Traffic patterns in the 1950s were relatively simple, compared to today!
Understanding patterns of traffic movement at signalized intersections can be very helpful to anyone who is crossing there.
However, for blind pedestrians who cannot see the traffic signal and must use the traffic movement to determine the appropriate time to begin crossing, this understanding is essential!
When orientation and mobility (O&M) programs were developed in the 1940s, traffic patterns at signals were simple and predictable, and so was the strategy that blind people used to determine when their signal was green. Traffic patterns were revised when Right-Turn-On-Red laws were enacted throughout the United States, as explained below.
However that law was just the tip of the iceberg of cataclysmic changes that require O&M specialists to reconsider the traditional strategies that served us so well 50 years ago. Traffic patterns have become more complex, making traditional strategies unreliable, especially with the introduction of protected left turns. We must update our understanding of them, and revise the strategies we teach.
So this section will deal with typical traffic patterns at modern traffic signals, including protected turns. That means not only the movement of vehicles, but also that of pedestrians, as they are part of the traffic movement too (yes, most transportation engineers and planners consider pedestrians and bicyclists to be modes of traffic - finally!).
VEHICLE MOVEMENT AT SIGNALIZED INTERSECTIONS
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Vehicle movement at intersections
- In the first phase, vehicles coming from both directions of one street all have the green signal and are permitted to move forward and enter the intersection to go straight, or turn right or left (shown in the drawing and the animated graphic to the right) or, if it is permitted, make U-turns. Vehicles that are turning left or making a U-turn must yield to drivers coming straight from the opposite direction, and vehicles turning right or left must yield to pedestrians. During this phase, all traffic on the other street waits.
- In the next phase, vehicles on the street that just had its turn must stop and wait, while vehicles from the other street all have the green signal to move forward and enter the intersection from both directions, and go straight, turn right or left, or make U-turns.
- south-bound vehicles approaching the intersection on the major street have to wait, and
- north-bound vehicles on the major street get a green signal AND a green left-turning arrow, which means that:
- vehicles that are turning left have the right-of-way (a protected left turn), and
- vehicles going straight have the right-of-way
- vehicles that are turning right must yield to pedestrians.
- At the same time, pedestrians on the east side of the major street (that is, the same side of the street as the moving vehicles) can cross the minor street, and
- pedestrians on the west side of the major street (in the lanes furthest from the moving traffic) are not allowed to cross the minor street, because the major street vehicles have a protected left turn and the right of way across their crosswalk.
- the northbound vehicles on the major street have to wait and
- southbound vehicles get their green signal AND a protected left turn (green left-turning arrow), and
- pedestrians on the west side of the major street (the side closest to the moving traffic) can cross the minor street, while
- pedestrians on the east side of the major street have to wait.
- Vehicles travel on the right side of the street on typical two-way streets in the U.S. and Canada.
When they reach most traditional intersections of two-way streets, they can go straight, turn left, turn right or make a U-turn.
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At signalized intersections, drivers usually are permitted to turn right, go straight and turn left only when facing a green signal.
However, in the U.S., where Right-Turn-On-Red is legal unless otherwise posted, drivers who want to turn right may do so when facing a red signal, as long as they come to a full stop first, and yield to pedestrians and to traffic in the nearest lane of the perpendicular street (that is, traffic in the lane that the driver plans to enter after turning right).
This is not true in New York City, where right-turn-on-red is not permitted.
Note that even though they are not permitted to turn right if they conflict with pedestrians who have a green signal or WALK signal, drivers who are turning right on red often do not look for pedestrians -- they look only toward their left to make sure there are no vehicles approaching in the first lane of the street they want to turn into. They move forward across the crosswalk and turn right while still looking left, without ever looking to their right. If that happens when you are crossing with the parallel street on your right, as you step out to cross in front of the vehicle, the driver may never look your way or see you before he moves forward to turn right (this is discussed later in this course in the section on Risks).
The cycle of traffic movement for some signals today has the same pattern that was prevalent the 1940s.
This cycle has two phases -- one for each street:
Learning diversion!
Just for fun (and to enhance your learning!) we have sprinkled a few little thought-provokers in here -- enjoy!
Or to put it in engineering terms, which movement (straight, left, or right) is a potential conflict with other legally moving vehicles? . . . Answer Question: With this simple traffic pattern, what happens to the drivers who want to turn left? How are conflicts with left-turning vehicles handled? That is, how did they avoid any collisions with other vehicles? . . . Answer |
Hopefully you've figured out that it is the left-turning vehicles that have potential conflicts, and so left-turning drivers need to wait for a gap in the opposing traffic before turning! Traffic engineers call that kind of turning "permissive left turns" - these drivers don't have the right of way, but they have permission to turn left or make U-turns when it's clear to do so.
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Many modern traffic signals provide drivers the right of way to turn left through what is called a protected left turn so they do not have to conflict with and yield to other vehicles or pedestrians.
Where protected left turns are installed, there is usually a separate lane provided for the left-turning traffic, and drivers have a green-arrow signal letting them know when they have the right of way to turn left.
The length of time for the protected left turns and the order and combination of vehicle movement can vary, either because of actuation or because of variations in the timing plan designed to accommodate demand in various lanes of the intersection. More about that later -- for now, let's look at two different possible traffic movements where protected phases for turning left are provided.
One is called split phasing, and the other is simply called protected left turns.
Are your solutions similar to the traffic pattern designs described below? |
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With split phasing, instead of vehicles from both directions on one street being permitted to move at the same time, they take turns.
First, vehicles coming from one direction get a green signal AND a green left-turn arrow while vehicles coming from the other direction have to wait, and then they switch.
To understand how this works and how pedestrians fit in, you can watch the video to the right, and/or see below an example of the pattern at a major north-south street intersecting a minor street.
First part of phase: The north-bound traffic gets a turn (illustrated to the right):
Second part of phase: South-bound traffic gets a turn (illustrated to the right):
Protected left turns provide the left-turning vehicles with a green arrow that gives them the right-of-way to make left turns, while other conflicting traffic must wait (including pedestrians!)
This can be done in a number of ways.
One common timing pattern is "leading (or lagging) protected left turns" (as shown in the first drawing to the right), in which the left-turning vehicles from BOTH directions of one street get a green arrow that gives them the right of way to make left turns, while all other traffic must wait (including pedestrians!). The other traffic on that street (including pedestrians) gets their green signal before or after the left-turning traffic gets theirs (as shown in the second drawing). It's called a "leading left turn" when the protected left-turners get their green arrows before the other traffic, and a "lagging left turn" when they get their green arrows last. The animation "Protected left turn" above illustrates a leading protected left turn.
Variations in protected left turn timing.
The vehicles waiting to turn left may not all turn at the same time. Here's how it happens:
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Left-turners from both directions may get the green arrows at the same time, but if the signal is actuated, timing of protected left turns can change from cycle to cycle, depending on the number of vehicles in each direction waiting to turn left.
Even at pretimed signals, left-turn traffic coming from one direction may be given a shorter time than those coming from the other direction, for example when more traffic is expected from one direction than the other (such as traffic going into town in the morning, and out of town in the afternoon).
Whatever the reason for the left-turn phase from one direction being longer than the other, when the left-turners from one direction have cleared before those from the other direction, the straight-through traffic beside the remaining left-turners may be allowed to start moving through the intersection while the straight-through traffic from the other direction is held until the remaining vehicles in the left-turn lane have cleared.
There are many variations on this pattern. For example, traffic from one direction (including left-turners) may have a phase at the beginning, then traffic from both directions can go straight or turn right, and then traffic from the other direction can turn left, go straight or turn right.
Video shows examples of modern traffic patterns explained in this section.
Traffic movements are described (using compass directions) for those who cannot see it.
Protected-permissive left turns
Traffic movements are described (using compass directions) for those who cannot see it.
- A protected left turn may be followed by a permissive left turn.
In other words, there may be a protected left turn (a green arrow), which is followed by the round green signal or a yellow flashing arrow for that turn lane, indicating that vehicles are allowed to turn left when there is not conflicting traffic (vehicles or pedestrians).
This is called Protected-Permissive Left-turn Phasing.
Sometimes the order is reversed, and the permissive left-turn phase comes first and then, just before the end of the cycle, the left-turning traffic gets a protected left phase and green arrow.
- As we just explained, regardless of whether there is split phasing, protected left or protected right turns, or protected-permissive phasing, pedestrians are NOT allowed to cross when the vehicles that have the right-of-way are turning across their crosswalk.
In other words, wherever drivers have a protected left or a protected right turn, they have the right-of-way through the crosswalk, so pedestrians are not allowed to cross at that time.
How can pedestrians know when they have the right-of-way, with no protected left- or right-turning vehicles across their crosswalk? It's very simple -- pedestrians have the right-of-way when they have the WALK signal.
- Of course the WALK signal is the most reliable cue to use to start crossing, so if you have access to it, you should use it.
But what if you do not have access to the pedestrian signal?
In that case, the most reliable strategy is to
- 1) press the pedestrian pushbutton appropriately and then
2) cross with the surge of the near-lane-parallel traffic.
Why is this such a reliable strategy for avoiding protected left turns? Well, if you think about it, you'll realize that when the traffic engineers provide a protected left turn to vehicles coming from one direction, they're not going to give a green signal to vehicles coming straight through from the other direction! That means you can cross with those straight-through vehicles if you are on their side of the street. This is true regardless of whether you're walking the same direction as that traffic (with the parallel street on your left), or you're walking the opposite direction (with the parallel street on your right). You can see how this works if you watch the pedestrians and vehicles in the animated video of split phasing, and there is a video demonstrating this strategy as well as a video about teaching it.
KEY PRINCIPLE:
The WALK signal is the most reliable cue to use to start crossing. If you do not have access to the pedestrian signal, the most reliable strategy for crossing with the WALK signal is to
2) cross with the surge of the near-lane-parallel traffic. More about this key principle is in the section "Strategies for Visually Impaired Pedestrians at Modern Signals." IMPORTANT!! Make sure you understand what "near-lane-parallel traffic" is, by going to our Basics page and take the quiz!!! |
Protected right turns
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We mentioned that at some intersections, right-turning vehicles get a green arrow (protected right turns) at the same time that left-turning vehicles on the intersecting street get green arrows (protected left turns).
But uh-oh! We have to tell you that at some intersections, right-turning vehicles get a green arrow at the same time that vehicles on THE SAME STREET are allowed to go straight! An example of this is seen in this video.
The reason we said "uh-oh" is that this is one of the exceptions to the Key Principle in the box above, as explained in the section Strategies for Visually Impaired Pedestrians at Modern Signals. This situation is rare but is occurring increasingly in some areas, such as the ones in Washington state noticed with alarm in the summer of 2017 by O&M specialist Joanne Laurent.
You need to know that
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you cannot rely on the traffic patterns remaining consistent at any given intersection, because the traffic patterns AND timing for many signals can be changed in an instant, from an office many miles from the intersection, such as at the Los Angeles' Automated Traffic Surveillance and Control System shown below.
Using these systems, engineers can monitor the traffic flow at intersections throughout their jurisdiction and temporarily change the timing pattern for signals wherever/where and when traffic is backing up.
- if there's a pedestrian signal and the pedestrian doesn't press the pushbutton to call the signal, the pedestrian may not get a WALK signal and does NOT have the right of way.
If there's a pedestrian signal installed, pedestrians are legally allowed to begin crossing only with the WALK signal.
- there are signals with timing plans that are unique and/or don't fall into any of these categories.
One such timing plan has an "exclusive pedestrian phase" when all vehicles have a red signal, and pedestrians have a pedestrian WALK signal to cross any street, including making diagonal crossings. At all intersections with unique or unusual traffic patterns, it is crucial that the pedestrian signals be accessible so that those who are visually impaired can become aware of when they have their turn to cross.
- it is never reliable to assume that whenever traffic on the street you are crossing stops, the traffic signal for that street must be red and therefore your signal is green.
There are many other reasons that traffic stops, such as congestion and, as we explained, traffic traveling in one direction often stops for a red signal while traffic traveling in another direction on the same street has the green signal (for example with split phasing).
STUDY QUESTIONS
for this section. |
Proceed to "Pedestrian signals and pushbuttons"