.e., the maximum number of passengers which can cross on average per second and per meter of the way's width. Its equivalent concept in Physics is Volumetric flux.
Directional flow.
Many public transport systems handle a high directional flow of passengers— often traveling to work in a city in the morning rush hour and away from the said city in the late afternoon. To increase the passenger throughput, many systems can be reconfigured to change the direction of the optimized flow. A common example is a railway or metro station with more than two parallel escalators, where the majority of the escalators can be set to move in one direction. This gives rise to the measure of the peak-flow rather than a simple average of half of the total capacity.