We all remember those days before the turn of the millennium when the Y2K bug was expected to wreak havoc on our world. We imagined chaos overtaking us as computers failed to keep order. We pictured traffic lights failing, leaving streets a mess of cars unable to negotiate busy intersections without guidance (a scenario that came true during the August 2003 blackout), and forced us all to navigate streets rammed with hapless motorists and pedestrians.
With this thought in mind, it is a wonder that we don't think more about how things like our traffic lights work. We come face to face with them every day, and though they often prove to be frustrating, slow, even inconvenient, we rely on them to keep us moving and moving safely.
There are three types of traffic signals in use today: Fixed-Time signals, Semi-Actuated signals, and Fully-Actuated signals. Fixed-Time signals are normally installed at the intersection of two major roads. These signals cycle in sequential order and do not rely on the detection of peripheral traffic such as pedestrians. Semi-Actuated signals are usually installed at the intersection of a major road and a minor road. The light remains green on the major road until a vehicle or pedestrian is detected on the minor road. Waiting vehicles are detected by electromagnetic wires embedded under the pavement near the stop bar, and function like metal detectors, sensing the metal of the stopped car above. Pedestrians are detected through pushbuttons. Fully-Actuated signals rely solely on the detection of currents of traffic, and sequence themselves based on actual traffic conditions.
Toronto uses two control systems for its traffic signals. A total of 83% of Toronto's signals are controlled by a central computer called MTSS (Main Traffic Signal System). This computer provides different timing plans for different times of day and allows transportation staff to monitor its operation. The rest of Toronto's signals are on the SCOOT (Split, Cycle and Offset Optimisation Technique) system, which is a demand-responsive urban traffic control system. It relies on the use of loop vehicle detectors located on all approaches to the intersection. The data produced is processed by a central coordinating computer, which can alter the traffic signals at a given intersection. This system allows the signal timings to change frequently depending on traffic flow, so that they cater to current traffic conditions.
Metro Transportation undertook a study of its SCOOT system in May and June 1993. It found on average that there was a reduction in journey time by 8%, of delays by 17%, and of stops by 22%. Fuel consumption also was reduced by 5.7%, begging the question: why doesn't Toronto use the SCOOT system more widely?
The answer is probably the cost. Since these systems can be prohibitively expensive, they just aren't practical. However, Toronto is taking steps to reduce the operating costs of its traffic lights by switching over to LED (Light Emitting Diode) technology for its lights at intersections. A 2003 pilot project using the new light technology at ten Toronto intersections found the new lights resulted in an 84% reduction in energy use. Over the next eight years the city plans to switch all 1,900 traffic and pedestrian signals to LED, saving $1.8 million in energy annually.
Some things to think about the next time you're waiting to cross the street.