In
traffic engineering, saturation describes the maximum traffic flow which can be handled by a junction. The saturation flow is the rate at which a continuous flow of vehicles can pass through a constant green signal, typically expressed in vehicles per hour or
PCUs per hour.
A formula to calculate saturation flows based on lane geometry is given in Transport and Road Research Laboratory RR67. However, the formula can over-estimate saturation flows at congested locations.
Degree of Saturation
The degree of saturation (DoS) of an intersection (typically under
traffic signal
Traffic lights, traffic signals, or stoplights – known also as robots in South Africa are signalling devices positioned at road intersections, pedestrian crossings, and other locations in order to control flows of traffic.
Traffic lights ...
control) or a link measures the demand relative to the total capacity. A DoS value of 100% meaning that demand and capacity are equal and no further traffic is able to progress through the junction. The formula to calculate DoS is:
*Degree of saturation = (demand x cycle time) / (saturation flow x effective green time)
Values over 85%-90% typically indicate
traffic congestion
Traffic congestion is a condition in transport that is characterized by slower speeds, longer trip times, and increased vehicular queueing. Traffic congestion on urban road networks has increased substantially since the 1950s. When traffic de ...
, with queues of vehicles beginning to form. The practical reserve capacity (PRC) refers to the available spare capacity at a junction.
Ratio of Flow to Capacity
For priority junctions including roundabouts, the equivalent measure to DoS is the ratio of flow to capacity (RFC).
[Essex Highways - Epping Forest Local Plan Highway Impact Assessment https://www.efdclocalplan.org/wp-content/uploads/2017/11/Essex-Highways-Tech-Note-1-Base-year-junction-capacity-modelling-Oct-2013-EB500A.pdf]
References
External links
Transport for London - Modelling Guidelines
Transportation engineering
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