Traffic Signals - Part 10 - Linked Modes of Operation

In addition to the isolated modes of operation, traffic signal controllers (TSC's) are equipped with operating modes that overcome the lack of co-ordination between adjacent sites.

Cableless Linking Facility (CLF)

CLF uses operating plans programmed directly into the TSC. These can be selected by time of day / day of week criteria programmed into the TSC’s timetable. Each plan has a predefined cycle time, and at specific points of this cycle it will allow particular stages to run. The plans are originated so that traffic flowing around the adjacent road network in the area, will benefit from a co-ordinated approach, utilising fixed offsets to provide estimated journey times between junctions. To achieve this, it is essential that the real-time clock, operating in the TSC is accurately set, so that the introduction of each CLF plan is synchronised with those operating in adjacent TSC's. To ensure this is maintained, it is common for the real time clock at each site to be updated on a regular basis. Although it is possible to achieve this by using a connection to an atomic clock via radio or GPS, traditionally this has been accomplished by using a special synchronisation signal from the Urban Traffic Control (UTC) system.

The operation of CLF suffers from a lack of flexibility to cope with changes in traffic flows or incidents, as plans and timetable entries are pre-programmed in the TSC, and use assumed fixed offsets for journey times to adjacent junctions. To make any alterations to the operation of CLF, the TSC has to be re-programmed on-site.

Urban Traffic Control (UTC)

UTC is very similar in operation to CLF. Each TSC is controlled by a plan, but instead of these running on individual controllers, the plans are run from a central UTC centre. Each TSC is connected to the UTC centre by a communications link, over which a constant stream of commands and confirmation replies are sent. Traditionally, a dedicated telephone 'leased-line' was used for this link, however, these are being phased out by telecoms providers such as BT. It is therefore becoming increasingly common to use Internet Protocol (IP) capable technologies, such as broadband, fibre optic and radio based systems instead.

The advantage of this system is the ability to be able to modify plans as traffic flows change, and to be able to intervene remotely in the operation of the network if an incident occurs.

Split Cycle and Offset Optimisation Technique (SCOOT)

SCOOT is a demand responsive UTC system, for networks of signalised junctions. A centralised computer monitors traffic flow information fed to it by a comprehensive network of detection sensors, situated across the area controlled by the system. These are normally located at the exit of the previous upstream junction so that the system is aware of the extent of traffic approaching subsequent traffic signals. The system frequently calculates the best signal timings for each TSC in response to current traffic conditions, to optimise traffic flow and reduce delays throughout the network. The system has the advantage of not requiring costly and time consuming manual plan updates, whilst achieving reductions in vehicle delays and improvements to traffic flow. However, it is essential that the original set-up and operation of the SCOOT system is accurately validated to achieve optimal operation.

Microprocessor Optimised Vehicle Actuation (MOVA)

MOVA is utilised at isolated junctions, and is housed within the TSC cabinet, usually within an Outstation Monitoring Unit (OMU) or integrated into the main processor unit. MOVA is an enhanced VA implementation, maintaining the green whilst vehicle flow is maintained at, or above, the saturation flow rate. Once the end of the saturation flow has been detected, a delay optimisation process begins. If one or more lanes are oversaturated, MOVA uses a capacity-maximising algorithm instead of the delay-optimising process.

The system has two principle modes of operation:

• In the un-congested mode, the system seeks to disperse queue formation.
• In congested mode, MOVA undertakes a capacity maximisation routine.

MOVA uses vehicle detection on each approach in a similar way to VA mode, but instead of using System D the detection requirement is more onerous. The system uses individual loops in each lane in order to ascertain traffic demand on a lane by lane basis. In addition to the possible use of a stop line loop, MOVA uses an X loop at around 39m, an IN loop and possibly a Q loop. The distance of the loops from the stop-line is dependant upon the average speed of vehicles on each individual approach. A facility has been added to MOVA in recent years which allows the system to be more readily utilised in urban environments. It can be problematic to install the IN loops where there is already a lot of other services in the ground, because of the long distance of ducting required. Compact MOVA (CMOVA) offers the opportunity to operate without an IN loop, but will have a slightly negative impact on junction performance. In the event of losing detection capability, MOVA also has the benefit of logging historic traffic flows, allowing this data to be used as a contingency instead to forecast timing requirements.

MOVA is the default method of control for TSC’s located on the trunk road network, unless they form an integral part of a UTC area.

It will normally be used with a connection to the local UTC centre for remote monitoring.

Remote monitoring

UTC systems also offer additional benefits. It is common to use the communication channel to report status information, such as lamp failures or more serious faults, using a Remote Monitoring System (RMS). The RMS allows faults to be passed on to the maintenance contractor automatically, reducing the time taken to respond to fault occurrences. UTC can also confirm and reset the TSC Real Time Clock automatically; ensuring operational synchronisation is maintained in the event of communications being temporarily lost. It is also possible to provide remote access, which allows operators at the UTC centre to interrogate the TSC as if they were on site using the engineer’s handset.

Incorporating other features

UTC systems can also operate with a range of different systems to encompass a host of requirements for traffic in urban areas. These can include CCTV, car park information, travel information systems and bus priority systems. However, the interoperability of disparate systems has traditionally caused problems. To overcome this, systems such as Urban Traffic Management & Control (UTMC), allow a vast array of different technologies and systems to operate with each other.

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