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Overlap Phasing

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### Signal Operations

With signalized intersections, there is a lot of terminology that must be known before much analysis or design can take place. The best place to begin explaining this terminology is with the concept of phasing, which covers phase assignments, definitions, and phase diagrams.

Phase Assignments

Intersection with NEMA phase labels

In the illustration above, we have a four way intersection with through lanes and separate left turn lanes. The numbers on the diagram each correspond to a movement, and those numbers are set by the National Electrical Manufacturers Association (NEMA). There are also numbers assigned to right turns and pedestrians, but we are not worrying about those here.

When intersections are signalized, that is, stop lights are installed, movements are often lumped together to run at the same time. These sets of movements are known as phases. There may be more than one movement served in a phase, but at least one NEMA movement number must be assigned. Two approaches can be taken to the assignment of NEMA movement numbers.

It is common to set the phase number equal to the lowest through movement number in the phase. For example, if all of the north-south movements were given the right of way at the same time, that could be one phase (diagram at right). Since the movements involved are 1, 6, 2, and 5, and 2 and 6 are through movements, the phase would be labeled as phase 2. The movements can also be assigned to separate phases, giving the south bound and north bound traffic separate phases, giving the through and turning traffic separate phases, or even giving each movement its own phase. For simplicity, we will refer to individual movements as "movement X", and phases as "phase X"

It is important to note that only some of the movements can safely go at the same time which is why we regulate these movements with traffic signals. Using signal indications that are changed in intervals, traffic signals can prevent conflicting movements from having the right of way at the same time. A signal indication is any one of the traffic lights at a signalized intersection and an interval is a time during which none of the indications change. Right-of-way is assigned using these signal indications, where a red light means that no right-of-way is given and a green light indicates that right-of-way is given. Note that it is not quite that simple, especially when dealing with left turning traffic, where a green ball indicates that left turn traffic may proceed but they must yield to opposing through traffic.  When this situation occurs, it is said that the movement is permitted.

To solidify how intervals and indications work at a signalized intersection, refer to the following table. The table shows a signal plan for the intersection at the top of the page. The time is the length of the interval, and the R (Red), G (Green), and Y (Yellow) refer to the indication during the specific interval. This table assumes that the eastbound and westbound left turn traffic has a left turn arrow and that the northbound and southbound left turn traffic does not.

 Interval Time (sec) NBLT NBTH SBLT SBTH WBLT WBTH EBLT EBTH 1 12 R R R R G R G R 2 4 R R R R Y R Y R 3 1 R R R R R R R R 4 25 R R R R R G R G 5 4 R R R R R Y R Y 6 1 R R R R R R R R 7 14 G G G G R R R R 8 3 Y Y Y Y R R R R 9 1 R R R R R R R R

The first interval in this table shows that the westbound and eastbound left turn movements have green left turn indications. All of the other traffic movements have red indications.After 12 seconds, the westbound/eastbound yellow left turn indication comes on to communicate to traffic that their turn for right-of-way at the intersection is about to end.As before, all of the other traffic movements have red indications. In interval three, all of the indications are red to provide time for the intersection to clear before the next phase has the right of way. This process of going from one interval to the next repeats itself over time. When the last interval is reached the signal controller returns to interval one, repeating the sequence of intervals over again.

Definitions

Now it is time to put forth some definitions using the table, figure and discussion above.

• Cycle: A cycle is a complete sequence of intervals. In the example provided, the sequence would go from interval one to nine and then another cycle would start with interval one.

• Cycle length:A cycle length is the time it takes to complete one cycle. In the table this would be the sum of the interval times, or 65 seconds.  The minimum time for a cycle length is generally 45 seconds, to limit the time lost starting and stopping traffic.

• Phase:A phase is the part of the cycle assigned to a fixed set of traffic movements. When any of these movements change, the phase changes. The first phase in the above table is comprised of intervals one, two, and three. The third interval is included even though no traffic movements have the right-of-way.

• Yellow Change interval:This is an interval in which yellow indications tell drivers in the phase with the right-of-way that their movement is about to lose its right-of-way. An example of this is interval five.

• Red Clearance interval:This describes the interval when all of the indications are red and is a safety measure designed to give the oncoming traffic enough time to clear the intersection before the next phase begins. An example of this is interval six.

• Intergreen time: This is the summation of the time allocated to the change and clearance intervals for a given phase (yellow and all red time).

Phase Diagrams

In putting together movements to make a phase, it is important to remember that not all movements can be put together. There are some movements that just cannot go at the same time. For this reason we put together phase and ring diagrams. A phase diagram is a diagram which groups movements into phases and each phase is shown in a single block. As we discussed above, this intersection has three phases. Let us consider the phases as Phase A, Phase B and Phase C as shown below.

Phase Diagram

If the phases change as a block, as is often the case in pretimed signals, the phase diagram is adequate to ensure that no conflicts arise However, the movements do not necessarily change as a block. For this reason, we have ring diagrams. Ring diagrams with barriers ensure movements do not conflict, thus ensuring driver safety. The ring and barrier structure for a signal with 8 movements is shown below.

Ring Diagram

Possible Four Phase Diagram

The two rings operate independently except that their control must cross the "BARRIER" at the same time (i.e. movements on the left side of the barrier must all be terminated before movements on the right side of the barrier can begin). All of the movements from one street must be assigned to the left side of the barrier and all of the other movements from the other street must be assigned to right side.

On both sides of the barrier there are four movements, two through movements and two left movements. A movement in a given barrier may be served simultaneously with either of the movements in the other ring, but can not be served with the movement that is in the same ring. The figure below shows the possible scenarios for both sides of the barrier.>

Compatible Movements

In summary, there are two basic rules that govern phasing using the dual ring diagram and they are listed below.

1. Movements from opposite sides of the barrier can not be served simultaneously.

2. Movements from the same ring can not be served simultaneously.

These rules can be used for defining phases for fixed time control and for governing the operations of actuated controllers, which is discussed in the actuated signals module.

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