Mini roundabouts
|
See ROUNDABOUT for source
A mini-roundabout in the United Kingdom, where a painted white circle is used for the centre. The arrows show the direction of traffic flow.
|
Feature |
1-lane Roundabout |
Clearwater's mini- roundabouts |
"Rock-Ring" Nano- roundabout
|
Inside Circle Diameter range |
80'-130' |
45-80' |
80'?? |
Essential Features |
|
|
|
Central island |
Yes |
Yes |
Yes |
Raised central isand |
Yes |
Yes |
No |
OK to drive over central island |
No |
No |
Yes |
Truck apron |
Yes |
Yes |
No |
Splitter islands |
Yes |
Yes |
Yes |
Raised splitter islands |
Yes |
No |
No |
Pedestrian crosswalks |
Yes |
No |
No |
Yield at entry |
Yes |
Yes |
Yes |
Horizontal deflection |
Yes |
Yes |
Yes |
Negative superelevation for circulating lane |
Yes |
Yes |
? |
Slows (calms) traffic |
Yes |
Yes |
Yes |
Many fewer vehicle/vehicle conflicts |
Yes |
Yes |
Yes |
Many fewer pedestrian/vehicle conflicts |
Yes |
Yes |
Yes |
Good design |
Yes |
Yes |
? |
|
|
|
|
Optional Features |
|
|
|
Vertical element in central island |
Sometimes |
Yes |
No |
Uplighting in central island |
Sometimes |
No |
No |
Texture/color contrasting truck apron |
Sometimes |
Yes |
No |
Texture/color contrasting splitter islands |
Sometimes |
No |
No |
Texture/color contrasting ped crosswalks |
Sometimes |
No |
No |
Context-Sensitive design |
Sometimes |
Yes |
Yes |
Landscaped |
Sometimes |
Yes |
No |
Opti-Curb markers on truck apron curb |
Sometimes |
Sometimes |
No |
1-way sign/chevron on central island |
Sometimes |
No |
No |
2nd YIELD sign on splitter island |
Sometimes |
No |
No |
Roundabout ahead signage |
Usually |
Yes |
? |
Free-flowing 20 hours/day |
Usually |
Yes |
Yes |
Can be modeled by SIDRA |
Yes |
Yes |
? |
Can be modeled by RODEL |
Yes |
Yes |
? |
Mini-roundabouts exist at smaller intersections to avoid the use of signals, stop signs or the necessity to yield in favour of one road of traffic. Mini-roundabouts can be a painted circle, a low dome, or often are small garden beds. Painted roundabouts and low domes can easily be driven over by most vehicles, which many motorists will do when there is no other traffic, but the practice is dangerous if other cars are present. Mini-roundabouts work in the same way as larger roundabouts in term of right of way. They can often come in "chains", making navigation of otherwise awkward junctions easier. There are usually different road signs used to distinguish mini roundabouts from larger ones.
Mini-roundabouts are also common in Irapuato, Mexico, usually marked with a tiny grassy circle enclosed edge paving, and in Calgary, Canada's inner-city Mount Royal and Rosedale neighbourhoods, where mini-roundabouts recently replaced intersections formerly controlled by stop signs to combat increasing cut-through traffic.
A slightly larger version of a mini-roundabout, sometimes called a "small roundabout", is designed with a raised centre surrounded by a sloped "overrun area" of a differen
t colour from the roadway and up to a meter in thickness called a "truck apron" or a "mountable apron". The truck apron's design discourages small vehicles from taking a shortcut over it while at the same time allowing the mini-roundabout to more easily accommodate the turning radius of larger vehicles (such as a truck which may have to navigate the roundabout). These are not well suited for bus routes, as mounting the apron can be somewhat uncomfortable to passengers.
In the UK the maximum diameter permissible of a mini-roundabout is 4m. Whilst it may be physically possible, it is illegal for vehicles like cars, which can turn around the mini-roundabout, to go over the painted island, or around the wrong way- vehicles should treat it like a solid island and proceed around it. (In practice, few motorists obey these rules). Some local authorities have installed double white lines around the island to indicate this, but these are not permissible. The centre island also must be able to be over-run by larger vehicles. If this is not possible, perhaps due to plants, or street furniture it is considered a small roundabout not a mini roundabout and as such must adhere to the stricter roundabout guidelines.
Part of the issue seems to be terminology and understanding of differences between true UK classified "Mini's" (and now other countries "minis" and "Seattle Style" traffic calming circles. They have profoundly different design criteria, driving rules and objectives.
There are no allway yields in the USA. The underlying philosophy is to stop people to save them the inconvenience of slowing down.
Is that first sentence a true statement? Isn't any uncontrolled intersection nothing more than an unposted all-way yield? Technically, I guess it's not; in an uncontrolled intersection, each approach implements a different right-of-way assignment depending on the orientation of incoming traffic. A two-way yield, on the other hand, would require one approach to yield whether traffic was coming from the right or the left. But how would a four-way yield operate?
"Filter-in-turn", meaning arrival priority with onside priority for simultaneous arrivals?
A lot of uncontrolled intersections still do exist, though neighborhood groups and ill-informed traffic engineers are apt to replace them with all-way stops, because they perceive AWSC to be safer. I personally find that AWSC encourages roll-thrus and creates a situation where drivers will feel less inclined to remain aware of their surroundings, but I think we all agree on that!
Benefits
Minis are a roundabout with all the safety benefits and performance of
"normal" roundabouts. The typical mini does have a raised central
area similar in nature to a traffic calming device since it is designed to
permit larger vehicles to make left hand turns. The 85 percent speed
aspect does not apply here since you are in an urban condition with a low
speed environment to begin with (note that the 85% rule used in
highway traffic evaluation is a fatally flawed measure). There are plenty
of minis in operation which anyone can visit (mostly in the UK) with
a few in the US and several in development. One of the design elements
which sets the design of minis at odds with American practice is
the use of two four-foot marked lanes at the entry for two lane minis. They
apparently work quite well.
It is unfortunate that in the US minis were not adopted early and often
instead of "normals." Minis are less expensive, get a bigger benefit
for the buck, and do not depend on funding from state and federal sources.
Again, we suffer from local public works directors in this regard
and the lack of interest in providing safe and supportive conditions for
peds.
Concerns
This is in the realm of only an option that may, or may not, be appropriate.
Personally, I cringe at the concept of any analogy giving equal, or even
approximate value to a 'mini' in comparison to a Roundabout intersection.
A mini may be an improvement over stop signs, and that they have merit under limiting conditions, but it is a
long leap to compare them to true Roundabouts. As noted in last week's
TIME article, Roundabouts are beginning to receive the respect they
deserve. I would hate to see public opinion of minis detract from the
benefits that I expect from Roundabouts.
I am curious as to how the mini was analyzed in SIDRA and Synchro. If the
only modification is that smaller parameters are entered as data, does
SIDRA 'know' that it is a mini and not a true Roundabout? I have never
attempted this analysis.
"Minis are a roundabout with all the safety benefits and performance of "normal" roundabouts". Are there any studies to support that statement? I don't
question that they have performance benefits over an all-way stop, but have studies shown them to have crash reduction comparable to normal roundabouts
at comparable intersections? Have studies shown the delay at minis to be as low as it would be with a normal roundabout with comparable volumes? I think
minis may have their place, but suggesting they be used instead of "normals" seems to me like comparing apples and oranges, but maybe I am wrong.
A mini is simply a all yield control intersection with a minimim size painted cirecle, or slighly raised, traffic calming device, to improve driver yielding behievor, and to help force more aggressive drivers (speeders) to slow down. The video on Mark's web site shows relatively high speeds by some drivers. Which would lead me to conclude that the crash history is higher, since speeding drivers cannont yield quickly enough, or as a string, dominate flow through intersection - in part due to higher speeding platoons. So mini's in my opinion only work well (safety wise) where the 85 percential speed is in the range of 15 to 20. And those mini's
with stronger traffic calming designs work better.
Certainly a full roundabout is also a yield controlled intersection, by the design is more aggressive.
So I consider mini's one step up from a two way or all stop intersection, rather than a step down from a complete roundabout.
Lessons Learned
http://www.teachamerica.com/RAB08/RAB08S5ASawers/index.htm
See also http://www.mini-roundabout.com/crossroads.html
which illustrates the sizes of central islands necessary.
Do NOT restrict the central island size as we did in the UK.
It's great to see so much interest in minis! I wish Frank Blackmore could see how things have changed.
At first, it was hard for me to believe that something so absurdly simple could also be so powerful, but, if we give the drivers a simpler task and use their brains and eyes, no computerized system can match the performance.
There's a lot of published research. The UK has more than 2000 minis, and decades of design experience and field data. Outside the UK, opinions are more common than actual minis, but we don't need to reinvent the things
based on intuition and trial and error. We can learn from their experience if we just read it.
Ken Todd is absolutely right: the OFFSIDE PRIORITY RULE is a crucial difference between a mini and AWSC or AWYC. The Dimondale mini operates as an offside priority system. Cars often enter and circulate simultaneously
there, so it has the same operating characteristic in Dimondale as in the UK. An American AWSC doesn't do that. (I've never heard of an All-Way-Yield.)
Regarding terms: The standard UK 'mini-roundabout' has a 4-meter traversable central island. Something with a larger traversable central island might be called a 'midi-roundabout', or if it has a small raised island it might be
called a 'small normal' or 'urban' roundabout. Clive may know of a better definition.
. UK minis are14-28m, but we might need a slightly different dimensions here. It would depends on the design vehicle and the site constraints.
Mark Johnson is right: Minis are VERY useful for capacity improvement.
Recommended conversion volumes range from 13,000 to 30,000 entering ADT. We can use them to scrap a lot of urban signals and save a lot of fuel.
- See Marlow, M., Conversion of rural and semi-rural major/minor T-junctions to offside priority, TRRL Lab Report 883, Department of Transport, TRRL, Berkshire, UK,1979.
- See Bramwell, F.J. BSc(Eng), MICE, MIHE: "20 Years of Roundabout Construction in Buckinghamshire", The Highway Engineer, June 1982.)
There are innumerable examples of minis used for capacity improvement, and lots of UK minis clearly serve that purpose. The Dimondale mini was intended for both delay and speed reduction, and it serves both purposes.
Capacity software is readily available to analyze minis. The ARCADY-6 (Analysis of Roundabout CApacity and DelaY) program is calibrated for miniroundabouts based on recent and extensive field capacity measurements at a wide variety of minis.
Clive Sawers points out very valuable information about central island design. The standard UK design uses a 4m central island as in Dimondale. UK data shows this works very well at 3 leg roundabouts, and less well at 4-leg roundabouts. http://www.mini-roundabout.com/ Clive offers some valuable lessons learned.
The Dimondale central island is a UK-typical 4m X 120mm truncated spherical section to deter high speed through movements. Drivers can drive over it - as Daredevil Dawayne is doing in the attached photo in his 9-axle, 70-ton, 60' Kenworth (at maybe 8 MPH) - but they don't do it at speed. Drivers of small vehicles tend to clip the side of the blob as they drive around it.
At the 3-leg WYE in Dimondale, we observe two different driver behaviors with regard to the central island (aka "the blob"). When other vehicles are present, they behave themselves and drive around the blob. When nobody else is around, some drive over it, enjoying the fact that they think they're misbehaving. Either way, it doesn't matter: the surrounding environment, vertical deflection, and horizontal deflection prevent high speed, and it operates safely.
Driver performance at Dimondale has also not been constant: area drivers they have become increasingly adept at using it. They used to stop more often.
Minis must be designed with care. They need to be visible and markings have to be maintained. Dimondale has done a good job of maintaining theirs, and it is highly visible (see recent photo by Eric Woodhouse). Some of the miniexamples others have shown are obsolete designs, worn out, invisible, or traffic has grown beyond the design year capacity.
The entries at Dimondale flare to 5.0m (16') at the yield lines. This was in part to accommodate the design vehicle (a combine), but it also enables more flexible entry paths which intuitively would (and empirically do) increase capacity. The actual capacity at Dimondale is unknown as it is operating at maybe 30% capacity and offers no queuing to allow capacity measurement. Delay is very low, and perhaps 10-20% of vehicles stop during peak periods.
Striping two narrow lanes at entries would have a number of effects on the entry performance, one of which would be to (intuitively) reduce entry speed as drivers feel "confined" by side friction and confronted by a lane choice
decision. At high RFCs, it would allow small vehicles to enter side by side. We did not divide the entry at Dimondale (capacity was not needed), but it would be interesting to try that and test the effect on approach speed and
driver entry position.
Minis are very suitable for many state highways in urban environments. A large design vehicle can be accommodated, and capacity performance is excellent. (Assuming the design is good.)
Contrary to some comments, the safety performance of minis is excellent and well documented in the literature. (See background references in http://www.teachamerica.com/roundabouts/RA054A_Waddell_ppr.pdf)
Minis do tend to have less effective deflection than 'normals,' so they need to be installed in low speed environments. (Since the entire purpose of deflection is to control entry speed, this is less critical in low speed environments.)
In tight urban environments, a painted roundabout is probably fine, but I would not do that without some other network characteristic to constrain speed. The new thermoplastic mini at the off-ramp in Maryland has me concerned. I wonder how it's doing?
I studied an intersection here in Portland with a queuing issue. Though the results are from simulations:
- Currently, with AWSC, LOS is F, the longest AM inbound queue is 1500 ft.
- ASWC with turn lanes takes it into the C to E range and 95th queue to about 300 ft.
- signals (60 and 90 second cycles) take it to D/B territory, but the V/C is near 1 and queues move up to 500-700 ft.
- A roundabout moves it to A with queue of 300 ft - constrained to operate at 10 mph in the circulating roadway. The 10 mph speed was used to
conservatively mimic a mini as the intersection is constrained and that is all we would likely be able to do.
The analysis used both Snycrho and SIDRA.
I think the change from stop to yield control can have a dramatic effect on efficiency without a significant change in safety and studies indicate the
changing from AWSC to roundabout has the smallest gain in safety. PDOT recommends the turn lanes or the mini, but the problem is political and
'do nothing' appears to be acceptable for now.
I accept that a mini can be analyzed with the standard RODEL parameters.
My observation was that Scott Batson analyzed minis in Portland with Synchro and SIDRA. Rahmi Akelik's comments identify some issues related the assumptions that must be made when using SIDRA for an AWYC. Rahmi also noted that applying normal Roundabout capacity analysis methods would be doubtful for minis. I still see several issues with minis, including the fatality at the mini near Seattle when alternative driving rules are
applied: http://seattlepi.nwsource.com/local/370578_rainierbeach12.html
See also: http://seattlepi.nwsource.com/transportation/373400_getthere04.html
Considering the above, and the other issues that have been noted, I still view minis as an improvement over AWSC that lacks the full benefits that Roundabouts offer.
Kenneth Todd's comment "The underlying philosophy is to stop people to save them the inconvenience of slowing down" is a jewel!
John H. Biendara
Rodel models the capacity of minis quite well.
The 6 geometric inputs used in the capacity prediction equations of:
- Diam.
- Entry Width
- Entry Angle,
- Radius-at curb return and
- Length of flare
- approach roadway width
Captures the geometry and therefore the capacity.
Mark T. Johnson, P.E.
Ken Todd's makes two startling, but rational, statements. It's a forest and trees (who can see the...) which we in the vanguard of
roundabouts either do not get or do not get into our role as advocates.
First, Todd speaks of off-side priority at roundabouts as almost a singular characteristic, different from a standard "yield" concept.
Second, far more important from this viewpoint, the U.K. which originated the roundabout in both normal and mini forms, abolished in rule
and practice the all-way stop and all-way yield intersections. This is a logical progression in policy based on experience and research on
all types of roundabouts. In low speed conditions (Todd defines this in British practice as under 30 mph, a little racy in this opinion) normals
or minis operate quite nicely, in far superior manner to all-way stop and/or all-way yields. Where normals and minis cannot be installed
then presumably either two-way stop (TWSC) or relatively unsafe signals are employed. Note the typical all-way stop/yield intersections are
relatively low volume in terms of roundabout capacities, i.e., capacity is not an issue for roundabout installations.
The problem in abolishing all-way stop controls in North America comes from local jurisdiction control over most of these intersections
which are primarily local/collector or collector/collector contexts. And, there is little profit for consultants and engineers for engineers and
consultantsSave compared to the fees and opportunities for normal roundabouts. So, shouldn't we suggest to the MUTCD people and the Green
Book people plus those who compose subdivision regulations to set rules abolishing the all-way stop and all-way yield intersection?
From: Clive Penn Sawers <PENNTRAFF@AOL.COM>
Subject: Re: Terminology: "Miniroundabout"
By the UK definitions NO because we don't officially use the term. I have
used the term "midi" for mini-roundabouts with central traversable islands
greater than our current max of 4m diameter, but I hope that will be
unnecessary in time. So your suggestion of midi for this may not be far off the
mark but there may not be a need to distinguish this from larger
intersections. Of course this must become a roundabout with yield lines etc which
appear to be absent at present.
Clive Sawers MA MICE CEng
Traffic Engineering Consultant
www.mini-roundabout.com
www.midi-roundabout.co.uk
www.penntraff.co.uk
------------------------------
Date: Tue, 9 Nov 2010 09:00:22 -0800
From: "tonyrvt@ACEWEB.COM" <tonyrvt@ACEWEB.COM>
Subject: Mini 2-Lane Entries narrower than two vehicles
Ed:
How about the behavior management at two-lane mini entries with pavement markings, each lane about four feet, so that vehicles"nudging" to entry as adjacent vehicles permit.
Be interesting to see MUTCD handle this approach to pavement markings.
Tony Redington
Montreal
------- Original Message -------
>From : Edmund Waddell[mailto:edmundwaddell@hotmail.com]
Sent : 11/6/2010 3:10:28 PM
To : ROUNDABOUTS@listserv.ksu.edu
Cc :
Subject : RE: Re: Terminology: "Miniroundabout"
Gary:
You're quite right. The purpose of large central islands is to provide deflection. The purpose of deflection is to control entry speed. If speed is already low (from surrounding street
environment), deflection is moot, and a large central island is not needed. Therefore, the space is not needed and the cost is much, much less (perhaps 90% less). Two and three
lane minis are perfectly OK in low speed (25-30 MPH) environments. The UK has lots and lots of them, and they work quite well.
That may need a second read to appreciate the logic: "There is no need for deflection if approach/entry speed is already slow enough."
The potential cost savings are enormous. It is hard to believe that a gallon of white paint can be so much more efficient than our best traffic signals, but there you are. Life is full
of surprises.
I've been through Frank Blackmore's linked mini-roundabout systems in Swindon and Hemel Hempstead. From the driver's perspective, they're just a series of intersections - no
big deal. They look odd from the air, but to a driver they appear the same as a corridor of minis, or a city block with minis at the defining intersections.
Ed Waddell,
AFINPREPA
Cheyenne, Wyoming, good ol' US of A
> Date: Sat, 6 Nov 2010 13:50:15 -0700
> From: abramvanelswyk@HOTMAIL.COM
> Subject: Re: Terminology: "Miniroundabout"
> To: ROUNDABOUTS@LISTSERV.KSU.EDU
>
> The Swindon "Magic Roundabout" is essentially five multilane minis (with fully-traversable islands of the traditional British "white painted circle" variety). Swindon has other
minis with multilane approaches as well:
>
> http://maps.google.com/maps?
f=q&source=s_q&hl=en&geocode=&q=swindon,+uk&sll=50.687758,0.186768&sspn=0.290158,0.464859&ie=UTF8&hq=&hnear=Swindon,+United+Kingdom&ll=51.554726,-
1.79895&spn=0.001119,0.001816&t=k&z=19
>
>
> ---A
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