Interlocking lefts at single signal?

[johndoe]

I drove through this the other day, and it struck me as very odd:


The pavement marking in the median makes me think this is to be treated as two intersections, but there is only one signal. 

The other part that seems weird is that the NB and SB lefts are interlocking (they drive behind each other), but people DON'T drive the EB and WB lefts as interlocking (they instead make lefts in front of each other).  It seems really weird that the pavement marking would apply only to the NB and SB.  Shouldn't EBL and WBL drivers technically make interlocking lefts too?

Am I crazy; is this common and I'm missing something?

[Mohkfry]

Here's an example in Chicago along Stoney Island Ave. The only difference is there is two sets of signals for the cross street, and not just one. AFAIK, it's not too common. In fact, this is the only street I know of where this set-up exists.

There is a "left on green arrow only" signal here as well.

https://www.google.com/maps/@41.733369,-87.5855769,3a,75y,130.07h,80.42t/data=!3m6!1e1!3m4!1sNppcUcqekiC_ed36nEBDFQ!2e0!7i13312!8i6656

[Brandon]

And to think there's already a great solution for this: https://en.wikipedia.org/wiki/Michigan_left

[jeffandnicole]

And to think there's already a great solution for this: https://en.wikipedia.org/wiki/Michigan_left

That's your simple solution for an otherwise normal intersection...requiring people to go thru an intersection twice?

[jakeroot]

The pavement arrows are most likely arrows which encourage traffic to pull into the intersection before they turn (to increase the flow of traffic). Interlocking lefts at permissible signals are actually pretty common outside of the US. I know in the UK, it's somewhat common because the visibility is better (that is, visibility of oncoming cars which you are yielding to). That said, none of that matters if you wait behind the line to turn. Also none of that matters if there are dedicated turn lanes leading up to the intersection and the turn lanes are pointing straight at each other. Also, there must be some median beforehand the width of a lane (you need to the equivalent of 3 lanes for a turn box to work effectively).

Here's a drawing that I've done up which demonstrates how an interlocking turn might occur. There are no left turn lanes beforehand, and traffic must yield. Ideally, traffic pulls out into the intersection, and then slides into the turn box where they wait. Cars behind you have to be careful to not block the opposite direction's left turners, which is really the only downside to this setup (otherwise, the visibility is great and it should be done more often).

It's worth noting that left-right traffic does not have this interlocking lefts because it is physically impossible with dedicated turn lanes. They'd have to make some awkward curve around each other and it would be far more dangerous than just putting the sides of your cars next to each other, and inching out just enough to see past their bumper (and into oncoming traffic so as to not pull into out into anyone).

[vtk]

Fortuitous you had that graphic handy, jakeroot.

I find this concept of interlocking lefts extremely awkward.

[jakeroot]

Fortuitous you had that graphic handy, jakeroot.

I find this concept of interlocking lefts extremely awkward.

Even more fortuitous is this video on interlocking lefts in Vancouver, BC, which I stumbled upon a few months ago while doing some research:

https://www.youtube.com/watch?v=DoYRMmbjWIc

Here's an overhead shot of the intersection with some markings showing how to turn left (followed by the incorrect movement):
 
 

The other part that seems weird is that the NB and SB lefts are interlocking (they drive behind each other), but people DON'T drive the EB and WB lefts as interlocking (they instead make lefts in front of each other).  It seems really weird that the pavement marking would apply only to the NB and SB.  Shouldn't EBL and WBL drivers technically make interlocking lefts too?

It does require some intuition...intuition such as "arrows that aren't oriented towards me probably don't apply to me". And like I explained above, interlocking lefts usually occur when there aren't dedicated left turn lanes, because of reasons that are, frankly, hard to explain beyond "it just doesn't feel right".

[jeffandnicole]

 

The other part that seems weird is that the NB and SB lefts are interlocking (they drive behind each other), but people DON'T drive the EB and WB lefts as interlocking (they instead make lefts in front of each other).  It seems really weird that the pavement marking would apply only to the NB and SB.  Shouldn't EBL and WBL drivers technically make interlocking lefts too?

It does require some intuition...intuition such as "arrows that aren't oriented towards me probably don't apply to me". And like I explained above, interlocking lefts usually occur when there aren't dedicated left turn lanes, because of reasons that are, frankly, hard to explain beyond "it just doesn't feel right".

However, a problem with these pictures is the placement of the vehicles.  In the first picture, the cars are shown more to their left.  In the 2nd picture, the cars are shown more to their right, to make it look as if they need to make harder lefts, crossing into each other's paths for a longer period of time.  If you were to draw a passing line thru the intersection, the car is practically straddling both lanes in that 2nd picture.

[johndoe]

Thanks for the responses... that Chicago offset-left example is a bit different, not sure I've seen that either.  Although that is pretty clearly treated as two intersections.

Jake's Vancouver example is good.  Notice how in the video one direction on the minor road has quite a few left turns.  Now imagine if both directions had that many lefts....they would yield to each other and throughput would go way down.  It would require more green time, so the major road's capacity would also go down.  As Brandon said, this is why Michigan Lefts can be useful.

As the driver went through, they were trying to be nice and not block the intersection.  Then you get this:

The red driver (like the one in the video) thinks by stopping they're helping the white driver, but this leads to lots of crashes where the pink driver hits the white driver at a right angle.

I'm guessing the Vancouver example doesn't allow normal lefts because the median isn't shaped quite right (notice where the green arrows are pointing in the "x-ed out" picture).

But notice the Vancouver example doesn't paint a centerline in the median as the original example does.  I feel like it's weird that EBL and WBL have to disregard that marking.  Are there any other examples where traffic is encouraged to ignore a centerline like that?

[Mohkfry]

Here's one that might fit this thread better. It is one intersection minus the yellow center line. Located in Port Orange, FL along S.R. 5A. I completely forgot about this one when I made my first post.

It's treated as an interlocking left, without the yellow center stripe.

Streetview: https://www.google.com/maps/@29.1447071,-81.0139899,3a,75y,230.86h,67.7t/data=!3m7!1e1!3m5!1swFy3twdCPYPUSdpcW7tD-Q!2e0!5s20150501T000000!7i13312!8i6656!6m1!1e1

Ariel view:
https://www.google.com/maps/place/South+Daytona,+FL/@29.1444594,-81.014462,88m/data=!3m1!1e3!4m2!3m1!1s0x88e6d9c95b4c4485:0x97966dfdde57904b!6m1!1e1

[jeffandnicole]

Are there any other examples where traffic is encouraged to ignore a centerline like that?

Well, almost everytime traffic enters a driveway they have to ignore a yellow center line & white shoulder line.

It's pretty clear the intention of the center line is to assist NB & SB turning traffic.  It's also pretty clear that it has nothing to do with EB & WB turning traffic. Even the left yellow line indicates to turning motorists to cut to the left, rather than go straight then make a 90' turn into opposing left turning traffic.

[vdeane]

This is a divided highway where there's enough space between the carriageways that it almost acts like two intersections.  Naturally, we wouldn't want people to turn onto the wrong carriageway and be going opposite the direction of traffic.

[cl94]

Stuff like this happens all over the place. While most of the cases have been removed or converted to split phasing, here is an example outside of Buffalo. It is treated as an interlocking left and signaled as such.

[realjd]

They call those "offside turns" in the UK. They even have pavement markings indicating where you're supposed to do it.

They often also don't offset their dedicated turn lanes the way we do in the US where they're aimed head first toward each other. They'll often have the right edge of both right turn lanes line up which makes this type of turn more natural, especially for cars pulling into the intersection anticipating the turn if there isn't a right turn arrow.

[johndoe]

Well, almost everytime traffic enters a driveway they have to ignore a yellow center line & white shoulder line.

It's pretty clear the intention of the center line is to assist NB & SB turning traffic.  It's also pretty clear that it has nothing to do with EB & WB turning traffic. Even the left yellow line indicates to turning motorists to cut to the left, rather than go straight then make a 90' turn into opposing left turning traffic.

I suppose it's true that driveways act like that, although IMO that's different because you're leaving a highway and entering a different facility type.  The yellow marking wrapping around the intersection is a good clue, but how many drivers even recognize that? (Now that I think about it, this stretch has yellow markings on the edgeline and the centerline...that's gotta be pretty rare too)

It still seems this is a strange combination of "single" and "dual"  intersection(s).  There are plenty of divided highway examples out there, but we haven't seen any others with a centerline marking in an intersection.  Let's say the median were 10' wider...do you still think it'd be obvious to EBL and WBL? 

[cl94]

Many states avoid this problem for left turns from the main highway by having the turn lane cross over. As far as side streets go, this configuration is very common if the median is too wide for opposing left turns to be "clean". There are a few intersections north of Buffalo that have opposing protected lefts from the side streets with a similar median width. Even with the side street being 4 lanes, you have to drive at a 30 degree slant in the oncoming lane for 100 feet. Drivers unfamiliar with the area often have issues navigating.

[mrsman]

Here's an example in LA, Barrington at San Vicente:

https://www.google.com/maps/@34.0536973,-118.4656686,3a,90y,139.65h,56.57t/data=!3m6!1e1!3m4!1s33Ydgjb3sfAKkei_cJgNuQ!2e0!7i13312!8i6656

Cars have to make left turns behind each other.  Despite the setup, the intersection works pretty well.  First, there is only one lane of traffic on each direction of Barrington, but due to parking lanes, the road is wide enough for through traffic to bypass left turners by going on the right.  Second, there are lagging lefts that allow for intersection clearance.  Third, the left turn lanes from SV to Barrington do not interfere with each other (and I beleive are protected only lefts).

[freebrickproductions]

Seems like this could be an issue with protected lefts, IMO.

[Brandon]

Many states avoid this problem for left turns from the main highway by having the turn lane cross over.

Illinois does this quite often.  Michigan, on the other hand, has a much different solution.  And, IMHO, it works much better (turn right to turn left).

[Brian556]

That setup is standard practice if the median is over a certain width, and is MUTCD required, along with stop or yield signs in the median at non-signalized intersections.