Intersection Signalization and Timing Plans

Intersection signalization

Intersections are generally designed to provide optimal vehicle traffic flow. Timing plans may be of two general types:

A signal at a given intersection may be designed to change from actuated to pretimed to flashing mode depending upon:

It is important for O&M Specialists to understand the signal design and terminology to teach these concepts to their students.

There is some variability in timing plans in different municipalities and in different locations, depending on the roadway needs and local practices.

Signal design terms

Phase — the right-of-way, yellow change, and red clearance intervals in a cycle that are assigned to an independent traffic movement or combination of movements

Interval — the part of a signal cycle during which signal indications do not change

Cycle — sum total of all phases at a signal

Pretimed (Fixed time) signals

Pretimed intersections operate in predetermined and predictable fashion.

Actuated signals

Figure 3-1. Photograph of vehicle detector loops in pavement

Figure 3-1. Photograph of vehicle detector loops in pavement

Figure 3-2. Photograph of vehicle detector loop in pavement

Figure 3-2. Photograph of vehicle detector loop in pavement

Actuated signals change the length and/or order of the phases in response to variations in vehicle or pedestrian traffic.

The extent of actuation is dependent on geometric and operational requirements, but is generally categorized as either semi-actuated or fully actuated.

Semi-actuated signals

Fully actuated signals

Basic turning phases

Protected turn

Figure 3-3. Protected left turn signal

Figure 3-3. Protected left turn signal

A protected turn is made without opposing through vehicular traffic or pedestrian crossing.

Permissive turn

Figure 3-4. Permissive green ball signal

Figure 3-4. Permissive green ball signal

A permissive turn is made across an opposing flow of through vehicles and/or pedestrians.

Design of turning movements

Concurrent (dual) left turns

Split or non-concurrent phasing

Figure 3-5. Illustration of split phasing

Figure 3-5. Illustration of split phasing

Example of northbound/southbound movements running under split phasing (see Figure 3-5):

  1. Northbound traffic, including traffic turning east and west, moves on one signal phase, (southbound traffic and all traffic on the E/W street have a red signal at that point). The pedestrian phase usually is provided at this time for pedestrians on the east crosswalk
  2. Northbound traffic receives a red light while all southbound traffic, including turning traffic, is allowed to go. The pedestrian phase usually provided at this time is for pedestrians crossing on the west crosswalk

Flashing operation

Signals may only operate during peak periods of the day and may switch to flashing operation at non-peak hours, late at night, or in response to a signal malfunction.

Coordinated systems

Coordinated systems provide automated control of signal timing to two or more signalized intersections.

Instead of looking at an intersection in isolation, coordinated systems look at an entire arterial or network of intersections and make signal timing adjustments that benefit (optimize) the operation of the entire system.

System changes are a result of traffic volume and travel times. Most often, a central controller (computer) provides the primary control and communicates with individual controllers located at each intersection.

Coordinated control has a number of advantages from a vehicle perspective:

Signals in a coordinated system can present problems for blind pedestrians:

Vehicular Signals and Timing

Meaning of signals

The use of particular traffic signal colors and symbols, and their meaning, is described in Part 4 of the MUTCD. Signs and pavement marking used at signalized intersections are covered in Parts 2 and 3 of the MUTCD, respectively.

Although this section presents basic traffic laws concerning signals, it is important to be well educated on the specific laws of the state of interest. Some laws, such as right-turn-on-red-arrow, vary from state to state. Most states provide a Drivers Handbook that presents this sort of information.

Steady green signal

Circular green (green ball):

Green Arrow:

Pedestrian movement:

Steady yellow signal

Pedestrian movement:

Steady red signal

Right turn on red:

Left turn on red:

Red arrow:

Pedestrian movements:

Flashing yellow

Flashing red

Flashing red arrow & flashing yellow arrow

Other Intersection Terminology

Figure 3-6. Channelized right turn lane

Figure 3-6. Channelized right turn lane

Channelized turn lane (slip lane) — a turn lane that channels turning drivers to a position where they will either yield to oncoming traffic or complete a "free flowing" turn, which means the turning vehicles have a dedicated lane on the road they are entering and therefore do not need to stop or yield to traffic.

Pedestrian Signals and Timing

Visual pedestrian signals

Pedestrian signal heads ("pedheads") are installed at some intersections to instruct pedestrians when it is lawful to cross. This is typically done where there is a significant amount of pedestrian activity or for safety-based reasons, such as the possibility of confusion for pedestrians taking cues from the traffic signal.

Figure 3-7. Typical pedestrian signal symbols

Figure 3-7. Typical pedestrian signal symbols

Pedestrian signals have three intervals:

Some locations also use pedestrian countdown signals. These signals provide the countdown in seconds for the remaining time allotted during the change interval.

Figure 3-8. Correct display of pedestrian countdown signal (counting down during flashing DONT WALK)

Figure 3-8. Correct display of pedestrian countdown signal (counting down during flashing DONT WALK)

Figure 3-9. Incorrect display because countdown is displayed during WALK interval

Figure 3-9. Incorrect display because countdown is displayed during WALK interval

Pedestrian signal timing

Pedestrian signal timing design deals with the length of the WALK and change intervals. The WALK interval is typically short (around 4 to 7 seconds). The change interval is designed to be long enough for a pedestrian to cross the street. This is typically calculated assuming a walking speed of 3.5 to 4 feet per second. Parking lanes might be excluded from the calculation.

The green time for the parallel traffic movement is calculated based on the time necessary for a pedestrian to cross the street (see equation below).

The figure 3-10 below illustrates how the vehicle and pedestrian phases overlap. Time is the horizontal axis in the picture. This is a typical signal timing diagram used by traffic engineers in the design of the signal timing.

Equation

Equation

Figure 3-10. Illustration of the overlay of vehicle signal and pedestrian signal timing

Figure 3-10. Illustration of the overlay of vehicle signal and pedestrian signal timing

Pedestrian phase actuation

Some signals are designed so that the pedestrian phase is actuated by a pushbutton.

Passive pedestrian detection

Leading pedestrian intervals

Exclusive pedestrian phasing

Ped recall

Rest-in-WALK

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