Saturation (traffic)

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. [1]

A formula to calculate saturation flows based on lane geometry is given in Transport and Road Research Laboratory RR67.[2] However, the formula can over-estimate saturation flows at congested locations.[3]

Degree of Saturation

The degree of saturation (DoS) of an intersection (typically under traffic signal 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, with queues of vehicles beginning to form. The practical reserve capacity (PRC) refers to the available spare capacity at a junction.[4]

Ratio of Flow to Capacity

For priority junctions including roundabouts, the equivalent measure to DoS is the ratio of flow to capacity (RFC).[5]

References
  1. DfT Traffic Signs Manual https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/851465/dft-traffic-signs-manual-chapter-6.pdf
  2. Kimber R M, Macdonald M & Hounsell N B, The Prediction of Saturation Flows for Road Junctions Controlled by Traffic Signals, Transport and Road Research Laboratory, Department of Transport, Research Report 67, 1986. https://www.eastleigh.gov.uk/media/8592/cd58.pdf
  3. TfL Modelling Guidelines v3.0, paragraph 2.4.7.1, http://content.tfl.gov.uk/traffic-modelling-guidelines.pdf
  4. The Norfolk County Council (Norwich Northern Distributor Road (A1067 to A47(T))) Order - NCC/EX/3 Glossary of Terms https://www.norfolk.gov.uk/-/media/norfolk/downloads/roads-and-transport/ndr/post-submission-documents/nccex3-glossary-of-terms.pdf?la=en
  5. 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
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