Traffic controller methodologies nema based vs interval based
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Signal Group University. Traffic Controller Methodologies NEMA-based vs. Interval-based. A lesson to attain a basic understanding of the two traffic controller programming methodologies used worldwide. NEMA.

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Traffic Controller Methodologies NEMA-based vs. Interval-based

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Traffic controller methodologies nema based vs interval based

Signal Group University

Traffic Controller MethodologiesNEMA-based vs. Interval-based

A lesson to attain a basic understanding of the two traffic controller programming methodologies used worldwide.

SGU- NEMA vs. Interval


Traffic controller methodologies nema based vs interval based

NEMA

  • NEMA stands for the National Electrical Manufacturers Association. It is a volunteer organization of manufacturers. It is NOT a government body. Section 3 of NEMA is designated as Traffic Systems, otherwise known as NEMA 3-TS.

  • Before 1989, there were no significant national traffic controller standards. NEMA published TS-1 (1989) that described a standard traffic controller for the general US market. Multiple revisions of TS-1 and subsequently TS-2 have been released and maintained by NEMA over the past 20 years.

  • NEMA TS standards are voluntary and are not required that agencies use them in their own specifications.

SGU- NEMA vs. Interval


Nema based

NEMA-based

  • A NEMA-based traffic controller meets certain hardware requirements, but also uses certain methodology and terminology common to the NEMA TS standards.

  • NEMA is often referred to as “phase based”, but this is not an entirely accurate way to look at it because all controllers use phases, not just NEMA controllers.

    • Note: The term “phase” is universally used as interchangeable with “vehicle movements”. (ex. A northbound through phase, or a eastbound left-turn phase).

  • A better description of NEMA is that it is phase “focused”. This is because the timings for a particular phase can be programmed independently for that phase and not dictated by some other grouping in which that phase belongs. (see next slide for further explanation)

SGU- NEMA vs. Interval


Nema phase focused

Possible Green Extension

Minimum Green Time

Amber

Time

Step Time

Maximum Green Time

NEMA phase “focused”

  • The green portion of NEMA phases are typically defined with a minimum time and a maximum time that can be extended up to the maximum in incremental “steps”.

  • The amber portion is a fixed length parameter.

  • If a phase is not green or amber, it is considered off (i.e. Red).

SGU- NEMA vs. Interval


Nema phase focused1

Φ1

Cycle Length

Φ2

Φ3

Φ4

Φ5

Φ6

Φ7

Φ8

NEMA phase “focused”

  • Each phase can be variable based on the parameters set for each individual phase.

SGU- NEMA vs. Interval


Nema sequencing

Φ1

Φ2

Φ4

Φ3

Φ1

Φ2

Φ6

Φ5

Φ4

Φ3

Φ8

Φ7

NEMA Sequencing

  • NEMA controllers use a ring structure to determine the sequence in which phases are activated, or “served”. It also uses this structure to determine compatibility between phases.

    • Compatibility simply means which phases can be on (green or amber) at the same time without causing an unsafe traffic situation.

    • NEMA allows multiple simultaneous rings. Think of them as stacked donuts.

SGU- NEMA vs. Interval


Nema compatibility

Φ4

Φ7

Φ6

Φ1

Φ5

Φ2

Φ3

Φ8

NEMA Compatibility

Common NEMA 8-Phase, 2 Ring Configuration

Concurrency

Group

Concurrency

Group

  • General Ring Compatibility Rules

  • Phases in same ring are NOT compatible (i.e. Φ1 and Φ2)

  • Phases in separate rings ARE compatible (i.e. Φ1 and Φ5, Φ6)

  • Phases are not compatible with phases in another concurrency group (Φ1, Φ2, Φ5, Φ6 are NOT compatible with Φ3, Φ4, Φ7, Φ8)

SGU- NEMA vs. Interval


Nema highlights

NEMA Highlights

  • So, basically, a NEMA-based controller sequences through the phases in a ring (or in parallel rings), while determining what phases can be on with what other phases.

  • As long as phases follow the correct sequence and compatibility rules, the factors that determine how “long” a particular phase is on depends upon:

    • The user definable phase parameters for each phase, and….

    • Outside influences that may require a phase to end sooner than normal, but no less than the minimum, like the need to serve a coordinated plan (the green wave of lights down a street), or emergency or transit preemption (a topic for later discussion).

SGU- NEMA vs. Interval


Interval based

Interval-based

  • The word Interval refers to an increment of time within the cycle time (time it takes to complete on complete rotation of the signal plan).

  • Many people refer to Interval-based control as “Pre time”, but this is also misleading (see section on Pre time vs. Actuated). It is best to call it Interval-based.

SGU- NEMA vs. Interval


Intervals

Intervals

  • Interval-based plans are typically programmed using a matrix approach with Phases as one legend and Intervals as the other.

  • Phases have no timings associated with them, but rather their activity is dependent on the timings of the Interval or Intervals that they belong.

  • In the diagram below, the user would specify that the phase should be green in Interval 1 and Amber in Interval 2. The times for each of the Intervals would then be set up. All phases assigned to an Interval are subject to the Interval time.

Interval

I-1

I-2

I-3

I-4

I-5

I-6

I-7

I-8

Φ

SGU- NEMA vs. Interval


Interval based timing plan

Interval-based Timing Plan

  • Sequence and Compatibility is dependent on the programming of the timing plan. You basically tell the controller what to do and when.

I-1

I-2

I-3

I-4

I-5

I-6

I-7

I-8

Φ1

Φ2

Φ3

Φ4

Φ5

Φ6

Φ7

Φ8

SGU- NEMA vs. Interval


Pre time vs actuated

Pre time vs. Actuated

  • Pre time means that the length of time and decision to service a phase or not is predetermined and does not change. It is part of the timing plan.

  • Actuated means that the length of time and/or decision to service the phase or not is variable based on demand (i.e. detectors).

  • Semi-Actuated is a mixture of the two approaches. Some phases will be predetermined (a constant time and/or on “recall”, which means always serviced regardless of demand). Some phases will be demand driven.

    • A good example of this is when a quiet side street intersects a busy main road. The phases for the main road will always be serviced each cycle, but detectors are installed at the side street and it gets serviced (i.e. green signals) only when vehicles are present.

SGU- NEMA vs. Interval


Pre time vs actuated cont

Pre time vs. Actuated (cont.)

  • As you can see, Pre time is NOT the same as Interval. You can have pre time and actuated Interval traffic controllers and you can have pre time and actuated NEMA (so-called phase-based) controllers. The terminology should be mutually exclusive.

SGU- NEMA vs. Interval


Nema interval

NEMA Interval

Summary

  • With Interval controllers You:

    • Assign phase outputs to Intervals 1, 2, 3, …etc.

    • Assign times to the Intervals

  • The controller runs your plan just as you entered it

  • With NEMA controllers You:

    • Build the ring sequence

    • Assign concurrency groups

    • Set-up phase parameters

  • The controller does the rest

SGU- NEMA vs. Interval


Nema interval1

NEMA Interval

  • Legacy controllers that were Interval style controllers are:

    • Honeywell HMC1000

    • TCT LMD40

    • Semex C200

  • Legacy controllers that were NEMA style controllers are:

    • Transyt 1880EL

    • TCT LMD9200

    • Peek 3000/3000E

    • Mulitsonics 820A

    • Traconex TMP390/390CJ

SGU- NEMA vs. Interval


Is the atc 1000 nema or interval

Is the ATC-1000 NEMA or Interval?

  • The short answer is…….Both.

  • This is the first controller that we know of that will do both methods without having to change firmware to convert from one to the other.

  • The ATC-1000 can change between the two traffic “engines” the same way it changes between timing patterns. This can be done on a schedule (time of day), by central override, or any other method of changing patterns.

  • This is helpful if there is a particular plan needed for an Intersection that cannot be accomplished with the traffic engine used during normal operations.

  • Each traffic engine (NEMA and Interval) have their own feature sets. They are basically like two traffic controllers living in the same box.

  • It is very important to remember that just because the controller has a feature in one traffic engine, it does not mean that it is in the other.

  • The International version of the ATC controller will focus on adding features to the Interval traffic engine, while most likely having a way to disable the NEMA traffic engine to avoid confusion for those not familiar with the NEMA methodology.

SGU- NEMA vs. Interval


Atc 1000 two controllers in one

ATC-1000Two Controllers in One

NEMA Traffic Engine

(Actuation Modes:

Pre time, Actuated, or Semi-Actuated)

Switches back and forth based on pattern control

Interval Traffic Engine

(Actuation Modes: Pre time only)

Features

Features

Interval side of database is all proprietary. Though we use the NTCIP object (i.e. OID) structure, NTCIP does not specify Interval operations.

NEMA side of database complies with NTCIP 1202 to support NEMA operation and “standard” NTCIP. Features not specified by NTCIP are proprietary.

NTCIP Database

SGU- NEMA vs. Interval


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