The goals in coordinated signal timing are improved safety, minimal delays, minimal stops, and minimal inconveniences for all roadway users in an equitable manner. These efforts also minimize air pollution and fuel consumption associated with vehicle travel. Signal timing, whether for a single intersection or a coordinated group of intersections, is a complex task. For example, the following are a few of the items that are considered.
- Traffic volumes in multiple directions and for multiple movements must be considered. These volumes may vary by time of day, day of week, and as overall travel in the area changes.
- The ability to coordinate signals may be significantly impacted by the spacing of the signals and the cycle length(s) used in the system. This coordination requires that the signals have the same cycle length or be multiples of the same cycle length. For example, if the system is primarily operated on a cycle length of 120 seconds, in some cases a 60 second cycle length may work best at some locations. Typically, there are a few critical intersections that require a particular cycle length and the others in the system must be compatible to allow coordination. It is rare that the same cycle length is optimal for all locations in the system.
- The impact of vehicle queues must be considered. For example, it is desirable to avoid vehicle stacking that blocks lanes.
- Pedestrian crossing times may be a critical component of the timing. A signal may need to give more time than is required for the vehicles on an intersection approach in order to allow pedestrians to cross the street safely.
- Not all drivers drive the same speed on a roadway, and speeds may vary as vehicle demands use more of the available roadway capacity.
- Construction projects may disrupt the coordination between signals by altering the speed of traffic, reducing the number of lanes, etc.
- Some movements (typically from lower volume side streets or driveways) may have alternate routes available and some may not. Those without other options may be more critical to the signal timing.
- Where a vehicle enters the system may impact when it is able to incorporate into a platoon of other vehicles.
- Vehicle platoons may be difficult to serve at the critical intersections that must give less green to the main streets.
- There are times when the congestion and delays to motorists may actually be worse when signals are coordinated. This is based on many of the items above. For example, large distances between signals may make coordination among signals less desirable, especially if the optimal cycle lengths of the signal are considerably different.
In our coordinated systems, direct observations, traffic counts, driving and recording delays and stops, monitoring of the green phases for individual traffic movements, specialized computer software, and physical characteristics of the roadways are used to assist in timing the signals. There are specific points in the cycle of the coordinated signals that are set. For example, the beginning of the yellow for the main street through movement is typically set to a specific time in the cycle for each signal. However, the green times for individual movements in the cycle may vary within certain limits based on second-by-second detection of vehicle and pedestrian demands.
Staff uses specialized software for developing signal timing. In addition, field reviews include direct observation of flows and, in some cases, collection of data on traffic speeds, delay, locations, etc. along the arterials. Machine and manual traffic counts are also beneficial.
Essentially, each traffic signal has a computer that contains its timing, adjusts for detected vehicles and pedestrians, monitors the lights to prohibit conflicting movements, and many other functions. Most of the coordinated signals are also connected to a central computer that provides for keeping the clocks consistent, assists in data collection, and other functions.
Of course it is generally impossible to have all lights green for all drivers at exactly the time they would like it. Even when timings are generated that attempt to "optimize" the intersections and systems, it must be recognized that the traffic conditions are not static. Also, it is not unusual for lights to be retimed with reduced motorist delays and congestion, but for the agency to only receive complaints. Any timing adjustments give some drivers more green and others less green. Typically, only those with less green will contact the agency. It is sometimes amazing that such a complex systems of computers, vehicle detection, etc. works so well when it must operate 24 hours a day under a wide variety of weather and traffic conditions.
Traffic Operations Center (TOC)
The TOC in the Development Services Building provides a centralized control of the signals that are interconnected through the signal communications system. Traffic Division staff are also connected to the TOC through individual computers that allow work on the system from their offices.
The use of a centralized traffic system allows monitoring of intersection operations in detail. Timings can be changed remotely; the history of individual green times for each movement can be recorded and analyzed; and the relationships among signals in coordinated groups can be monitored. Alarms are also passed through the system to alert technicians of the need to respond to field conditions, resulting in quicker response. For example, power losses, signal going into flash mode, and railroad preemptions may be viewed.