Traffic signals maintain a level of order and control on America’s roadways. Children on their way to school wait for the traffic signal to give them the okay before they cross the busy road to their waiting buses. Pedestrians use traffic signals to help them safely cross a through-fare teeming with speeding cars. Drivers place their own safety and the safety of their passengers in ability of a traffic signal to allocate the right-of-way at any given time.
Traffic signal integration accomplishes the following objectives:
- Reduces any delays caused by pedestrians and opposing lines of traffic
- Reduces the likelihood of accidents at intersections
- Improves traffic flow on to and off of the main road
- Reduces travel times by synchronizing traffic flow for maximum effectiveness
- Regulates the amount of traffic on the roadway at any given time
Traffic signal controls operate by regulating the flow of traffic between intersections to prevent accidents and to ensure smooth integration between opposing and perpendicular routes. The signals ensure that the available roadways are used as effectively and safely as possible. Traffic signals are composed of four main components: the display, the controllers, the detectors and the supports.
The display is what we call the actual “traffic light”. In the United States, the order of color from top to bottom is red, amber and green. In the typical traffic light, the circular color indicator is a foot in diameter, over three feet tall and can weigh up to 50 pounds.
The signal controller is the brains behind the light and is responsible for coordinating the flow of conflicting traffic streams in a process known as phasing. Technological advancements now permit controllers to use real-time information gathered from roadway sensors and cameras to determine the appropriate amount of green time for each traffic stream. The most sophisticated controllers use software programs to minimize stops and delays at adjacent intersections.
Loop detectors are buried beneath the road and are often defined by the square black lines visible when approaching an intersection. The loop detector is a thin wire that sends a signal to the controller when a vehicle drives over it. The software in the controller analyzes the information from all the loops of that particular intersection and adjusts the signal timing as needed.
Although they have certain advantages, traffic signals can also have their drawbacks:
- They require frequent maintenance to repair broken lights, faulty wiring or malfunctioning equipment.
- They require routine monitoring to ensure that they are regulating the traffic in the most effective way possible without causing bottlenecking or unnecessary delays.
- They can create delays during low-traffic times that may cause disruption in traffic flow.
- A signal breakdown due to mechanical malfunction or loss of electricity can lead to serious interruption in traffic flow and possible accidents.
- Rear-end collisions are more likely when drivers make sudden stops to respond to a changing traffic signal.
Synchronization and Timing
As with other types of highway engineering, the decision of where to place traffic signal controls and how to synchronize them is key to ensuring a safe and effective means of traffic control. The ratio of demand to capacity for a given flow path plays perhaps the most important role in determining the setting of the traffic signal. The capacity of a specific flow path is determined by its saturation flow. A saturation flow is defined as the maximum possible traffic flow capable of crossing the intersection at the speed limit with a green time of 100 percent.
Most of the time traffic signals operate on a fixed-time sequence. Fixed-time sequences allow the same percentage of green time to each approach to the intersection per cycle regardless of traffic conditions. A cycle is defined as one complete sequence of traffic signal operation. Some fixed-time traffic control systems use different cycle lengths based on the time of day, e.g. rush hour.
Actuated signal controllers use loop detectors to determine the appropriate amount of green time for each cycle. Actuated signal controllers are usually located at intersections where traffic volume fluctuates considerably or when access to the main road must be maximized. Coordinated network controllers maintain traffic flow through a series of signals set at pre-determined speeds to minimize stops and delays. The computer systems within the controllers monitor the flow of competing lines of traffic to provide a smooth flow in two or more lines of traffic.