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.