41.2. Definitions and notations

A signal cycle is one complete rotation through all of the indications provided.

A signal cycle represents the complete set of signals (like red, yellow, and green) that a traffic signal can display. When a signal goes through all its color phases for each direction of traffic, this constitutes one full cycle. This concept is crucial in understanding timing and coordination in traffic signal design.

Think of a traffic signal cycle like a musical piece where each section plays a different part—when the green light is on, it’s like a joyful melody playing for cars to move, followed by a cautionary yellow note before the red light signals a stop.

Cycle length is the time in seconds that it takes a signal to complete one full cycle of indications. It indicates the time interval between the starting of green for one approach till the next time the green starts. It is given by the symbol 'C'.

Cycle length represents the total time it takes for one complete series of signal changes. This period includes all light changes from green to yellow to red and back to green for the same lane. Understanding cycle length is vital for traffic engineers as it determines how often vehicles can proceed through an intersection.

Imagine you are on a rollercoaster. The cycle length is like the time it takes the ride to go from the highest point, down to the lowest, and back to the starting point again. Just like how thrill-seekers want more rides, drivers want shorter cycle lengths for quicker movement.

Thus it indicates the change from one stage to another. There are two types of intervals: change interval and clearance interval. Change interval, which is also called the yellow time, indicates the interval between the green and red signal indications for an approach. Clearance interval, which is also called 'all red', is included after each yellow interval indicating a period during which all signal faces show red and is used for clearing off the vehicles in the intersection.

Intervals are specific time segments within the cycle that manage traffic flow. The change interval (yellow light) warns drivers to stop safely as the signal is about to turn red. The clearance interval ensures that all vehicles have left the intersection before any new movement is allowed. Both intervals are crucial for preventing accidents and ensuring smooth transitions in traffic.

Consider a play where the yellow light is like a stage hand signaling the actors to prepare for their next move. The clearance interval acts as the curtains closing to prevent any stragglers from staying on stage before the new act begins.

Green interval: It is the green indication for a particular movement or set of movements and is denoted by G.i Red interval: It is the red indication for a particular movement or set of movements and is denoted by R.i

The green interval allows vehicles to move, while the red interval indicates when they must stop. Each interval is essential for regulating traffic flow and ensuring safety at intersections. The appropriate lengths of these intervals are calculated based on traffic volume and safety requirements.

Imagine a game of 'Red Light, Green Light.' The green light means you can run forward (your green interval), while the red light means you must freeze in place (your red interval). Just as the game leader controls when players can move, traffic signals control vehicle movements.

A phase is the green interval plus the change and clearance intervals that follow it. Thus it is the assigning of conflicting movements into separate groups. It allows a set of movements to flow and safely halt the flow before another set of movements.

A traffic phase combines multiple intervals (green, change, and clearance) for specific sets of movements, ensuring that conflicting traffic flows do not occur simultaneously. Phasing is essential in managing traffic and ensuring safe crossings at intersections.

Think of phases as different lanes at a relay race where each runner cannot start until the previous one has completed their leg. Just like runners need their designated time to prevent collisions, traffic phases ensure vehicles move safely without interference.

It indicates the time during which the intersection is not effectively utilized for any movement. For example, when the signal for an approach turns from red to green, the driver of the vehicle which is in the front of the queue will take some time to perceive the signal (usually called reaction time) and some time will be lost here before he moves. That reaction time is referred to as lost time.

Lost time refers to wasted time during signal changes when vehicles are unable to move. This includes the reaction time needed for drivers to respond to the signal change. Minimizing lost time is critical for efficient traffic flow and signal performance.

Imagine you're at a concert waiting for your favorite performer to take the stage. When the lights change and the cue to move is given, there's a moment of hesitation and cheering that delays entry. That pause is 'lost time,' similar to what drivers experience at a traffic signal.

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