At most intersection with dedicated left-turn lights the sequence for who gets green goes:
North-South Left Turns
N-S Straight
East-West left turns
E-W straight
But there is one near me that goes:
North straight and left
South straight and left
East-west left
East-west straight
Is there is a reason based in traffic flow management for this pattern? I can’t think of anything about this intersection that makes it unique in terms of flow needs, but maybe I don’t know what to look at.
For those in “Drive on the left” countries (like me), CookingwithGas is talking about drive on the right situation, where “turn left” is the “turn across the centre of the road” , “across the traffic”… , the right turn for the “drive on the left”.
There are a bunch of options.
The main thing is about the symmetry or asymmetry of traffic flow at peak hours…
It would be inefficient to have “north south turn across traffic” times if the traffic flow is known to be highly skewed to one direction, they want to allocate more time to the flow from that direction , so they give it the green to go to turn left, right or go straight…
So while you may observe the traffic light gives equal time , that is just at the time you observed it.
It may give different times during peak hours… so they leave it at the same mode, and only adjust, eg extend, the time for one direction (eg by detecting traffic level with sensors. )
Another factor, they may want to bunch up the vehicles turning across traffic (by having them wait for the “both directions turn across traffic” time ), or they may want to avoid having the vehicles queuing to turn across traffic blocking the other traffic, by having them proceed at the same time as the other traffic. There’s no point allocating a lot of time to turning across traffic if the traffic queued at the red blocks their way…
You can evaluate and balance safety concerns, queue lengths, chances (and results) of queues causing blockages, and traffic flow expected for the time…
Vehicles per hour on that road, vehicles per hour turning left, going straight, turning right ?
The ratio of time allowed… if you get the ratio wrong , then the queues will block traffic creating time that light is green but no one going.
My guess would be that the N-S street has a large amount of traffic making left turns off of that street, leading to what may be a longer light-protected left turn than the “usual” pattern.
Alternately, there might be a history of accidents on the N-S street, with cars trying to turn left on the “straight” green, and possibly not seeing oncoming traffic due to the cars waiting in the opposite left-turn lane. The pattern you describe would not ever have left-turners making such a turn while traffic is coming towards them from the opposite direction, thus avoiding that collision threat.
But if both N and S have a lot of L-turning traffic, you could achieve this with the “standard” pattern, just making the left turn period longer.
However, if there is a lot of left-turning traffic AND traffic is significantly heavier in one direction (more traffic Northbound than Southbound or vice versa) then this pattern allows Northbound to have a longer green time than Southbound.
It’s called split-phasing when one direction gets an all-way green. Generally yes it’s used where there’s a high volume of left turning traffic (at least close to straight-through traffic), but having four separate split phases seems to be uncommon because it makes the total cycle time quite long and it complicates pedestrian crossings. Simply making the left turn phase longer may not be an option because of the length of the left turn lane. It’s no use having a long green left arrow if drivers are stuck behind others waiting to go straight. Split-phasing prevents that because you can have combined left/straight lanes and combined right/straight lanes and nobody will get blocked. With standard phasing you need dedicated lanes for dedicated signals to get maximum effect.
This definitely feels like the answer.
Lengthen the protected left turn phase without physically lengthening the left turn lane would waste the extra time. Lengthen the protected straight-and-turning green will allow more left turners to get to the intersection and make their turn.
Thank you for the clarification. I had forgotten that the headers on the posts here do not show the member’s location as they do on some other forums where I post.
FTR this intersection does not have any combined lanes. All lanes are straight-only or left-only (although the right lanes also allow right turns, of course).
But that just moves the point of contention upstream to where the dedicated left turn lanes begin.
E.g.: If the dedicated left turn lane is only 5 cars long, but typically 10 cars want to turn left per cycle, the last five will be stuck in the leftmost straight-through lane upstream of where the dedicated left turn lane branches off.
We have several intersections around here like that were do to short blocks or other issues, the left turn pockets cannot be made even close to long enough to support the peak demand for left turns. So would-be left turners are routinely trapped where they can’t get into the left turn lanes after the left turn traffic has drained and the signals are still allowing dedicated left turns.
Smarter signals would of course detect this scenario and cut off the empty dedicated left turn signal in favor of opposite direction straight-through traffic. Net of all the other factors peculiar to that intersection.