Why do left turns go first?

In most jurisdictions in Ontario, the convention is that left turns will precede the straight-ahead (through) movements in the same direction.  Why is this the case?  There’s actually more to it than meets the eye.

Disclaimer: this post is fairly technical, so you may need to refer to my definitions page.  But for your convenience here are some keywords you’ll need:

Leading left turn: The left turn precedes the corresponding through movement
Lagging left turn: The left turn follows the corresponding through movement
Permissive turn: Drivers wait for a gap in traffic to proceed
Protected-permissive turn: Drivers may proceed on a green arrow with priority, but can also make a permissive turn at other times in the cycle.
Fully-protected turn: Drivers may only proceed when they have green arrow (typically there’s a separate Left Turn Signal).

Advantages of Leading Left Turns
Leading left turns are probably popular since they are simpler and more efficient to operate than lagging left turns.

Determining phase lengths
Left turn phases are geometrically interchangeable with the through phase in the opposite direction.  For example with leading left turns, the eastbound left phase can end and the westbound through phase can begin regardless of what the westbound left and eastbound through are doing.  But all four east-west phases need to end before any north-south phases can begin.  Here’s a time diagram of a typical left turn setup.  I’ve arbitrarily shown the eastbound left as being longer than westbound just to illustrate the flexibility.
Note that in North America, convention is to group interchangeable phases within “rings” and separate incompatible phases with “barriers” (more details at this page from the University of Idaho).

Usually the left turn phase gets just enough time to clear the number of cars waiting, and the rest of the time goes to the opposing through movement.

With leading left turns, it’s pretty simple. Just keep each left turn phase on until there’s no more activity on the vehicle detector in that left turn lane (or the allocated phase time runs out).  Then switch to the opposing through phase.

But with lagging left turns it gets a bit more complicated. Since left turn phases in both directions need to end together before switching to the next group of phases, it’s actually the beginning of the left turn phase that needs to vary based on the number of waiting cars.  Which means that the duration of the phase needs to be calculated in advance, rather than in real-time as with leading lefts.  This could be done by putting a vehicle detector at the start of the left turn lane and counting the cars passing over it.  But that setup is less efficient since it is based on an estimate of how long it would take a given number of vehicles to pass through the intersection, rather than the actual time it takes those vehicles to clear.  There are also some other disadvantages.  If a car shows up after the length of the phase has been determined, then it won’t have enough time to make it through and will have to wait till the next cycle.  Similarly, cars could easily get missed if they enter the left turn lane downstream of the vehicle detector.

Avoiding the Yellow Trap
At a typical traffic signal when the light changes to yellow, people waiting for a gap to turn left will assume that the opposing direction is also facing a yellow.  As a result, they may expect that traffic to stop, and they probably feel the need to promptly get out of the middle of the intersection.

But if there is a lagging left turn phase in the opposing direction, that may not be the case, and the opposing traffic may still be flowing through on a green indication – a recipe for a very serious crash.  This dangerous situation is called the Yellow Trap, and is strictly forbidden in Ontario.
In order to avoid the yellow trap, the direction whose through green ends first cannot have permissive left turns.  The simpler saying I like to remember is:

The left turn opposing a lagging left turn phase must be prohibited or fully-protected.

Actually, in the United States, a new form of signal control – the flashing yellow arrow – is becoming popular as a method of preventing the Yellow Trap while still allowing permissive turns.  But flashing yellow arrows are not yet recognized in Ontario, so my statement above still applies.

Fully-protecting or prohibiting a left turn is a fairly high-impact change, so in order to justify it, there would need to be some clear benefit.

Benefits of Lagging Left Turns
Lagging left turn phasing in itself doesn’t really have many advantages.  The real benefit from its use comes from the flexibility of using either leading and lagging left turn phasing depending on context.

Using a leading left turn in one direction and a lagging left in the other (lead-lag phasing), the through greens in the opposite directions actually start and end at different times from each other.  That opens up a lot of possibilities for signal co-ordination, making it easier to create a green wave in both directions on a two way street rather than just one (or neither).  For a clear example of two-way signal progression using lead-lag phasing, check out Main Street West in Hamilton if you’re in the area.

A more specific use for lagging left turn phases is with Transit Signal Priority (TSP) implementations that are prioritizing a left turn movement.  Having the option of serving the left turn phase either as leading or lagging opens up more opportunities for it, increasing the effectiveness of the TSP system.

My take on it all
Given that lagging left turns aren’t inherently that great, I don’t really mind that leading left turns are ubiquitous in Ontario.  The main missed opportunities for lagging left turns are with TSP to clear left-turning vehicles off of streetcar tracks, and with lead-lag phasing to improve vehicle progression through closely-spaced signals.  But these are simply local implementations.  At a provincial regulatory level, it all looks good to me.

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5 Responses to Why do left turns go first?

  1. Ontario, and the rest of Canada for that matter, could and should implement coloured arrows other than green, flashing green in some provinces like my own, and a solid yellow following a protected permissive phase. Flashing ambers have the same indication as a green ball, red arrows mean don’t turn, even after stopping, right turn on red should be prohibited universally, if the visibility is so poor to require a stop before turning, then it’s not suitable to have this sort of system, and solid amber arrows should be used instead of yellow balls. And bicycle signals being shaped like bicycles. Countdown timers would also be a thing to add.

    I also suggest adding yield (and occassionally stop) signs to traffic light poles, with the idea being if the light fails or the the signal is under flashing mode, the yield sign is able to let the intersection be a two (and sometimes one way) yield with the yellow being a cautionary symbol. Stop signs can do the same thing as a two or one way stop. It is used in the Netherlands to allow signals to turn off at night or when the volume is low enough to act like a sign controlled intersection. The intersection has to be designed well, like speed tables designed for the speed limit, median refuges for cyclists, pedestrians and cars if possible, and tight corner radii for cars, and good sightlines, but it makes people look outside their car for once.


    • We do have solid yellow following protected-permissive phases in Ontario. I think that was introduced in the early 2000s. And special bicycle-shaped lenses were legalized this year, which is exciting! You can look forward to a post about a real-world example if I ever find time to head out and see it.

      The prohibition of red arrows and solid amber on dedicated turn signals is indeed silly and counterproductive. I’m a bit torn on the flashing amber though, I think we’re okay without them. They do open up some interesting possibilities, but I fear they would result in even worse service for buses at signals than currently.

      As for flashing red arrows, I simply disagree. If visibility is an issue, then the movement needs to be fully-protected. Stopping then proceeding doesn’t make the visibility any better.

      Yield signs on signals are just confusing. They are used in the Netherlands because they sometimes turn signals off entirely at certain times of day. In Ontario we never do that, we would only put them in flash, which does pretty much the same thing as a yield sign without being confusing during the times the signals are active. The legal meaning of a blank signal in Ontario is all-way stop, thereby eliminating the need for any signs to allocate priority in the case of failure.


      • There isn’t a way to create a part time traffic signal with two way yield signs as the control when the signal isn’t active. That’s the idea behind the yield signs part. And I don’t see it as very confusing. There are 2 way flashing yields and in the other directions flashing reds exist, but the meaning of that is side street(s) stop and then yield, main road proceeds with caution. And more signals could be put into flash mode in Ontario, and elsewhere.

        I did mean that flashing red arrows should not be used ever. If just yielding on the right turn, even if a right on red was allowed, then a flashing yellow right arrow would suffice. If the visibility is too poor to do that, then it shouldn’t even be allowed to turn right without the green arrow.

        And as for the existence of flashing yellow arrows, I think the statistics from the US are promising, and nobody is saying that buses and streetcars in the middle should have green when the left turners have flashing yellow. Separating the turn should still be the standard method, but at some quite small intersections or where the volumes are so low but there are benign reasons to signalize, like a busway crossing, where otherwise it wouldn’t be signalized, for for some similar reason, it would work on flash mode alone, then it could work.

        Just a couple hours ago I was driving, and I was turning left, and there was a pedestrian crossing in front of me. It would be much more useful if there was a setback between pedestrian (and bicycle) crossing of a car’s length and the main roadway in the other direction so I could turn left, yielding as I do, wait on the other side of the road for pedestrians and cyclists, and then proceed.


  2. dunkin63@yahoo.com says:

    Dear Ontario Traffic Man. I think one benefit of the lag left arrow was omitted. If both directions face a lag arrow, then neither direction is forced to turn left during the yellow at the end of the signal cycle. It is when a driver is forced to turn on yellow (that missed a lead arrow) that they are hit by a red light runner. With the lag left, if you don’t find a safe gap in traffic, you are still rewarded by a green arrow at the end of the signal cycle- no forced turn… and you don’t have to wait until the next signal cycle. Also, it does not seem efficient when all of the through traffic must wait for one turning driver and the opposing lead arrow to go, especially if the left turning driver could have turned once all of the through traffic has cleared. There are benefits to both, but those lag arrows are awesome once you get used to them!


    • That’s an interesting point about the impact of phase order on the psychology of waiting to turn left. But I’m not sure it would really address the safety issue at the end of the through green, since the person turning left wouldn’t know whether or not there will be a left turn phase following the through phase until it’s too late. Typically it would be about 6 seconds between the start of amber and the start of the lagging left turn arrow, by which point the left-turning driver within the intersection will likely have already turned.

      The delay to through traffic is only affected by the duration of its red signal: the sum of all other phases. It doesn’t directly matter what order in which those phases occur. It’s actually better for through traffic for back-to-back left turns to be leading, because as I described in the article, the duration of the green can more easily be calculated in real-time, which means less wasted left-turn green and therefore more green time for everyone else.

      (FYI, in many Ontario jurisdictions a protected-permissive green arrow will only be shown if there are at least 3 cars waiting to turn left, in order to avoid making lots of people wait for just one or two cars to turn).


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