Asymmetrical Roundabouts

[corco]

http://goo.gl/maps/JrSBs

Is this an odd thing, or does this happen all over the place? The north and west sides have fewer lanes than the south and east.

[vdeane]

It's a stub, but still: http://goo.gl/maps/uMe4y

[SteveG1988]

Traffic Circle: http://goo.gl/maps/YlZWU

NJ has several, thats the first one that comes to mind, there used to be one on fort dix at the end of NJ 68 but it has been closed off to traffic,

http://goo.gl/maps/Ahl6P

Fort dix road stayed four lanes going into it, if i remember, NJ68 always was four lanes into it

[briantroutman]

...one on fort dix at the end of NJ 68 but it has been closed off to traffic,

http://goo.gl/maps/Ahl6P

Do you know the history of this one–did the base simply grow and consume what was previously area open to the public? Based on the clearing (and the names), it looks like the through routing of Pemberton Wrightstown Road was straight through General Circle.

And I took a quick look on street view–I'm surprised there isn't a sign on NJ 68 just prior to the Fort Dix intersection saying something like "Restricted Military Post Ahead - All Civilian Traffic Must Turn Right". Unless there is something like that and I missed it. The way it's set up, it looks too easy to be driving down 68 and not realize you're heading into an army base until you're wasting some guard's time as he directs you out.

[NE2]

NJ 68 was driveable to the end before Sekrit Muslims Pearl Harbored us. I think at that time, Pemberton Wrightstown Road was realigned.

[silverback1065]

Traffic Circle: http://goo.gl/maps/YlZWU

NJ has several, thats the first one that comes to mind, there used to be one on fort dix at the end of NJ 68 but it has been closed off to traffic,

http://goo.gl/maps/Ahl6P

Fort dix road stayed four lanes going into it, if i remember, NJ68 always was four lanes into it

I don't think that first link is a traffic circle, it's just an elliptical roundabout, it brings this one to mind. https://www.google.com/maps/preview#!data=!1m4!1m3!1d523!2d-86.1644408!3d39.9561294!2m1!1e3&fid=7

[theline]

Can I get a clarification on the definitions of traffic circle and roundabout? I've heard them used interchangeably.

Here's an unusual one from South Bend. Although the pavement may be of uniform width, it's striped with a varying number of lanes. The funky shape results from converting an ordinary intersection that is squeezed between 2 cemeteries.

[Brandon]

Can I get a clarification on the definitions of traffic circle and roundabout? I've heard them used interchangeably.

Beats the crap out of me other than one term came from the UK.  However, whatever term you use, they should never be used with stop signs:

Aerial: https://maps.google.com/maps?hl=en&ll=41.532405,-88.098719&spn=0.000383,0.00066&t=h&z=21

Streetview: https://maps.google.com/maps?hl=en&ll=41.532415,-88.098393&spn=0.000383,0.00066&t=h&z=21&layer=c&cbll=41.532415,-88.098393&panoid=Pfe6mcuvbC4-wIRP3QyC-g&cbp=12,276.72,,0,5.48

Especially when the advance warning sign is for a traffic circle/roundabout: https://maps.google.com/maps?hl=en&ll=41.532428,-88.097548&spn=0.000383,0.00066&t=h&z=21&layer=c&cbll=41.532428,-88.097548&panoid=2B0jNwrPhE14CNUTTxhIcA&cbp=12,294.67,,0,5.85

[NE2]

Roundabout is a marketing term like light rail.

[J N Winkler]

To be precise, in American technical usage roundabout is positioned as a contraction of modern roundabout, i.e., a circular intersection with yield-on-entry, (usually) outward drainage, relatively small inscribed-circle diameter, and relatively low circulating speeds, designed according to principles developed in Britain between 1955 and 1965.

The original intention was to establish a distinction between the modern roundabout and old-school AASHO rotaries (large inscribed-circle diameter, high circulating speeds, merge-on-entry), traffic circles like the one around the Arc de Triomphe (large diameter, high speed, entering traffic has priority), traffic calming circles, and other forms of circular intersection of dissimilar design.

The term roundabout itself was apparently developed in the 1920's by an American working for the BBC, and was intended as a convenient substitute for the term gyratory circus.  It is properly written as one word, without hyphens, not as "round-a-bout," as a lot of uninformed engineers and design technicians do in the US (probably because it is a convenient back-construction from "RAB," which is becoming a popular abbreviation).  In original British usage, it applied to circular intersections of whatever shape and priority rule.  Yield-on-entry did not become the norm in Britain until 1965, and for at least a decade beginning in the mid-1930's, official design advice actually favored roundabouts with a rounded-rhombus shape (now squareabout in SABRE parlance), with the larger traffic volumes receiving the lesser deflections from a straight course, though several influential experts disagreed and continued to advocate a circular shape.

[agentsteel53]

merge-on-entry

what does this mean?  who has priority?  entering traffic, or already present traffic?

[tradephoric]

Great explanation J N Winkler.  Another distinction is modern roundabouts don't have pedestrian crossings to enter into the central island while a traffic circle will often allow pedestrians to cross into the central island.

[1995hoo]

merge-on-entry

what does this mean?  who has priority?  entering traffic, or already present traffic?

I've always understood it to refer to how older "traffic circles" often have the approaches and exits arranged as tangent movements. In a left-hand-drive country (USA or Canada), someone making a right turn (what in a roundabout is called taking the first exit) often wouldn't actually enter the main circulation on the circle itself. He'd stay in a dedicated entrance/exit lane. If he wanted to go further, he'd have to merge to his left (changing lanes). I suppose it's kind of similar, in a way, to the weave area between the loop-around ramps at a cloverleaf interchange, if that makes sense–traffic entering the circle and traffic exiting the circle must cross over each other's paths. (Some roundabouts use a bypass lane for people taking the first exit. Works pretty well most of the time.)

I've seen "traffic circles" that give priority to entering traffic (traffic already on the circle must yield), others that give priority to exiting traffic (traffic entering must yield, similar to a roundabout), and others that didn't sign it clearly at all such that people making the "tangent" maneuver apparently didn't have to yield but people entering or exiting the circle did. In Europe if you visited the Continent you ran into a further problem due to the "priority to the right" principle. In a left-hand-drive country, priority to the right at a circle or roundabout means that you have priority when you enter, but when the next road enters you in turn are subject to the entering vehicle's priority. It made exiting a massive problem.

Ultimately I understand that part of the motivation behind the modern roundabout was to eliminate the problems that arose from inconsistent designs and inconsistent traffic rules. "Yield to traffic already on the roundabout and signal when you're exiting" is very simple and straightforward.

Some people still don't seem to get it, though. VDOT installed a series of four roundabouts around the intersection of US-15 and US-50 in Loudoun County within the last few years and most people agree they're a huge improvement over the crummy traffic light that used to be there, but some people now complain that people on the roundabout don't slow down to let them merge. Well, they're not supposed to do that! But the biggest problem I still see is people who don't signal their exit (it freezes entering traffic because you don't know whether you have to yield) and, to a lesser degree, people using the wrong lane. The latter is less of an issue here in Northern Virginia due to a lack of two-lane roundabouts. My sister-in-law lives in Viera, Florida, and is terrified of the roundabout on Wickham Road because she almost got run into by an old man who wanted to use the outside lane to "make a left turn" (go to the third exit). You're not supposed to do that–you only use the outside lane if you're going to the first or second exit, because at the second exit people on the inside lane are permitted to exit. My sister-in-law will drive 15 minutes out of her way to avoid that roundabout.

[Alps]

Hell, I've even been involved in designing an asymmetrical roundabout. It's all based on traffic volumes, so if one half is lighter than the other, you do what you need to.

[DaBigE]

Hell, I've even been involved in designing an asymmetrical roundabout. It's all based on traffic volumes, so if one half is lighter than the other, you do what you need to.

Agreed. In fact, out of all of the roundabouts I've been involved with (several dozen), symmetrical lane configurations have been fairly rare (single-lane roundabouts excluded). Unfortunately, the general, non-roadgeek public has a hard time grasping this concept and feels that all roundabouts should look exactly the same.

[johndoe]

Your Idaho example is quite weird.


1.  Who would ever need the lane with the purple question mark?  When they went back (I'm guessing) with the yellow paint on the west side (reducing the portion in the middle from 2 to 1 lane), why didn't they do the same on the east?

2.  There used to be two WB lanes east of the roundabout, why did they take one out?  They could have both lanes going through, like the EB.

[english si]

1.  Who would ever need the lane with the purple question mark?

Traffic heading from south to north.

When they went back (I'm guessing) with the yellow paint on the west side (reducing the portion in the middle from 2 to 1 lane), why didn't they do the same on the east?

As the northern and western arms are the busy ones, and they wanted 2 lanes on those kind of flows.

The same question and answer also applies to north and south pair of sides.

[DaBigE]

A few more asymmetrical roundabouts for your viewing pleasure, along the US (I)41 corridor in NE Wisconsin: Breezewood Ln/Bergstrom Rd, Bell St/Harrison St, ramp terminals at CTH F, Shawano Ave/Taylor St. There will be ~40 roundabouts along the 41 corridor (Oshkosh - Green Bay) when construction is complete.
They only seem to get more asymmetrical the further you head north.

[english si]

How about this one?

South side is three lanes due to the exit not long after - one lane is signed for the exit. West side is zero lanes due to that side being pointless (not that it stops them elsewhere).

[colinstu]

1.  Who would ever need the lane with the purple question mark?  When they went back (I'm guessing) with the yellow paint on the west side (reducing the portion in the middle from 2 to 1 lane), why didn't they do the same on the east?

2.  There used to be two WB lanes east of the roundabout, why did they take one out?  They could have both lanes going through, like the EB.

1. This is why.


2. Why not? Lets them extend the curb out which increases pedestrian safety, less confusion for multiple cars entering the roundabout at once from that direction, etc.

[SteveG1988]

...one on fort dix at the end of NJ 68 but it has been closed off to traffic,

http://goo.gl/maps/Ahl6P

Do you know the history of this one–did the base simply grow and consume what was previously area open to the public? Based on the clearing (and the names), it looks like the through routing of Pemberton Wrightstown Road was straight through General Circle.

And I took a quick look on street view–I'm surprised there isn't a sign on NJ 68 just prior to the Fort Dix intersection saying something like "Restricted Military Post Ahead - All Civilian Traffic Must Turn Right". Unless there is something like that and I missed it. The way it's set up, it looks too easy to be driving down 68 and not realize you're heading into an army base until you're wasting some guard's time as he directs you out.

NJ 68 was driveable to the end before Sekrit Muslims Pearl Harbored us. I think at that time, Pemberton Wrightstown Road was realigned.

The history, as i was in middle school just down the road from the circle on 9/11, Suddenly a makeshift barrier was put up right beyond the school, if you look to the west of the intersection you will see two schools, barely 100 feet off the school property they put up barriers, They also closed off PointVille Road on nearly the same line, and closed off Juliustown Road on both ends, the comprimise is that they would put up a fence along the southern side of the road, closing off the entireity of Juiustown road between Fort Dix Road, and Pointville Road, routing fort dix road to NJ68 just north of the gate, prior to 9/11 fort dix was an open base, Texas Road was closed off, range road was later re-routed away from the McGuire (always closed off but had a road along the fence) Flight Line

https://www.google.com/maps?q=Pemberton+NJ&hl=en&ll=39.999413,-74.595687&spn=0.013397,0.027874&sll=39.956129,-86.164441&sspn=0.00237,0.003484&t=h&hnear=Pemberton+Township,+Burlington,+New+Jersey&z=16 The part that is open to the public is the section to the north of the buildings, the diagonal section is unpassable.

[johndoe]

1.  Who would ever need the lane with the purple question mark?

Traffic heading from south to north.

There is only 1 NB approach lane, so why would a second lane be needed in the circulating roadway?

When they went back (I'm guessing) with the yellow paint on the west side (reducing the portion in the middle from 2 to 1 lane), why didn't they do the same on the east?

As the northern and western arms are the busy ones, and they wanted 2 lanes on those kind of flows.

1. This is why.

You've perfectly illustrated the problem.  You can't just assume that because there is a second lane in the circulating roadway you can make that left turn from the right lane.  Street view shows that the left lane can go left or through, and the right lane can go through or right.

To me, this is the problem with multilane roundabouts; if engineers and road geeks don't get it right, will the general public?

[johndoe]

A few more asymmetrical roundabouts for your viewing pleasure, along the US (I)41 corridor in NE Wisconsin: Breezewood Ln/Bergstrom Rd, Bell St/Harrison St, ramp terminals at CTH F, Shawano Ave/Taylor St. There will be ~40 roundabouts along the 41 corridor (Oshkosh - Green Bay) when construction is complete.
They only seem to get more asymmetrical the further you head north.

Cool, thanks for sharing!  The Bell/Breezewood interchange is serious business!

[english si]

There is only 1 NB approach lane, so why would a second lane be needed in the circulating roadway?

traffic already on the roundabout making the left turn from west to north.

That and south-west left turns.

The markings (even to my non-American eyes) make it clear that the manoeuvre drawn by colinstu isn't what they want you to do - the dashes would follow the circulating carriageway otherwise. I'd imagine the advance signage would make it obvious that it is left lane for left and straight on, right lane for right and straight on when approaching the roundabout from the west.

[DaBigE]

1.  Who would ever need the lane with the purple question mark?

Traffic heading from south to north.

There is only 1 NB approach lane, so why would a second lane be needed in the circulating roadway?

When they went back (I'm guessing) with the yellow paint on the west side (reducing the portion in the middle from 2 to 1 lane), why didn't they do the same on the east?

As the northern and western arms are the busy ones, and they wanted 2 lanes on those kind of flows.

1. This is why.

You've perfectly illustrated the problem.  You can't just assume that because there is a second lane in the circulating roadway you can make that left turn from the right lane.  Street view shows that the left lane can go left or through, and the right lane can go through or right.

If that is the way it's signed, there should only be one circulating lane in front of the east leg. Apparently they tried to do a minimalist reconfiguration. Based on that lane usage, there is no reason to have the second circulating lane at that point. It probably hurts more than it helps.

You also wouldn't typically have that kind of exclusive right turn SB>WB either. Typically, (as seen in many of the WI examples I posted), those right turns would be better served in a partial or full bypass format. The way this one is designed, you're losing capacity (and increasing the risk of crashes), due to the exit path overlap from the single circulatory lane transition to the exit. I'm guessing that they couldn't get the ROW necessary to do it properly.

To me, this is the problem with multilane roundabouts; if engineers and road geeks don't get it right, will the general public?

Exactly. This is why I am glad Wisconsin has a tiered roundabout design program. Newbies (Level 1) cannot design more than a basic single-lane roundabout. Level 3s work on the most complex. That's not to say that bad designs don't ever get built...like this one: http://goo.gl/maps/nq9lv. The NB partial right turn bypass gets used almost as much to enter the circulatory as the proper lane.