TABLE OF CONTENTS The term capacity when referring to a highway link or junction is its ability to carry, accommodate or handle traffic flow. Traditionally, capacity has been expressed in numbers of vehicles or passenger car units (PCU). (Vehicles vary in their performance and the amount of road space they occupy. The basic unit is the passenger car and other vehicles are counted as their PCU equivalent, e.g. a bus might be 3PCUs and a pedal cycle 0.1PCU.) In recent years public transport operators have applied pressure to consider highways in terms of their passenger-handling capacity and thus give a greater emphasis to the benefits of using high occupancy vehicles, such as buses or trams.
There is no absolute capacity value that can be applied to a given highway link, traffic lane or junction. Maximum traffic-handling capacity of a highway depends upon many factors including:
The highway layout including its width, vertical and horizontal alignment the frontage land uses, frequency of junctions and accesses and pedestrian crossings.
Quality of the road surface, clarity of road marking, signing and maintenance.
Proportions of each vehicle type in the traffic flow and their general levels of design, performance and maintenance.
The numbers and speed of vehicles and the numbers of other road users, such as cyclists and pedestrians.
Ambient conditions including time of day, weather and visibility.
Road user levels of training and competence.
The capacity of a road junction is dependent upon many of the features that govern link capacity with the...
