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.