In traffic engineering

Traffic engineering (transportation)

For the engineering of communications and computer networks, see Teletraffic engineering.Traffic engineering is a branch of civil engineering that uses engineering techniques to achieve the safe and efficient movement of people and goods on roadways...

, the practical reserve capacity (PRC) of a traffic signal junction is a commonly used measure of its available space capacity.

The practical reserve capacity is related to the degree of saturation

Degree of saturation (traffic)

In traffic engineering, the degree of saturation of an intersection or road is a measure of how much demand it is experiencing compared to its total capacity....

 of a traffic signal junction. A positive PRC indicates that a junction has spare capacity and may be able to accept more traffic. A negative PRC indicates that the junction is over capacity and is suffering from traffic congestion

Traffic congestion

Traffic congestion is a condition on road networks that occurs as use increases, and is characterized by slower speeds, longer trip times, and increased vehicular queueing. The most common example is the physical use of roads by vehicles. When traffic demand is great enough that the interaction...

.

A PRC value is calculated by the commonly used traffic signal modelling software LINSIG

LINSIG

LINSIG is a software tool which allows traffic engineers to model traffic signals and their effect on traffic capacities and queuing. As well as modelling the effects of traffic signals LINSIG also optimises signal timings to reduce delay or increase capacity at a junction or group of interlinked...

.