Action at a distance to restrain traffic from a critical area
The objective of "gating" is to control the inflow of traffic into sensitive areas where it is particularly important to prevent serious congestion. It can also be used to relocate queues in conjunction with bus priority measures. Gating is not universally applicable; there is often nowhere suitable to relocate queues to, although it is appropriate to help prevent serious exit blocking of junctions, and particularly gyratory systems, where the exit blocking delays cross-traffic.
To implement gating, SCOOT takes "action at a distance" by modifying the traffic signal settings at junctions which may be some way from the problem area. In some situations, such as where major problems in one region may cause congestion to spread rapidly to adjoining areas, the use of gating could provide overall benefits to the network as a whole.
The gating logic allows one or more links to be identified as critical, or "bottleneck", links. A critical link can affect the green time on the gated links. The gated links are those that have been designated to store queues which would otherwise overload the sensitive area defined by the dritical links. When a Critical link is too busy (i.e. goes over the target saturation or congestion for that link), the green time is reduced on the gated links. The traffic engineer would specify a link as critical if it were important to avoid large queues on the link and those queues could be relocated to one or more gated links.
Use gating to
- relocate queues to more acceptable positions e.g. where there is space for a bus lane; where queues will not exit block important junctions, particularly where there is danger of locking up a gyratory system
- protect sensitive areas e.g. shopping areas with many pedestrians
- take action at a distance, restrain traffic in advance of the critical area
- define the area to be protected and the allowable conditions in the area
- define where queues should be relocated to and the maximum restraint allowed there
- SCOOT gating will operate automatically as required by conditions in the protected area