Intersection Signalization and Timing Plans
Intersections are generally designed to provide optimal vehicle traffic flow. Timing plans may be of two general types:
- fixed time (or pretimed)
A signal at a given intersection may be designed to change from actuated to pretimed to flashing mode depending upon:
- time of day (peak periods vs. non-peak periods);
- day of week; and
- malfunctions due to power outages.
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
- In other words, a phase is the time allotted to a specific movement, such as northbound traffic, whereas an interval is how long the light stays green, yellow, or red for vehicles or WALK, flashing DONT WALK, or DONT WALK for pedestrians.
- Busier intersections typically have separate phases for left turn movements (i.e., protected left turns). When a major road intersects a minor road, the green intervals for the major road will be longer than those for the minor road to accommodate the heavier traffic on the major road.
- Although the MUTCD gives specific definitions to "phase" and "interval", these terms are often used interchangeably by traffic engineers.
Cycle — sum total of all phases at a signal
- A cycle is timed from the start of one phase to the start of that same phase when it comes around again.
- Larger, busier intersections will commonly have longer cycles.
Pretimed (Fixed time) signals
Pretimed intersections operate in predetermined and predictable fashion.
- Regularly repeated sequence of phases (often 30 seconds or more), regardless of traffic flow
- Length of phases may change at different times of day, based on a consistent timing plan, for example, one street may have longer phases at peak hours than non-peak hours
- Still found in many locations, particularly in downtown areas
Actuated signals change the length and/or order of the phases in response to variations in vehicle or pedestrian traffic.
- Detectors monitor traffic and send signals to the traffic signal controller. Detectors are most often inductive loops (electric), though they may be magnetic, microwave, video, and other detection technologies.
- Pushbuttons are most often used for pedestrian detection, though other "pedestrian sensing" technologies (microwave, infrared, piezoelectric) may be used as well.
- Used where traffic volumes fluctuate or where it is desirable to minimize interruptions to traffic flow on the major street. Detectors are often placed on minor roads and in turn lanes to detect when a motorist is waiting to make a turn.
- Vehicular actuation allows the cycle to skip phases, so pedestrians with visual impairments cannot accurately predict, based on previous experience, when in the cycle the pedestrian phase will begin.
- Some actuated signals may provide very short phases to accommodate a single vehicle, without provision of a pedestrian phase during that cycle. A pedestrian who is blind crossing parallel to that vehicle may not realize that a pedestrian phase is not provided during that vehicle's movement.
The extent of actuation is dependent on geometric and operational requirements, but is generally categorized as either semi-actuated or fully actuated.
- Common at the intersection of a main road and a minor side street
- Main roadway signal stays in green until a side street detection is received, causing the traffic signal to change (the WALK signal for the crossing the main roadway may not come on unless pedestrian pushbutton is pressed)
- Vehicle detectors only on the side street; pedestrian detectors are also possible only on the side street
- Pedestrian activation (usually use of pushbutton) activates signal to give WALK interval for pedestrian phase to cross the major street
- Pedestrian signal for side street crossing may "rest-in-WALK" (gives the WALK indication during the green signal for the major street) for the visual pedestrian signal with no pushbutton installed
Fully actuated signals
- Common at the intersection of two main roads (arterials)
- All movements/ phases are actuated
- Detectors for vehicles on all approach lanes
- Pedestrian pushbuttons or detection to activate WALK signal for pedestrian crossings
- Used when traffic volumes on each approach vary by time of day
- Used to skip phases within a cycle when vehicles are not present, thus minimizing delay
- Changing traffic volume can result in difference in timing and sequence of phases for every cycle
Basic turning phases
A protected turn is made without opposing through vehicular traffic or pedestrian crossing.
- Denoted for motorists by a green arrow
- Typically activated by a vehicle detector
- The signal phasing "protects" vehicles by prohibiting the opposing movements, including pedestrian movements
- Protected turns require a separate signal phase, which leads to multiphase signalization (more than two phases at the intersection)
A permissive turn is made across an opposing flow of through vehicles and/or pedestrians.
- Typically denoted for motorists by a circular green (green ball) signal
- The driver is "permitted" to cross the opposing through flow, but must select an appropriate gap in the opposing traffic stream through which to turn
- The driver must also yield to pedestrians who are crossing lawfully within the intersection
- This is the most common type of left-turn phasing at signalized intersections, and is used both when left-turn volumes are not excessive and where adequate gaps of sufficient size exist in the opposing traffic to accommodate turns safely
Design of turning movements
Concurrent (dual) left turns
- Two directions of turning traffic (eastbound and westbound) proceed together as the opposing through traffic on the same approaches is stopped (protected left turn phasing)
- Can be activated either before or after the opposing through flows have had their green phase. If the left turn comes before the opposing through movement, it is called a "leading left turn". If it comes after, it is called a "lagging left turn". Leading left turns, which are much more common than lagging left turns, can create safety problems for blind pedestrians, since the surging left turn traffic may be mistaken for the parallel through traffic surge.
Split or non-concurrent phasing
- Split phasing provides separate green time to vehicles on opposing approaches
- In typical signal design, the northbound and southbound through movements run simultaneously, as do the eastbound and westbound through movements. At offset intersections and locations where there are heavy turn movements, split phasing may be used to allow movements on each approach to move independently of other approaches.
- Pedestrian phases for parallel crosswalks will be activated at different times. The pedestrian phase for a crosswalk will coincide with the through traffic movement immediately adjacent to that crosswalk.
- Where there is split phase timing, the surge of parallel vehicles beside the pedestrian may be mistaken as indicating the onset of the WALK interval and blind pedestrians may cross into the paths of left turning vehicles. In addition, the heavy flow of turning traffic may be mistaken for the surge of traffic on the street beside the blind pedestrian, when the traffic is actually on the street the pedestrian is crossing.
Example of northbound/southbound movements running under split phasing (see Figure 3-5):
- 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
- 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
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.
- Signals no longer operate under stop-and-go sequencing
- Signals usually flash red for side streets and flash yellow for the main street or flash red for both streets
- Pedestrian signal heads (WALK/DONT WALK signs) are dark and APS are silent
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 can be controlled from a central traffic management center
- The detection elements of the system can be used to predict future flows within the network and adjust the signal timing proactively instead of reactively
Signals in a coordinated system can present problems for blind pedestrians:
- The green time given for vehicles on the intersecting road (not the road whose signals are coordinated) may be less than normal to fit into the timing scheme of the coordination
- This shorter time can be insufficient for pedestrians to cross the major road. Pedestrian who are blind will not know that there is insufficient time and may directly conflict with the approaching platoon of traffic on the major road. This demonstrates a need for APS at the intersection.
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):
- Traffic, except pedestrians, may proceed straight through or turn right or left except as such prohibition signs or markings modify movement. Vehicles turning right or left shall yield the right-of-way to pedestrians lawfully within the intersection.
- Traffic may make the movement indicated by the green arrow. Opposing vehicle and pedestrian movements will be given a red signal or DONT WALK indication.
- Pedestrians may cross unless a green arrow indicates conflicting traffic will cross into their path or unless a pedestrian signal indicates otherwise
Steady yellow signal
- Warning that the green interval has ended and the red signal will begin.
- Pedestrians do not have enough time to cross and should not initiate a crossing.
Steady red signal
- Traffic must stop at the stop line, before the crosswalk lines, or before the intersecting street.
Right turn on red:
- Unless a sign or local law prohibits right turn on red, vehicles must come to a complete stop but are allowed to then proceed with the turn if a safe gap in traffic is available.
- Turning vehicles must yield right-of-way to pedestrians and traffic already in the intersection.
- Right-turn-on-red makes it harder to determine the surge of traffic at the onset of vehicular green on the street parallel to the crossing direction. Blind travelers must wait to hear a car traveling straight across the intersection to determine that the light has changed, so they frequently are delayed in initiating crossings while they determine that parallel traffic flow has begun.
Left turn on red:
- This maneuver involves a left turn from a one-way street onto another one-way street on a red signal (same procedure as stated above for right turn on red). This is not allowed in some states.
- Vehicles must stop at a stop line, before the crosswalk lines, or before the intersection. Some states allow vehicles to turn right on red after stopping.
- Pedestrians should not enter the roadway in the direction of travel controlled by a steady red signal
- Proceed with caution, treated like Yield sign
- Stop, then go, treated like Stop sign
Flashing red arrow & flashing yellow arrow
- Typically have the same meaning as flashing circular signal indication, except they apply only to vehicular traffic intending to make the movement indicated by the arrow.
- In some states, flashing circular yellow and yellow arrow indications may be used during stop-and-go traffic signal operations for permissive left-turn indications (same control as a green ball for left turns)
- In some states, flashing circular red and red arrow indications may be used during stop-and-go traffic signal operations for permissive left-turn indications
Other Intersection Terminology
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.
Pedestrian signals have three intervals:
- WALK interval — White WALK or symbol of a person walking issued to indicate that pedestrians should begin crossing, after yielding to vehicular traffic still legally in the crosswalk.
- Change interval — Orange flashing DONT WALK or symbol of a flashing hand is used when pedestrians are not supposed to begin a crossing because there is not enough time left in the phase for most pedestrians to get all the way across the street. Pedestrians that have already begun to cross should finish crossing.
- DONT WALK interval — Steady orange DONT WALK or symbol of a hand is used when pedestrians are not supposed to be in crosswalk
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 3-9. Incorrect display because countdown is displayed during WALK interval
- Countdown signal supplements the WALK/DONT WALK signals, rather than replacing them.
- MUTCD guidance stipulates that the countdown should only be displayed during the flashing DONT WALK interval because of inconsistencies of the countdown during WALK at actuated signals. However, many cities are still using signals that display the countdown during the WALK interval (see Figure 3-9). Pedestrians with low vision have had problems distinguishing the countdown numbers from the orange flashing hand symbol when they are displayed alongside the white walking-man symbol.
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.
Pedestrian phase actuation
Some signals are designed so that the pedestrian phase is actuated by a pushbutton.
- Pressing the pushbutton calls a pedestrian phase which allows enough time for the pedestrian to cross at average walking pace
- Without pushing the button, there may not be enough time programmed into the vehicular phase for a pedestrian to cross the street. If the button is pressed, the pedestrian phase may begin immediately or will begin at a certain point during the following cycle.
- Extent of the delay before it begins will vary depending on the programming of the phases for that intersection and when the button was pushed within the cycle
- Blind pedestrians have traditionally waited through a light cycle to assess and refine their heading by listening to vehicular trajectories, before crossing on the next pedestrian phase. At a pedestrian-actuated intersection, that is not possible because pedestrians have to cross on the next pedestrian phase after pushing the button. If they do not cross at that time, it is necessary to locate and push the button again (and re-establish their alignment).
Passive pedestrian detection
- Pedestrians may be detected passively (without pushing a button) as they approach the crosswalk area through the use of microwave, infrared or piezoelectric technologies.
- Future developments may impact whether or not an APS with a locator tone is necessary at an intersection that uses passive detection. One issue for consideration is that pedestrians may not realize they have been detected.
Leading pedestrian intervals
- Provides a pedestrian WALK interval 2 to 4 seconds before the vehicular green, allowing pedestrians a head start so they are in the intersection before vehicles start up.
- However, this can be a disadvantage to pedestrians who are blind or visually impaired who rely on the surge of traffic to recognize when the signal is green. If these pedestrians begin crossing with the surge of parallel traffic where a leading pedestrian interval is used, they will have less time to cross than was designed. Also, when pedestrians do not initiate their crossing at the onset of the WALK interval, drivers may interpret this to mean that the pedestrians are not intending to cross.
- Pedestrians who are blind or visually impaired do not know about the leading pedestrian interval at unfamiliar intersections unless there is an APS installed.
Exclusive pedestrian phasing
- All vehicles have a red light during the WALK interval and all crosswalks have the WALK signal at the same time.
- Typically done to increase pedestrian safety. At some locations, right-turn-on-red is allowed during the pedestrian phase.
- Crossings may be made diagonally for pedestrian efficiency.
- Exclusive pedestrian phasing may be followed by an extended time for one or more of the crosswalks.
- Exclusive pedestrian phasing may be beneficial to pedestrians with mobility impairments and cognitive disabilities as it allows time to cross when no or few vehicles are moving through the intersection. However, it is a disadvantage for pedestrians who rely on traffic sounds to determine the signal phases. In addition, initial alignment and maintaining alignment during crossings may be difficult due to the absence of parallel moving traffic.
- WALK indication will be given every cycle (as if someone were always there pushing the button).
- Pedestrian signal to cross the minor street remains in WALK as long as the major street has green, and there is no call on the minor street
- When a vehicle approaches on minor street and is detected, the pedestrian signal to cross the minor street changes to flashing DONT WALK
- APS during Rest-in-WALK: instead of sounding constantly, some APS manufacturers provide a limit switch that limits the length of the audible WALK indication to seven or eight seconds, but recalls the audible and vibrotactile indications of the WALK if the button is pressed when there is adequate clearance time remaining.
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