Traffic Signal Definitions
To develop new state transition logic, it is necessary to have a set of definitions that are both general in application and specific in detail. “State Transition” is defined as the process that determines the change from one interval to the next interval. The following are offered for the purposes of developing an understanding of present worldwide practice and future possibilities. Most notable in this approach is the consistent use of terminology. The definition of phase is NOT intended to connote how movements are serviced. The current “standard” NTCIP definitions are presented at the end for comparison.
STATE TRANSITION PROCESS
There are at least two aspects to state transition. The first is to determine the sequence of movements. Sequence implies as set of procedures (or more commonly rules) by which a selection of the next movements to be serviced is determined. Second, the duration of service needs to be determined. This may or may not be tied to the order of movements. The duration can be determined by either simple inputs (e.g., vehicle actuations) or complex procedures that consider priorities among competing requests.
In order to understand competing frameworks, a set of
definitions is proposed below. These definitions
include standard concepts applicable to all approaches (e.g., the definition of
an indication), and those definitions unique to particular approaches (e.g.,
rings and barriers in
FUNDAMENTAL CONCEPTS
The following terms are fundamental, not differing in meaning
between
MOVEMENT
Movements reflect the user perspective. Movements can also be broken down into classes (car, pedestrians, buses, LRT, etc.). Typical movements are left, through and right.
Movement is an activity in response to a “go” (green ball, green arrow, walk, white vertical transit bar) indication. Typical movements include left turns, through traffic, right turns, pedestrian crossings, and others. A green ball typically serves a through and right movement. It may also service a permissive left turn movement depending on the type of signal group. In addition, it may also accommodate pedestrians in the absence of pedestrian indications. Movements may be exclusive (e.g., LRT on its own right of way), or shared (e.g., trucks, buses and cars using the same lane). Movements may also be protected (i.e., have the designated right of way), or permissive (i.e., they must yield to other movements because they do not have the right of way and therefore conflict with another movement). Movements can have priorities based on the class of vehicle requesting service.
Movements can share one indication (e.g. buses and cars share green ball if bus travels on the same lanes as cars) or have several indications associated with them (e.g. exclusive left turn with green ball and green arrow).
INDICATION
Any kind of signal used to present information to the road users in order to control movements. The most common form of signal is a red, yellow or green indication. However, a walk signal is an indication as is a LRT white bar signal. An audible pedestrian signal is also indication.
The state of indications can be steady, flashing, variable, or audible. The “countdown” pedestrian signal face is an example of a variable indication. Multiple indications may be controlled by a single phase.
There are several classes of indications:
- Vehicular Traffic
- Protected
- Permitted
- Dynamic Lane Assignment
- Emergency Vehicles
- Confirmation Beacons
- Warning lights (red revolvers)
- Light Rail Indications
- Transit Priority Indications
- Audible pedestrian
- Ped buttons that provide a confirmation indication that the call has been latched by controller
STATE
The state of an indication can be
either active or off. When active, an
indication can be steady, flashing, variable, or audible.
The following are considered to be the
smallest intervals for specifying indications:
- Green, yellow and red (arrows and balls)
- Walk (including audible walk)
- Flashing Don’t Walk (including count-down indication and/or audible don’t walk)
- Flashing Beacon Active (Prepare to stop when flashing sign)
- Sign Indication Active (No Left turn, changeable lane designation..)
SIGNAL GROUP
A combination of indications (e.g., red, yellow, green, green arrow, audibles, etc.) grouped together for controlling one or more movements.
INTERVAL
The duration of time during which the indications do not change their state (active or off). In the pedestrian signal above, the flashing don’t walk interval is the time during which the flashing hand is displayed and the count down indication active. Typically, one or more timing parameters control the duration of an interval. The pedestrian clearance interval shown is determined by the pedestrian clearance time. The green interval duration is controlled by a number of parameters including minimum time, maximum time, gap time, etc.
PHASE
A timing unit associated with the control of one or more indications. A phase may be timed considering complex criteria for determination of sequence and the duration of intervals.
Note: Pedestrian timings may be associated with vehicle timings or may be provided in a separate phase.
DURATION
The total length of time a phase is active including yellow and all red times.
EVENT
The point in time that a phase or phases begin or end.
CALLS
A request for service. Separate classes of movements may have separate calls. For example, a priority call is one where the normal phase timing may be altered due to a particular class of movements.
SEQUENCE
The order of phases. The order of phases may be fixed or dynamic. The normal sequence is the order of phases with only non-priority calls.
CYCLE TIME
The duration of a complete sequence of phases in the absence of priority calls. In an actuated controller unit, a complete cycle is dependent on the presence of calls for all non-priority phases. Some indications may be serviced more than once in a cycle. Occasionally, an indication may not be part of a normal cycle (e.g., a left-turn arrow may only be displayed during railroad preemption).
OVERLAP
An overlap allows a signal group to be controlled by one or more phases. For example, when the overlap is the result of two conflicting phases, the overlap handles the issue that the overlap does not display a clearance indication between the phases if they time sequentially.
PARENT PHASE
A phase or phases used to determine if an overlap should be active. Parent phases may be in different rings or may be on different sides of a barrier.
LEADING OVERLAP
A signal group that starts before the parent phase. An example application of a leading overlap is for the control of “Prepare to Stop when Flashing” signs.
TRAILING OVERLAP
A signal group that ends after the parent phase. An example application of a trailing overlap is to provide interior clearance at two “offset T” intersections.
U.S.
Modern
RING
A sequence structure consisting of two or more sequentially timed and individually selected conflicting movements so arranged as to allow flexibility in the relationship between compatible movements in different rings.
BARRIER
A reference point in the sequence in which 2 or more rings are interlocked. Barriers assure there will be no concurrent selection and timing of conflicting movements in different rings.
Preemption
Per NTCIP 1202 Version 2, the transfer of the normal control (operation) of traffic signals to a special signal control mode for the purpose of servicing railroad crossings, emergency vehicle passage, mass transit vehicle passage, and other special tasks, the control of which requires terminating normal traffic control to provide the service needs of the special task. For the purposes of this project, preemption is unchanged.
Priority
Per NTCIP 1211 (Draft 33), the preferential treatment of one vehicle class (such as a transit vehicle, emergency service vehicle or a commercial fleet vehicle) over another vehicle class at a signalized intersection without causing the traffic signal controllers to drop from coordinated operations. Priority may be accomplished by a number of methods including the beginning and end times of greens on identified phases, the phase sequence, inclusion of special phases, without interrupting the general timing relationship between specific green indications at adjacent intersections
OTHER PRACTICES
The following are concepts from non-US practice.
STAGE
A combination of non-conflicting movements controlled by a single phase that can be sequenced in a single ring type operation to produce a complete cycle. In US practice, “split phasing” would be analogous to a two stage operation.
PHASE BASED CONTROL
Each individual phase is a separate main phase candidate for being active. Subordinate phases are selected based on their inclusion in the timing strategy for the main phase and compatibility with other subordinate phases that have already been selected. Various criteria can be used for selecting the next phase including timeframe to be served and priority to select the next phase. In order to maintain coordination, the phase selection must generally be consistent with a system cycle length.
REFERENCE DEFINITIONS
The following are current NTCIP 1202 definitions.
Barrier: A barrier (compatibility line) is a reference point in the preferred sequence of a multi-ring CU at which all rings are interlocked. Barriers assure there will be no concurrent selection and timing of conflicting phases for traffic movement in different rings. All rings cross the barrier simultaneously to select and time phases on the other side.
Cycle: The total time to complete one sequence of signalization around an intersection. In an actuated controller unit, a complete cycle is dependent on the presence of calls on all phases. In a pretimed controller unit it is a complete sequence of signal indications.
Interval: The part or parts of the signal cycle during which signal indications do not change.
Overlap: A Green indication that allows traffic movement during the green intervals of and clearance intervals between two or more phases.
Phase Sequence: A predetermined order in which the phases of a cycle occur.
Phase, Active: The indicated phase is currently timing. A phase is always active if it is Green or Yellow (Walk or Pedestrian Clear for Pedestrian Phases). It is also active if it is timing Red Clearance. It may be considered active during Red Dwell.
Phase, Conflicting: Conflicting phases are two or more traffic phases which will cause interfering traffic movements if operated concurrently.
Phase, Nonconflicting: Nonconflicting phases are two or more traffic phases which will not cause interfering traffic movements if operated concurrently.
Phase, Pedestrian: A traffic phase allocated to pedestrian traffic which may provide a right-of-way pedestrian indication either concurrently with one or more vehicular phases, or to the exclusion of all vehicular phases.
Phase, Traffic: Those green, change and clearance intervals in a cycle assigned to any independent movement(s) of traffic.
Phase, Vehicular: A vehicular phase is a phase which is allocated to vehicular traffic movement as timed by the controller unit.
Ring: A ring consists of two or more sequentially timed and individually selected conflicting phases so arranged as to occur in an established order.
Sequence, Interval: The order of appearance of signal indications during successive intervals of a cycle.