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Freeway Traffic Management Systems

COMPASS

Compass is basically a large integrated freeway traffic management system intended to improve safety, optimize the real capacity of the highway and provide a better level of service to motorists without the addition of more traffic lanes. This improvement is accomplished by faster detection and response to incidents on the highway and through balancing of traffic volumes between the highway and other viable alternate routes. (For example, the express and collector system on Highway 401 within Toronto.)

A typical freeway traffic management system can comprise the following major subsystems:


Traffic Operations Centre (TOC)

Traffic Operations Centre

For large-scale freeway traffic management systems like Compass, the traffic operations centre is the focal point for all communications and control. It receives information on traffic conditions, analyzes and evaluates it, and make decisions as to what controls should be used. The traffic operations centre usually consists of the central computer system and its related peripheral accessories, communications equipment, CCTV monitors and camera control equipment, colour display components, and radio equipment for dispatching emergency agencies and maintenance vehicles to the problem locations.

The Compass Traffic Operations Centre for Highway 401 is located within the Downsview complex of the Ontario Ministry of Transportation at Keele Street and Highway 401. The centre is in operation 24 hours a day, 7 days a week and it is responsible for all Ministry freeway management activities within the Greater Toronto Area.

The operator workstation of Compass is equipped with several pieces of equipment. The function of each is discussed as follows:

Graphics Terminal
CCTV
  • Computer Terminal - Each workstation has two computer terminals to access the Compass System software. Typically, one terminal is used to notify the user of any system-detected events. The other terminal is used to access all other functional controls of Compass equipment.
  • Graphics Terminal - A colour graphics terminal allows the user to have an overview of traffic data relating to the entire freeway within system limit. Real-time traffic information such as traffic volumes, occupancy and vehicle speeds detected at various sections of the freeway can be displayed in a graphical format. In addition, current content on changeable message signs can also be shown instantaneously on the colour graphics terminal.
  • Monitor Banks - CCTV monitors are usually stacked together in a monitor bank and placed in the control centre directly in front of the operator workstations. The succession of monitors shows all available views of the freeway through the CCTV cameras.
  • Workstation Monitors - Two small monitors are located at the operator workstation. The operator can display and control any two of the available CCTV cameras on these monitors for close monitoring of incidents or special events.
  • CCTV Remote Control/Video Monitor Switch Keypad - The keypad is used to select the desired camera displays for the workstation monitors and to manoeuvre and control the cameras.
  • Video Cassette Recorder - A VCR is provided to allow the user to videotape any camera display especially during incident occurrence.
  • Wall Map - A complete wall map illustrating the configurations of the freeway within the system limit. The wall map indicates the locations of all system components such as CMS, VDS, and CCTV.

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Central Computer System

COMPASS Computer

The hardware configuration of the Compass Computer is a Digital VAX system with a LAN, a full backup system and associated peripherals. The software is written in a high level language and includes major modules for the various subsystems. The FTMS computer contains programs that coordinate and handle all the communications among the Vehicle Detector Stations Subsystem (VDS), Changeable Message Signs Subsystem (CMS), and Colour Graphics programs.

The computer system is the nucleus of Compass and its main functions are as follows:

  • Gather information, calculate parameters, make decisions, and output control commands.
  • Provide decision support for the Compass operators.
  • Perform automatic system operations in real-time with minimal human intervention.
  • Report system performance in printed form for permanent records as well as in display form to allow operator interaction with the system.
  • Control various kinds of systems, including signing and ramp metering rate, by turning on, turning off, or selecting equipment, based on control and decision logic programmed into the computer.
  • Maintain and store historical traffic data in different time intervals (20 second, 5 minute, 15 minute, hourly, daily, monthly) for future retrieval and analysis.
  • Perform hardware monitoring of all VDS, CMS, RMS equipment connected to the system and automatically alarm operators of equipment failure.

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Closed Circuit Television (CCTV)

Photo of COMPASS camera
Photo of COMPASS camera atop a post

CCTV is the video subsystem that provides live images between cameras located along the highway and monitoring equipment at the Traffic Operations Centre (TOC). Features such as zoom, pan, and tilt are achieved through the use of camera control units located at the TOC and at each camera location. The camera control signal is transmitted along the high speed communications subsystem.

The primary purpose of CCTV is to confirm incidents and facilitate incident management. CCTV cameras are normally mounted on the top of 15 m poles at approximately 1 km apart along the freeway. However, CCTV can also be mounted at vantage point on top of tall buildings to provide larger coverage areas. CCTV should provide almost 100% continuous video coverage of the freeway being monitored.

The information received from the CCTVs is primarily used to supplement and confirm the information received through the vehicle detector station subsystem and to provide information on local conditions which are affecting traffic flow. During off-peak periods or when traffic conditions are light, CCTV is probably the most effective tool to detect incidents since traffic flow may not be slow enough to be noted as a problem by the vehicle detector subsystem.

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Vehicle Detector Stations (VDS)

Photo of advanced traffic controller

Vehicle detector stations are the major elements of practically all freeway traffic management systems. Inductance loops are the most widely used detectors in freeway traffic management systems because of their reliability in data measurements and flexibility in design.

The main function of VDS is to detect the passage and presence of vehicles on the freeway. Data collected at vehicle detector stations is initially processed by a micro processor (Advanced Traffic Controller) located at the side of the highway. The processed data contains traffic volumes, vehicle speeds, occupancy, and vehicle length information. This information is then transmitted at regular polling intervals to the Central Computer System located at the Traffic Operations Centre via the communications subsystem. The computer system will use the data to monitor traffic patterns and identify traffic incidents as they occur.

Photo of diamond-shaped inductance loop

An inductance loop detector system basically consists of three components:

  • a loop embedded in the pavement consisting of multiple turns of wire;
  • a lead-in cable which connects the loop wire to the input of the loop detector amplifier;
  • a detector amplifier that intensifies the electrical energy produced by the detector loop.
Photo of vehicle detector video display

The loop detector shape may vary depending on its requirements:

  • diamond shaped loops are used in multiple-lane mainline sections because they are less likely to pick up vehicles on adjacent lanes;
  • rectangular shaped loops are used in freeway ramps where only one travel lane exists. The wide detection zone ensures detection of vehicles around the curves.

Vehicle detector stations that have a double-loop arrangement are specifically designed to measured vehicle speeds and lengths in addition to the traffic volumes and occupancy information. Stations with one loop per lane are capable of directly measuring traffic volumes and occupancy information only.

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Changeable Message Signs (CMS)

Photo of Changeable Message Sign

The CMS Subsystem is the primary FTMS interface with freeway motorists. The messages on the signs are controlled by the system operator and the content of the messages is based on the data received from the vehicle detector stations and CCTV camera subsystem. The messages generally advise the freeway motorists of the traffic conditions ahead and suggests alternative routing which is more time efficient and safer. The messages are displayed at specific locations in order to allow all the freeway motorists to have enough information to select a route when arriving at a decision point.

Each CMS is connected to a CMS controller located in the field cabinet. The CMS controllers are connected to the FTMS Central Computer at the traffic operations centre via the communications subsystem. Using the traffic data collected from vehicle detector stations, the central computer is capable of recommending specific messages on the appropriate CMS.

Photo of Changeable Message Sign

CMSs within the Highway 401 Compass System are primarily used to provide information to motorists with respect to collector/express diversion, diversion to and from connecting freeways, as well as advising motorists of adverse traffic conditions ahead. The sign display face is made up of 2 full matrix graphics panel with one on each end, plus 3 line matrices in the middle. The CMS messages are governed by the 3 line and 25 character per line limitation. The number of characters will be reduced by 5 for each graphics symbol panel being activated. A typical Highway 401 Compass System CMS has the following dimensions and specifications:

  • 13 metres wide
  • 3 metres tall
  • 0.5 metre deep
  • Each sign weighs approximately 3000 kg
  • The display face is composed of two 35 pixels by 35 pixels full matrix panels on each side plus 3 lines of line matrix in the middle with 105 pixels by 7 pixels each. The total number of pixels on each CMS is 4655.
  • Each pixel is made up of high intensity amber LEDs (Light Emitting Diodes).

CMSs in Mississauga and Burlington Compass Systems are primarily fibre-optic/flip disk technology. This type of CMS works on the principle of exhibiting the message via the light from quartz bulbs which are recessed behind the fibre optic tubes in the sign and by the reflection of car headlight or sunlight. The flip disks have fluorescent tape on one side and black on the other. When a message is exhibited on the CMS, the appropriate disks flip so that the fluorescent side faces the motorists. The disks which are not flipped will have the black side facing out and are designed to block the light from the fibre optic tubes from being visible to the motorist. There are a combination of 2 and 3 lane signs that vary from 20 to 22 characters in width.

Unique to the Burlington and Mississauga Compass system are two special walk-in CMSs. The benefits in providing a sheltered environment for maintenance staff to work within the sign are so overwhelming that all future CMSs being implemented in Ontario will be walk-in type.

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Ramp Metering Stations (RMS)

Photo of Ramp Metering Station

Metered ramp traffic is controlled by a traffic signal at some point on the ramp upstream of the freeway merge point. Traffic waiting to enter the freeway queues at the signal. The rate at which traffic is allowed to enter the freeway is determined by freeway traffic conditions measured at the mainline detector stations and the number of vehicles waiting on the ramp.

A ramp metering station comprises the following components:

  • Stop Bar - to show vehicles where to stop. It is located far enough from the end of the ramp to provide sufficient acceleration distance to ensure smooth merging procedures.
  • Passage Loop - to monitor vehicles released from queue.
  • Demand Loop - to determine vehicular demand at ramp signal.
  • Ramp Control Signals - two sets of signal heads facing upstream, one on either side of the ramp, downstream from the stop bar.
  • Storage area - to ensure that the maximum queue expected will not interfere with the arterial traffic.
  • Queue Loop - to monitor the growth of the queue and act as a trigger to override RMS stop signal. The intent is to prevent the queue from interfering with traffic on the arterial.
  • Mainline VDS - to provide data on lane occupancy on the freeway upstream of the entrance ramp in order to determine appropriate ramp metering rates.
  • Equipment Cabinet - to house control equipment for ramp signals and detector loops.
  • Ramp Metering Warning Sign - to warn drivers of ramp metering.

Ramp metering facilities for traffic entering the freeway from arterial roads are designed to control the rate of traffic entering the freeway. The objective is to maintain a predetermined level of service on the freeway by adjusting traffic volume on either an isolated ramp or a system wide basis. Typical waiting times at ramp metering signals are between 5 to 6 seconds per vehicle.

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Field Provisions Subsystem

The field provision subsystem consists of the duct work for the communications and other cables, general power supply, cabinets and cabinet bases, field traffic controllers, loops, loop sensor units, footings for CMS support structures, CCTV poles and maintenance areas.

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Communications (Fibre-Optic)

Photo of woman looking into microscope

The Highway 401 Compass System in Toronto relies exclusively on fibre-optic cable for its communications between the central computer system and all local field controllers. Fibre optics has an inherent immunity to interference. It also has a much larger bandwidth compared to co-axial, which leaves more room for system expansion.

Data communication is typically full duplex, operating asynchronously at 9600 bits per second. Communication between controllers and the FTMS Computer uses a poll/response protocol over multiple access communication circuits. The camera control transmitter and the FTMS Computer provide the access protocol to the communication circuits.

The data communication system is a two-tier system with a high speed backbone Data Ring Network supporting low speed (9600 bits per second) distribution from drop and insert nodes.

Photo of optical fibres

The high speed system consists of nodes interconnected by optical fibres. The optical fibres originate at the TOC and loop across the highway at the geographical ends of the FTMS, thereby creating Data Ring Networks. The communications subsystem is a high speed data ring interfacing to fibres installed in previous contracts. The nodes operate in a dual ring configuration so that each data ring network can maintain full connectivity between nodes and the TOC in the event of a cable, fibre or node failure.

The nodes drop and insert data from preselected time slots in a high speed time division multiplexed data aggregate channel and provide physical access to these time-slots from data channel ports connected to the node. Distribution from the data channel ports to field controllers is either local or through a combination of port sharing devices, fibre optic modems, fibre optic splitter and fibre optic taps. Port sharing devices allow multi-drop low speed circuits to share a data channel port. Fibre optic modems allow the creation of a multi-drop fibre optic low speed circuit or point to point fibre optic low speed communication.

The advent of the Advanced Traffic Controllers (ATC) to replace the current Type 170F controllers will remove the requirements to communicate at low speed (9600 bits per second). A study is under way to look at a new high speed communications system.

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Communications (Coaxial)

Photo of coaxial cables

Coaxial cables are high frequency transmission cables that consist of a centre copper core surrounded by insulation and then a grounded shield of braided wire. The coaxial cable has the capacity to transmit information 80 times faster than the standard twisted pair.

The QEW Mississauga and Burlington Compass systems utilized a broadband coaxial system for communications between the central computer system and all field controllers. The coaxial communication system is configured in a "sub-split" design using 5 to 30 MHz as the outbound bandwidth (TOC to remote) and 50 to 300 MHz as the inbound bandwidth (remote to TOC).

The amplifiers are designed to handle duplex communications for data, voice and television signals within various bandwidth configurations. The control signal data originates at RS-232-C data ports at the TOC and at field equipment stations. The data signals are modulated and demodulated at the appropriate RF transmit/receive frequencies via data modems with the communications subsystem technically transparent to the data protocol.

The television signals are unidirectional towards the TOC and are amplitude modulated and up- converted onto television channel frequencies. The data system includes field modems driving camera control receivers(CCTV), sign controllers (CMS), traffic controllers (VDS, RMS) and master modems in the TOC interfacing with the computers or master controllers. Outbound data for polling or control of field equipment is generally in a low frequency band (near 30 or 120 MHz). Inbound data for reporting field data is generally in a higher frequency band near the CCTV channel frequencies.

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