Roundabout Design 3

[endameshaft]

Could somebody please tell me the percentage gradient that is allowed coming into and going away from a Roundabout.

I am working to the Irish design Guidelines.

 

[oudst]

i think it will be different for every country but i would keep a 2% grade as a maximum. But i havent been working with Irish Guidelines.

 

[ACtrafficengr]

I think the US (FHWA) guidelines say to keep the approach grades at 3% or less.

------------------------------------------
"Come to think of it, there are already a million monkeys typing on a million typewriters, and the Usenet is NOTHING like Shakespeare.

- Blair Houghton

 

[ruggedscot]

Forget roundabouts.....

we have them in the UK and they are now throwing traffic lights at them to try and keep control of them.

they only work up to specific amount of cars and then well.

Rugged

 

[TD2K]

There's some of them starting to show up here in Washington State, USA and in B.C. Canada.

They've worked quite well in my experience though the initial prophecies were for mass confusion and accidents. We have one here in my town that has really eliminated the traffic backup that used to exist with the four way stop signs previously there (now, that was an exercise in frustration watching people try to figure out when they should go at an intersection controlled by 4 way stop signs).

 

[francesca]

Signalized roundabouts are common in England on roads with very high traffic volumes, multiple inward and outward lanes on frequently more than four intersecting roads and multiple lanes around the roundabout. Where you have more than four legs on an intersection, all of them with high traffic volumes, a plain signal is still going to cause inconvenience and delay. Whether or not the delay would be the same with an intersection of the same size as the roundabout is a point to consider, but such an intersection would no doubt be no less confusing; not to speak of the trouble during construction.

All intersections only work for a specific amount of cars.

Undoubtedly, the most confusing roundabout I've ever seen is at Hatton Cross, just outside Heathrow Airport. It has five legs to it, with a bus depot and tube station located between two of the legs. Where each leg of the roundabout meets the circulating lanes, there is a mini-roundabout, with traffic on the circulating lanes having to yield to inbound traffic. The circulating lanes go in both directions.

I've found a photograph. Click on it to bring up the full size. (It's approximately 250kB.)

 

[teeman]

Are you sure the Hatton Cross traffic circle qualifies as a roundabout? The size of the central island makes it look more like an old fashioned rotary. It was my understanding that the size of the central island in a roundabout has an upper limit in order to keep the speed of circulating traffic in the 20 to 25 mph range. Larger radii contribute to higher circulating speeds, requiring larger gaps for entry and lower operational efficiencies.

 

[francesca]

There may be an upper limit in the US, but in the UK, colloqially speaking in any event, (I was responding to ruggedscot's post,) they're all called roundabouts -- even the ones at motorway (interstate) offramps that you could put a football field in the middle of. These large ones are the usual candidates for signals, as ruggedscot pointed out happens frequently now.

I designed a few large roundabouts in my time in London, but anything with signals was analyzed in TRANSYT, an old text-based network analysis software (that has been upgraded recently, it seems), rather than traditional roundabout analysis software (ARCADY, also from the TRL family). The really big monsters sometimes go by "Gyratory", as the Hangar Lane Gyratory, which I had the joy of modeling in either TRANSYT or SATURN, I can't recall which. I also had the misfortune of trying to navigate [/LINK]that one in a car, but evidently I came out unscathed.

I would suggest that the biggest argument for not considering Hatton Cross's invention a roundabout would be the fact that traffic circulates in both directions.

Regardless of its classification, however, I take my hat off to the engineer who came up with that solution and had the temerity to present it to the local authority. I've been around that roundabout many times at different times of the day and I've never seen any problems with its operation!

 

[ACtrafficengr]

In the US, "roundabout" is used to separate modern designs from the earlier "traffic circles," a term which brings terror to the minds of people all over the northeastern U.S.

Many folks have seen an aerial photo of a new roundabout being constructed inside an old circle.

Traffic in the old circle moved at speeds up to 45 mph, with resultant chaos, including a reported crash every three to four days. Now that speeds are in the 20 mph range things are much better.

It also distinguishes them from "Seattle Circles," a sort of mutated mini-roundabout native to the Pacific Northwest.

These are used primarily for traffic calming, with intersection control as an afterthought.

So, in the grand American tradition of calling a spade an entrenching tool, they are roundabouts, not traffic circles.

------------------------------------------
"Come to think of it, there are already a million monkeys typing on a million typewriters, and the Usenet is NOTHING like Shakespeare.

- Blair Houghton

 

[CaliforniaTraffic]

Has anyone dealt with roundabouts in a region with severe dense fog? In the Central Valley of California, the winter months can bring thick tule fog that at times does not allow the driver even 50' or 100' of visibility. Some of the local developers are very keen to install roundabouts due to the attractive landscaping and hardscape opportunities offered by the roundabouts. They also are drawn to having a focal point for their development. However, driver unfamiliarity with an intersection is a concern for us where the driver is hugging the left edgeline or centerline to see where they should be going.

 

[francesca]

Other than a lack of driver familiarity (which will quickly remedy itself in a residential area), I can't see how roundabouts will perform any differently to priority (stop-sign) intersections in the fog, except that circulation speeds are much slower than traffic speeds on the primary leg of a priority intersection.

 

[CaliforniaTraffic]

One of our senior plan checkers is concerned that someone will drive smack into it because the roundabout requires you to curve around the perimeter whereas 95% of our streets are all N-S or E-W. I think the splitter islands will alleviate this, but he still describes the roundabout as an obstruction.

 

[francesca]

Provide adequate signage and that should allay his fears. If not, take him on a ride around a roundabout. It might revolutionize his life.

 

[CaliforniaTraffic]

Unfortunately we have a "traffic circle" in the most popular shopping center in town that is often cursed and frowned upon by some people. Of course professionals can point to the difference between the two! We will have to see how people drive them in the dense winter fog. Worst case is that we have problems and go back in future years to demo the roundabout and install a conventional traffic signal, but that would be a $300,000+ experiment...