Zipper Merge News

[kalvado]

For those early mergers, I'm curious what you all think of the frequent expansion of roads at "choke points" only to return to the smaller size right after them. Should we not use the expansions designed to alleviate traffic?

For example, when too many people are trying to turn left onto a road or highway, they frequently install a second left turn lane and a few hundred feet of a second lane on the road to the left to allow people to merge without requiring the left turn signal to be green and impeding the opposing thru traffic for so long?

Also, I know in Palm Beach County, they frequently install a 2nd or 3rd lane for a few hundred feet before a busy intersection only for it to end a few hundred feet after, so people can "skip the line" and not get stuck missing a light cycle. Doing this seems to successfully allow the road to handle much more traffic without the expense of expanding the entire road to 3 lanes, such as if you look at eastbound Clint Moore here: https://www.google.com/maps/@26.4077098,-80.1464466,195m/data=!3m1!1e3

They usually seem to try to expand all the intersections first to alleviate traffic, and only after that begins to fail, connect the 3rd lanes at the intersections (which also makes that eventual 3rd lane construction much less disruptive, as they don't need to rebuild the intersections at that point). If we're all supposed to early merge, wouldn't we never use these lanes specifically designed for us to enter so we can pass the choke point?

I know what you're talking about - and my feeling is that idea may work, but actual design is usually awful over here.  Of 5 or 6 spots I can think of, only one somewhat works. Well, local road designers are only in third decade of learning curve on the matter...
Overall, for the concept to work as designed, you need vehicles in the "main" lane to pass coke point - intersection or turn - at a rate of 1 car every 4-5 seconds. Looks like even most underpowered US vehicles (and from my observations, Jeep Patriot tends to be the worst among passenger cars) can do less than 3 seconds on level road. Maybe different in more fuel economy - conscious places. So from my experience those lanes can in fact be used only to pass a known slow vehicle - e.g. trash truck or dump truck. 

Once again to reiterate - in order for the concept to work, you need cars spaced by much more than 2 seconds, so there is enough interval to merge.  Otherwise all those bypass lane do is slowing down flow upstream.  Usually cars pass intersection at much smaller intervals making those lanes meaningless. Some limited visibility or uphill turns may use the concept, though.   

[UCFKnights]

For those early mergers, I'm curious what you all think of the frequent expansion of roads at "choke points" only to return to the smaller size right after them. Should we not use the expansions designed to alleviate traffic?

For example, when too many people are trying to turn left onto a road or highway, they frequently install a second left turn lane and a few hundred feet of a second lane on the road to the left to allow people to merge without requiring the left turn signal to be green and impeding the opposing thru traffic for so long?

Also, I know in Palm Beach County, they frequently install a 2nd or 3rd lane for a few hundred feet before a busy intersection only for it to end a few hundred feet after, so people can "skip the line" and not get stuck missing a light cycle. Doing this seems to successfully allow the road to handle much more traffic without the expense of expanding the entire road to 3 lanes, such as if you look at eastbound Clint Moore here: https://www.google.com/maps/@26.4077098,-80.1464466,195m/data=!3m1!1e3

They usually seem to try to expand all the intersections first to alleviate traffic, and only after that begins to fail, connect the 3rd lanes at the intersections (which also makes that eventual 3rd lane construction much less disruptive, as they don't need to rebuild the intersections at that point). If we're all supposed to early merge, wouldn't we never use these lanes specifically designed for us to enter so we can pass the choke point?

I know what you're talking about - and my feeling is that idea may work, but actual design is usually awful over here.  Of 5 or 6 spots I can think of, only one somewhat works. Well, local road designers are only in third decade of learning curve on the matter...
Overall, for the concept to work as designed, you need vehicles in the "main" lane to pass coke point - intersection or turn - at a rate of 1 car every 4-5 seconds. Looks like even most underpowered US vehicles (and from my observations, Jeep Patriot tends to be the worst among passenger cars) can do less than 3 seconds on level road. Maybe different in more fuel economy - conscious places. So from my experience those lanes can in fact be used only to pass a known slow vehicle - e.g. trash truck or dump truck. 

Once again to reiterate - in order for the concept to work, you need cars spaced by much more than 2 seconds, so there is enough interval to merge.  Otherwise all those bypass lane do is slowing down flow upstream.  Usually cars pass intersection at much smaller intervals making those lanes meaningless. Some limited visibility or uphill turns may use the concept, though.

Even if it causes a slowdown upstream, its past the choke point so the congestion isn't pushed further back. At some of these intersections, before the expansion, during rush hour, it would not be able to clear every vehicle that comes to the light. Once that starts to happen, it gets progressively worse every single light cycle during rush hour... making it worse once people can no longer get into the left or right turn lanes and then it continues to back up further into other intersections. After the small expansion, the light clears all traffic nearly every cycle, and while sometimes upstream of the light at the merge, traffic is slowed down or someone stops in the merge lane, its after the light, and can clear itself after the light is red.

[kalvado]

Even if it causes a slowdown upstream, its past the choke point so the congestion isn't pushed further back. At some of these intersections, before the expansion, during rush hour, it would not be able to clear every vehicle that comes to the light. Once that starts to happen, it gets progressively worse every single light cycle during rush hour... making it worse once people can no longer get into the left or right turn lanes and then it continues to back up further into other intersections. After the small expansion, the light clears all traffic nearly every cycle, and while sometimes upstream of the light at the merge, traffic is slowed down or someone stops in the merge lane, its after the light, and can clear itself after the light is red.

Let's not behave like traffic designers, lets talk engineering.
What is the throughput of a single downstream lane during green cycle? what is the throughput of a choke point, what are the limiting factors? Throughput at merge point?
What are the possible  penalties associated with merge - like longer yellow phase to allow cars to vacate intersection?
Is there an advantage or disadvantage  of arranging approach as a wider multilane section, as opposed to a longer turn lane? What is the distance past choke  point required to achieve smooth merge of two dense streams, and would that end up being different from adding extra lane throughout the road?

If I had some time to spare, I would go and take a movie of a busy intersection to show my points a bit better. Without solid numbers, this is all doesn't worth server bandwidth..

[jeffandnicole]

For those early mergers, I'm curious what you all think of the frequent expansion of roads at "choke points" only to return to the smaller size right after them. Should we not use the expansions designed to alleviate traffic?

For example, when too many people are trying to turn left onto a road or highway, they frequently install a second left turn lane and a few hundred feet of a second lane on the road to the left to allow people to merge without requiring the left turn signal to be green and impeding the opposing thru traffic for so long?

Also, I know in Palm Beach County, they frequently install a 2nd or 3rd lane for a few hundred feet before a busy intersection only for it to end a few hundred feet after, so people can "skip the line" and not get stuck missing a light cycle. Doing this seems to successfully allow the road to handle much more traffic without the expense of expanding the entire road to 3 lanes, such as if you look at eastbound Clint Moore here: https://www.google.com/maps/@26.4077098,-80.1464466,195m/data=!3m1!1e3

They usually seem to try to expand all the intersections first to alleviate traffic, and only after that begins to fail, connect the 3rd lanes at the intersections (which also makes that eventual 3rd lane construction much less disruptive, as they don't need to rebuild the intersections at that point). If we're all supposed to early merge, wouldn't we never use these lanes specifically designed for us to enter so we can pass the choke point?

I know what you're talking about - and my feeling is that idea may work, but actual design is usually awful over here.  Of 5 or 6 spots I can think of, only one somewhat works. Well, local road designers are only in third decade of learning curve on the matter...
Overall, for the concept to work as designed, you need vehicles in the "main" lane to pass coke point - intersection or turn - at a rate of 1 car every 4-5 seconds. Looks like even most underpowered US vehicles (and from my observations, Jeep Patriot tends to be the worst among passenger cars) can do less than 3 seconds on level road. Maybe different in more fuel economy - conscious places. So from my experience those lanes can in fact be used only to pass a known slow vehicle - e.g. trash truck or dump truck. 

Once again to reiterate - in order for the concept to work, you need cars spaced by much more than 2 seconds, so there is enough interval to merge.  Otherwise all those bypass lane do is slowing down flow upstream.  Usually cars pass intersection at much smaller intervals making those lanes meaningless. Some limited visibility or uphill turns may use the concept, though.   

In my experiences, too few people use them.  That said, since the lane is often empty, I'll use it and can usually move up several spots, and if I'm lucky, I'll get by the slowpoke car...the one that should have really been using that auxiliary lane in the first place.

The worst thing about the extra lane...those self-righteous drivers who stay in the regular lane of traffic and who think others shouldn't be using it so they tailgate, as if the extra lane is for decorative purposes only. 

[kalvado]

In my experiences, too few people use them.  That said, since the lane is often empty, I'll use it and can usually move up several spots, and if I'm lucky, I'll get by the slowpoke car...the one that should have really been using that auxiliary lane in the first place.

The worst thing about the extra lane...those self-righteous drivers who stay in the regular lane of traffic and who think others shouldn't be using it so they tailgate, as if the extra lane is for decorative purposes only.

Well, if people in main lane can tailgate - it means that a bypass lane is for decorative purposes only. Because whole purpose of that is to supply cars to fill voids in traffic past the choke point. And if there are no voids...

[jeffandnicole]

In my experiences, too few people use them.  That said, since the lane is often empty, I'll use it and can usually move up several spots, and if I'm lucky, I'll get by the slowpoke car...the one that should have really been using that auxiliary lane in the first place.

The worst thing about the extra lane...those self-righteous drivers who stay in the regular lane of traffic and who think others shouldn't be using it so they tailgate, as if the extra lane is for decorative purposes only.

Well, if people in main lane can tailgate - it means that a bypass lane is for decorative purposes only. Because whole purpose of that is to supply cars to fill voids in traffic past the choke point. And if there are no voids...

There isn't a void because someone is tailgating to eliminate that void.  That doesn't mean the lane isn't needed, and the situation is made worse by someone who is illegally tailgating. 

Besides...that one spot may have been lost, but there's still room in front of and behind other vehicles.

[kalvado]

In my experiences, too few people use them.  That said, since the lane is often empty, I'll use it and can usually move up several spots, and if I'm lucky, I'll get by the slowpoke car...the one that should have really been using that auxiliary lane in the first place.

The worst thing about the extra lane...those self-righteous drivers who stay in the regular lane of traffic and who think others shouldn't be using it so they tailgate, as if the extra lane is for decorative purposes only.

Well, if people in main lane can tailgate - it means that a bypass lane is for decorative purposes only. Because whole purpose of that is to supply cars to fill voids in traffic past the choke point. And if there are no voids...

There isn't a void because someone is tailgating to eliminate that void.  That doesn't mean the lane isn't needed, and the situation is made worse by someone who is illegally tailgating. 

Besides...that one spot may have been lost, but there's still room in front of and behind other vehicles.

So get behind that tailgaiting vehicle, what is the problem? There should be a double-long spot behind them. Or if there is no room there as well, and you actually want others to make room for you.. If drivers have to slow down to accommodate traffic from bypass lane - it is likely a net loss for throughput, and decorative lane is just a waste.

Of course, it is nice to be able to jump past a slowpoke - but in a grand scheme of things it is probably insignificant. 

[jeffandnicole]

In my experiences, too few people use them.  That said, since the lane is often empty, I'll use it and can usually move up several spots, and if I'm lucky, I'll get by the slowpoke car...the one that should have really been using that auxiliary lane in the first place.

The worst thing about the extra lane...those self-righteous drivers who stay in the regular lane of traffic and who think others shouldn't be using it so they tailgate, as if the extra lane is for decorative purposes only.

Well, if people in main lane can tailgate - it means that a bypass lane is for decorative purposes only. Because whole purpose of that is to supply cars to fill voids in traffic past the choke point. And if there are no voids...

There isn't a void because someone is tailgating to eliminate that void.  That doesn't mean the lane isn't needed, and the situation is made worse by someone who is illegally tailgating. 

Besides...that one spot may have been lost, but there's still room in front of and behind other vehicles.

So get behind that tailgaiting vehicle, what is the problem? There should be a double-long spot behind them. Or if there is no room there as well, and you actually want others to make room for you.. If drivers have to slow down to accommodate traffic from bypass lane - it is likely a net loss for throughput, and decorative lane is just a waste.

Of course, it is nice to be able to jump past a slowpoke - but in a grand scheme of things it is probably insignificant. 

The problem goes back to post 1 in this thread...how to properly zipper.

[kalvado]

In my experiences, too few people use them.  That said, since the lane is often empty, I'll use it and can usually move up several spots, and if I'm lucky, I'll get by the slowpoke car...the one that should have really been using that auxiliary lane in the first place.

The worst thing about the extra lane...those self-righteous drivers who stay in the regular lane of traffic and who think others shouldn't be using it so they tailgate, as if the extra lane is for decorative purposes only.

Well, if people in main lane can tailgate - it means that a bypass lane is for decorative purposes only. Because whole purpose of that is to supply cars to fill voids in traffic past the choke point. And if there are no voids...

There isn't a void because someone is tailgating to eliminate that void.  That doesn't mean the lane isn't needed, and the situation is made worse by someone who is illegally tailgating. 

Besides...that one spot may have been lost, but there's still room in front of and behind other vehicles.

So get behind that tailgaiting vehicle, what is the problem? There should be a double-long spot behind them. Or if there is no room there as well, and you actually want others to make room for you.. If drivers have to slow down to accommodate traffic from bypass lane - it is likely a net loss for throughput, and decorative lane is just a waste.

Of course, it is nice to be able to jump past a slowpoke - but in a grand scheme of things it is probably insignificant. 

The problem goes back to post 1 in this thread...how to properly zipper.

You cannot have a cake and eat it too.
There are intervals between cars, 2 seconds or slightly less. And if one lane is able to supply that much traffic, you cannot squeeze in traffic from the second lane.

I bet the way you think about it is "if they slow down a tiny little bit and let me in, nothing will change". Incremental change is not that great - but if you look at the end result, you will see the price to be paid.

[kalvado]

You do understand that these lanes exist, and yes, traffic from the second lane can squeeze in because, as we all know, drivers don't leave 2 second gaps.

In fact, in reference to that, in most cases that's just a suggested safety gap.  I'm not sure if there's any actual law anywhere requiring it.  So, what is tailgating?  Basically, it's in the eye of the beholder.

Many bad things exist in this world - Trump, Billary, cancer and roundabouts just to name a few...
That is not the point, point is if there is any good in existence of those bypass lanes, or bypass lanes belong to the same group. 

Previously I showed actual studies - highway lanes choke at 2000 VPH, or 1 vehicle every 1.8 seconds. A little bit more, a little bit less - still works about the same way, probably lower for streets.
So question is: what is the actual rate of vehicle flow through the intersection compared to flow in a straight lane downstream, and how long does it take for a consistent flow to establish. I don't have any data handy. Of course, I can just park a car with dashcam running near the intersection for 15-20 minutes, but I am lazy...
But my impression is that traffic through the single-lane intersection still flows in a fairly solid formation quarter mile past the light. Solid as in no easy merge spots, and fairly dense spacing. That would be pretty much the answer.

Or, if you will, a question is how bad of a choke point an average intersection is, and how fast can cars depart from a stopped lane.
My impression, once again, is that once cars further behind start moving, entrance rate is approximately the same as lane flow rate.

[thenetwork]

Zipper merging only works if you have two lanes merging into the middle, and then traffic moves where it needs to.  I hate the budgers so much, the ones who do it every day in non-construction zones

I agree on that (merge into the middle straddling both lanes), and think a zipper merge should require flashing (diagonal down) yellow arrows on both sides of the single open lane to indicate that traffic should alternate at the zipper point.  I have yet to see any zipper merge use the latter idea.

[empirestate]

With some trepidation, I thought I'd bump this topic since it seems to be in vogue these days, with various articles on the subject currently going around social media.

Most of these articles contain what I see as a fatal flaw: they only present the issue in terms of two possible options. One is to merge early and be courteous, and the other is to be a jerk, but correct. So, some drivers reading these articles will undoubtedly decide to merge early anyhow, knowing that it's not the best way, because they don't want to appear discourteous. But one of the articles links to this MoDOT page, which seems to do the best job at explaining the situation clearly and in full.

I also have to wonder, to what extent would it help simply to deprecate signage that refers to either the right or left lane ending (regardless of the actual configuration), to enforce the idea that there's no "correct" lane to be in and thus discourage early merging in case where it's not appropriate, as well as to eliminate the basis for vigilanteism?

[jeffandnicole]

I also have to wonder, to what extent would it help simply to deprecate signage that refers to either the right or left lane ending (regardless of the actual configuration), to enforce the idea that there's no "correct" lane to be in and thus discourage early merging in case where it's not appropriate, as well as to eliminate the basis for vigilanteism?

As I've probably said before, for the most part, NJDOT will generally only sign lane closures 1,500 feet in advance (and I've seen no more than 1,000 feet in advance too).  The most you'll get is about 1/4 mile of unused lane.

It's hard to say that there's no one particular lane that's ending, because it would invite confusion.   It's rare to be able to have an equal merge because you somehow have to instruct people to ride over the line, not between the lines, which motorists aren't used to doing.  There are people that refuse to cross a solid line even when there's an obstruction in the road, and they squeeze as best they can between the obstruction and the solid line.

[hbelkins]

In my mind, the concept of the zipper merge works best if you have frequent signs that say "Use Both Lanes Up To Merge Point" and then signs at the merge point instructing drivers to take turns advancing into the open lane.

[empirestate]

It's hard to say that there's no one particular lane that's ending, because it would invite confusion.   It's rare to be able to have an equal merge because you somehow have to instruct people to ride over the line, not between the lines, which motorists aren't used to doing.  There are people that refuse to cross a solid line even when there's an obstruction in the road, and they squeeze as best they can between the obstruction and the solid line.

Not sure I follow you; what do you mean by "riding over the line"? What I'm thinking of would only involved changing the wording on signs.

[kalvado]

With some trepidation, I thought I'd bump this topic since it seems to be in vogue these days, with various articles on the subject currently going around social media.

Most of these articles contain what I see as a fatal flaw: they only present the issue in terms of two possible options. One is to merge early and be courteous, and the other is to be a jerk, but correct. So, some drivers reading these articles will undoubtedly decide to merge early anyhow, knowing that it's not the best way, because they don't want to appear discourteous. But one of the articles links to this MoDOT page, which seems to do the best job at explaining the situation clearly and in full.

I also have to wonder, to what extent would it help simply to deprecate signage that refers to either the right or left lane ending (regardless of the actual configuration), to enforce the idea that there's no "correct" lane to be in and thus discourage early merging in case where it's not appropriate, as well as to eliminate the basis for vigilanteism?

Fatal flaw of the article is saying "capacity is maximized" right on a picture. I suspect "safety is increased" is also wrong. 

[jakeroot]

Fatal flaw of the article is saying "capacity is maximized" right on a picture. I suspect "safety is increased" is also wrong.

Based on what?

[kalvado]

Fatal flaw of the article is saying "capacity is maximized" right on a picture. I suspect "safety is increased" is also wrong.

Based on what?

Based on all discussion above along with common sense.
Otherwise, a peer reviewed research paper comparing two merge modes may be an argument, but - as always - no time to learn how to do things right and waste money on research, we need just finalize the budget.

[jeffandnicole]

It's hard to say that there's no one particular lane that's ending, because it would invite confusion.   It's rare to be able to have an equal merge because you somehow have to instruct people to ride over the line, not between the lines, which motorists aren't used to doing.  There are people that refuse to cross a solid line even when there's an obstruction in the road, and they squeeze as best they can between the obstruction and the solid line.

Not sure I follow you; what do you mean by "riding over the line"? What I'm thinking of would only involved changing the wording on signs.

If you have two lanes, one lane *has* to merge into the other.  Is the right merging into the left, or left merging into the right?  You can't just post "lane ends" signage for both lanes.  Or no signage at all. 

If there's two lanes, and they both have an equal merge removing half their lane, you'll be driving over the center line that divides the two lanes.

Now, I'm thinking construction based, where the lane merge is temporary.  If you're thinking of a permanent lane ending, then you definitely need some clarification, because for the most part traffic will be moving fine.  It's not like there's going to be a 24 hour backup where a zipper merge is always necessary.

[jakeroot]

Fatal flaw of the article is saying "capacity is maximized" right on a picture. I suspect "safety is increased" is also wrong.

Based on what?

Based on all discussion above along with common sense.

Oh, right. This is the sixth page of this discussion.

No need to go around and around again. We know each other's opinions on the matter.

[empirestate]

It's hard to say that there's no one particular lane that's ending, because it would invite confusion.   It's rare to be able to have an equal merge because you somehow have to instruct people to ride over the line, not between the lines, which motorists aren't used to doing.  There are people that refuse to cross a solid line even when there's an obstruction in the road, and they squeeze as best they can between the obstruction and the solid line.

Not sure I follow you; what do you mean by "riding over the line"? What I'm thinking of would only involved changing the wording on signs.

If you have two lanes, one lane *has* to merge into the other.  Is the right merging into the left, or left merging into the right?  You can't just post "lane ends" signage for both lanes.  Or no signage at all.

That's what I'm not following–why not? Whichever lane it is that's actually closed, you just don't mention that on signage; instead you'd just say "lanes merge ahead" (or similar). You wouldn't have to configure the lanes any different physically, in my scenario.

If there's two lanes, and they both have an equal merge removing half their lane, you'll be driving over the center line that divides the two lanes.

I see. Yeah, that's not part of what I'm wondering about; I'm talking only about changing the wording on signs. Now, a logical next step in the discussion could involve actually changing the physical configuration, which I think would necessarily involve temporary re-striping to avoid what you describe. But I'm not quite at that step.

Now, I'm thinking construction based, where the lane merge is temporary.  If you're thinking of a permanent lane ending, then you definitely need some clarification, because for the most part traffic will be moving fine.  It's not like there's going to be a 24 hour backup where a zipper merge is always necessary.

Construction based, yeah, or any scenario where traffic is slowed such that a zipper merge is appropriate. Maybe you could imagine a permanent lane drop, with changeable signage that reflects current traffic speeds–but again, I'm not quite that far along in my hypothesis yet.

[mrsman]

In most cases, one lane ends and it is subservient to the other.  If you are in the lane that ends you must safely merge in and there is no requirement for traffic in the other lane to let you in.  This of course, is standard at most freeway entrances and is shown by signs that denote right lane ends (or left lane ends)

The alternate merge, or zipper merge, is great and is designed that neither lane has priority over the other.  You see this on the NJ turnpike where both roadways merge together and the right lane of the car lanes merges with the left lane of the car-truck-bus lanes.  I also see this regularly on I-70 merging into I-695 north near Baltimore.  The alternate merge works great here because it is carefully worded and everyone understands what to do.

[empirestate]

In most cases, one lane ends and it is subservient to the other.  If you are in the lane that ends you must safely merge in and there is no requirement for traffic in the other lane to let you in.  This of course, is standard at most freeway entrances and is shown by signs that denote right lane ends (or left lane ends)

So my question is simply whether, in work zones especially, it makes sense NOT to have signs that denotes which lane ends, and simply to state that the lanes merge together. This being one step in a process, mind you, not an entire solution.



iPhone

[jp the roadgeek]

In most cases, one lane ends and it is subservient to the other.  If you are in the lane that ends you must safely merge in and there is no requirement for traffic in the other lane to let you in.  This of course, is standard at most freeway entrances and is shown by signs that denote right lane ends (or left lane ends)

So my question is simply whether, in work zones especially, it makes sense NOT to have signs that denotes which lane ends, and simply to state that the lanes merge together. This being one step in a process, mind you, not an entire solution.



iPhone

Another solution might be to put cones between the lane that ends and the lane that it merges into for a certain distance up until just before the closure point.  That way you won't have cars in the lane that doesn't close coming to a stop so that other cars can merge in 2 miles before the closure and then have perceived "cutters" zoom along at 70 until just before the merge and expect to be let in by the vehicles they just passed.

[paulthemapguy]

An aspect of this topic that has not been discussed...
What if an exit ramp is the choke point?
How should three lanes worth of freeway traffic start forming a single file line (while allowing the occasional thru car to go by at speed)?

Sounds like a question for whichever dipshit designed the Hillside Strangler at I-88 and I-290 by Chicago.