VEHICLE MOVEMENT AT SIGNALIZED INTERSECTIONS

Although new intersection designs are being created as we speaks, the standard intersection with two streets (four "legs") either has "permissive left turns" or "protected left turns," as described here:

Many signals today have the same pattern that was prevalent the 1940s, where drivers can turn left whenever there is a gap in the oncoming traffic and no pedestrians in the crosswalk. 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.

This cycle has two phases -- one for each street:
• 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.
  
  
Pedestrians: At these simple, traditional signals, pedestrians usually have the right of way when crossing parallel to the traffic that has the green signal. This is because drivers who turn right or left from the parallel street are supposed to yield to pedestrians in the crosswalk, but some do not (this is discussed later in the section on Risks).

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 called exclusive left turns.

Split phasing

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):
• 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.
  
  
Second part of phase: South-bound traffic gets a turn (illustrated to the right):
• 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.
  
  

Exclusive left turn phasing

Exclusive left turns provide the left-turning vehicles from both directions with a green arrow that gives them the right-of-way to make left turns, while other conflicting traffic must wait (including pedestrians!) While these drivers are turning left, all other traffic (including pedestrians!) have to wait. This exclusive left-turn phase might come before the others can go ("leading exclusive left turns") or afterwards ("lagging exclusive left turns") as shown in the first drawing to the right), The other traffic (including pedestrians walking parallel to the street) get their green signal before or after the left-turning traffic gets theirs (as shown in the second drawing). The animation "Exclusive left turn" below illustrates a leading exclusive left turn.

Variations in exclusive left turn timing

The vehicles waiting to turn left may not all turn at the same time. Here's how it happens:

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.

It should be noted that sometimes, while the left-turning vehicles from both directions of one street are having their turn, vehicles on the cross street get a green arrow that allows them to turn right at the same time (called a protected right turn). Of course whenever there is a protected turn, whether it's a protected left or right turn, there is no WALK signal for pedestrians in the crosswalk that the vehicles are crossing.

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
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.


How do pedestrians fit in here?
As we just explained, 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.


How can blind pedestrians be sure that 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.


There are some exceptions where this strategy does not work, which is why the WALK signal is more reliable. But crossing with the near-lane-parallel traffic surge can assure you that you are not in conflict with any protected left turns.

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 1) press the pedestrian pushbutton appropriately (explained here) and 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
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.


Well, congratulations, you've learned what you need to know about patterns of vehicle movement at traffic signals! But wait . . . there's more!

You need to know that
• 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"