Shropshire Council

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Dynamic traffic controls

Traffic lights can be controlled in different ways, depending on the location, day of the week or time of day. Methods of control used in Shropshire include the following:

Fixed timing

The most basic form of traffic control employs fixed timings between each signal phase or traffic movement, and is serviced in a programmed sequence repeated throughout the day. With this system the main street flow receives a fixed amount of green time followed by the amber and red clearance intervals. The same interval timing is then repeated for the minor or side street.

VA (vehicle actuated)

This is the simplest form of intelligent traffic light control, with each lane leading to a junction having a detector, either buried in the ground or on top of a pole. A stage is first called by a vehicle and then extended as more vehicles are detected. The maximum duration of the stage is fixed, but is normally changed by time of day to reflect typical traffic conditions and to balance delay for all road users at the junction.

MOVA (microprocessor optimisation vehicle activated)

This system is very similar to VA but is a lot more intelligent. It too uses detectors buried in the ground but it also counts cars, can tell the speed they're travelling and is able to vary the maximum cycle time in response to actual traffic flows, rather than the flows assumed for that time of day. It can also adjust the individual timings for one approach in response to conditions all-round the junction.

Unlike traditional VA, where individual approaches can be extended up to their maximum green periods, MOVA uses an algorithm to assess the benefit to the junction as a whole, in terms of overall delay, of curtailing one approach in order to give right of way to another approach. This produces significantly more efficient control, particularly at multi-lane approaches.

SCOOT (split-cycle offset optimisation technique)

A system which links several consecutive traffic signal sites together and will judge the amount of vehicles over the whole road network, rather than just one junction. The linking provides benefits in reducing congestion, and makes sure that any queues that emerge aren't just moved to the next set of lights.

SCOOT is able to treat large areas or even whole towns as one super junction, and work out the best choice of action for the whole network, not just one set of lights.

Shropshire currently operates two distinct SCOOT corridors, one along the Smithfield Road between Frankwell and the railway station in Shrewsbury, and the other between Arlington Way, along Battlefield Road and Ditherington Road, to Mount Pleasant Way in Shrewsbury.

In both cases these corridors are only operating under SCOOT technology during times of peak traffic flows, and revert back to MOVA at other times as this system operates more effectively when the traffic volumes are smaller.