Degree of saturation (traffic)

In traffic engineering, 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, with queues of vehicles beginning to form. The term practical reserve 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.