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 degree of saturation of an intersection (typically under traffic signal control) or road is a measure of how much demand it is experiencing compared to its total capacity.

The degree of saturation (%) is a ratio of demand to capacity on each approach to the junction, with a value of 100% meaning that demand and capacity are equal and no further traffic is able to progress through the junction.
Values over 85% are typically regarded as 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...

, with queues of vehicles beginning to form. The term practical reserve capacity

Practical reserve capacity

In traffic engineering, the practical reserve capacity of a traffic signal junction is a commonly used measure of its available space capacity....

(PRC) is often used to refer to the available spare capacity at a junction. A negative PRC indicates that the junction is over capacity.