The highway or traffic density is defined as the total number of vehicles that can be accommodated on a unit length of the road.
In a mixed traffic flow, the traffic capacity is generally expressed as Passenger Car Unit (PCU) per hour and the traffic density as PCU per kilometre length of lane.
The highway capacity, depending upon a number of prevailing roadway and traffic conditions, are as follows:
a) Basic Capacity: The basic capacity is the maximum number of passenger cars that can pass a given point on a lane or a roadway during one hour under the most ideal roadway and traffic conditions that can possibly be attained.
b) Possible Capacity: The possible capacity is the maximum number of vehicles that can pass a given point on a lane or a roadway during one hour under the prevailing roadway and traffic conditions. The possible capacity of a road is generally much lower than the prevailing conditions referred to under the possible capacity are ideal if
i) There are at least two lanes for the exclusive use to traffic moving in one direction.
ii) All the vehicles move at the same uniform speed.
iii) The width of lanes, shoulders and clearness to vertical obstruction beyond the edge of the traffic lane are adequate.
iv) There are no restrictive sight distance, grades, improperly super-elevated curves, intersections or interference by pedestrians.
c) Practical Capacity: The practical capacity is the maximum number of vehicles that can pass a given point on a lane or a roadway during one hour with creating unreasonable delay under the prevailing roadway and traffic conditions. The values of the practical capacity is less than the possible capacity.
d) Design Capacity: It is the practical capacity or a smaller value determined for use in designing the highway to accommodate the design hourly volume (D.H.V). It is a term, normally, applied to existing highways.
e) Theoretical Capacity : The theoretical capacity is the number of vehicles passing any point in one hour per lane. It depends upon the average length of the vehicle and the average spacing of the moving vehicles. Mathematically, theoretical capacity,
V = Design Speed of the vehicle in kmph
S = Center to center spacing of moving vehicle
= Reaction distance + Average Length of Vehicle