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The central island of a mini-roundabout is indeed a truck apron and should
be made as large as necessary to deflect light vehicles. My presentation at
Kansas covers much of the detail and accident risk at crossroads.
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http://www.teachamerica.com/RAB08/RAB08S5ASawers/index.htm
See also http://www.mini-roundabout.com/crossroads.html
www.mini-roundabout.com
www.midi-roundabout.co.uk
www.penntraff.co.uk
which illustrates the sizes of central islands necessary.
I thought it a good idea to try to provide a definition of a mini-roundabout. One is appended.
For more information on the official UK guidelines on roundabouts see TD16/07
http://www.standardsforhighways.co.uk/dmrb/vol6/section2/td1607.pdf
For mini-roundabouts see TD 54/07
http://www.standardsforhighways.co.uk/dmrb/vol6/section2/td5407.pdf
and for DfT guidelines on mini-roundabouts see:
http://www.dft.gov.uk/pgr/roads/tss/gpg/miniroundaboutsgoodpractice.pdf
But see also my websites below for reasons why there is much wrong with the above...
Clive Sawers MA MICE CEng
Traffic Engineering Consultant
www.mini-roundabout.com
www.midi-roundabout.co.uk
www.penntraff.co.uk
But as I have warned before, there are many flaws in these documents. I have yet to work on the first of these but my comments on the mini-roundabouts may be seen on (http://www.mini-roundabout.com) .
Clive Sawers MA MICE CEng
Traffic Engineering Consultant
_www.mini-roundabout.com_ (http://www.mini-roundabout.com/) _www.midi-roundabout.co.uk_ (http://www.midi-roundabout.co.uk/)
_www.penntraff.co.uk_ (http://www.penntraff.co.uk/)
2 Colehayes Park Cottages, Bovey Tracey, Newton Abbot, Devon, UK TQ13 9LD
+44 (0)1626 830225, mobile 07967 148794
Penntraff is part of Moor Value Ltd
Reg. Company 05018871
UK Department of Transportation "Mini-Roundabouts - Good Practice Guidancehttp://www.dft.gov.uk/pgr/roads/tss/gpg/miniroundaboutsgoodpractice.pdf">
http://www.dft.gov.uk/pgr/roads/tss/gpg/miniroundaboutsgoodpractice.pdf">http://www.dft.gov.uk/pgr/roads/tss/gpg/miniroundaboutsgoodpractice.pdf
Before relying on information in the UK guidelines which are free to download, see the comments on these on www.mini-roundabout.com. There are sadly MANY FLAWS in the recent UK official publications.
(Note: Do NOT restrict the central island size as we did in the UK.)
Painted Chevrons
Included in that presentation is a site in Sidcup, SE London. Here are some
more images of Sidcup taken at a lower level slightly above driver eye
height. These illustrate the poor visibility/interpretation of painted chevrons
very much as illustrated on the zip from NY, which features a good layout in
principle and I particularly commend the raised island shape which would prevent
much of the unnecessary overrunning that you see at Sidcup. But seriously,
don't bother with the chevrons. Michael's comment about let the geometry speak
for itself applies - make the central island a clear circle/dome etc.
Clive Sawers MA MICE CEng
UK Traffic Engineering Consultant
Here is a link to a few video's of minis both in UK and US.
The US example was or is perhaps the first mini in US and has the raised and
painted dome and follows UK mini design principles as applied in design by
Barry Crown.
http://mtjengineering.com/video_minis.html
Mark T. Johnson, P.E.
MTJ Engineering, LLC
Achieving Traffic & Transportation Solutions:
Specializing in Modern Roundabout Planning and Design
>From : Philip Demosthenes[mailto:PDemosthenes@parametrix.com]
Comment if you disagree, but distilled down, 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.
From: "tonyrvt@aceweb.com" <tonyrvt@ACEWEB.COM>
While I would hazard a guess that pedistrains feel a bit more at a disadvantage with a mini during peak hours, if it is a busy intersection,
what does the ped/vehicle crash rate indicate, or better yet, a measure of close calls? Or simpy do not apply mini design at a busy
interseciton.
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.
While they have significant traffic calming benefits, one reason for many
mini-roundabouts in the United Kingdom is that all-way-stops are not permitted
there. Thus many locations where we in North America might have an all-way-stop
those in the UK have a mini-roundabout even if traffic calming is not needed.
Alan Taylor, P.Eng., Transportation Planner
From: "Britnell, William W." <William.Britnell@PO.STATE.CT.US>
I must admit I have never been a fan of mini's, not from actual experience
but from a perception that drivers will just ignore the painted circle or
domed center and drive it like a conventional intersection. I must also say
that, as a State DOT engineer, my experience is with arterials and very
little with neighborhood residential roads where I believe minis are most
appropriate. My question is with Tony's first sentence: "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.
From: Roundabout Research On Behalf Of tonyrvt@aceweb.com
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.
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.
- Scott Batson
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.
I concur with Philip Demosthenes that 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.
John H. Biendara, Project Engineer
I should've included this Google Map link to the Clearwater
mini-roundbouts...so you could see the suburban residential neighborhood
context. There are three mini-roundabouts in a row on the E-W street
(Ridgewood St); the link is to the middle one. The one to the east is a
"T" intersection.
Google Maps Clearwater Fl Dridgewood & Baker
Trucks and cars towing trailers have to turn left in front of the central island.
Ken Sides, PE
From: "tonyrvt@aceweb.com" <tonyrvt@ACEWEB.COM>
Subject: Re: Mini Roundabouts--WHY NOT THE UK PRACTICE HERE?
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
consultants 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: Andrew O'Brien <andrew@OBRIENTRAFFIC.COM>
I am somewhat bemused by the statement from "tonyrvt" that -
"Local/local intersection treatments are at most traffic calming and
have no relevance to this listserv".
My 35 years involvement in rbts came from their safety benefits. The
first B&A study in Victoria of changes in crashes was for rbts in
local streets - 67 fatal/injury crashes in 7 years at 30+ sites
compared to 0 fatal/inj crashes in a 3 year after period at the same
sites. Almost all sites were STOP or YIELD controlled
cross-intersections. This is hardly something to neglect!
Andy
From: kennethatodd@AOL.COM
Starting with Virginia in 1952, eight jurisdictions and the UVC adopted the rule that entering drivers yield to those in the traffic circle. A few other states merely put yield signs at the entries, as everybody is doing now.
As for stop versus yield, you find something in the ITE Journal, May 1993, pp. 369-41,and July 1994, p.8, The July 2008 issue has a Letter about the yield-to-right rule.
KT
From: Rahmi Akcelik <rahmi.akcelik@SIDRASOLUTIONS.COM>
Here is a little more on mini roundabouts. I've analysed a mini
roundabout case study using SIDRA INTERSECTION US version quickly. The
case study was described in a paper by Waddell and Albertson presented
at the Roundabout Conference, Vail CO, 2005. This single-lane
T-roundabout has an inscribed diameter of 21 m (69 ft). Year 2020
volumes were used, generally resulting in low v/c ratios and delays.
I've analysed the mini roundabout as a roundabout as well as an All-Way
Yield Control (AWYC) case which was possible to set in SIDRA
INTERSECTION by making assumptions about movement Yield rules. I've
also analysed the AWSC alternative.
Roundabout analysis vs AWYC control gave close results. This finding
depends on the assumptions I made about Yield rules (rough guess) as
well as the critical gap and follow-up headway values used in the case
of AWYC. In the analysis as an AWYC, I used critical gap and follow-up
headway values close to those reported for single-lane roundabouts in
the NCHRP 3-65 US roundabout research (5.0 s and 3.2 s, respectively).
Note that the inscribed diameter values of single-lane roundabouts
included in the NCHRP 3-65 research database are in the range 32 - 59 m
(105 - 194 ft) so mini roundabouts fall outside the size range (if the
UK max 28 m is accepted). However, the SIDRA INTERSECTION roundabout
model with an Environment factor of 1.2 (default for the US version)
found critical gap and follow-up headway values similar to those used
for the AWYC analysis. Note that the inscribed diameter values of
single-lane roundabouts in the Australian research database are in the
range 16 - 32 m (52 - 105 ft) and cover the mini-roundabout size range.
Using the FHWA model in SIDRA INTERSECTION (similar to the UK / RODEL
analysis), capacities were significantly higher as expected since it was
not calibrated for US research results. Nevertheless, the time gains
reported by Waddell and Albertson are generally confirmed (i.e. I found
similar values). It should also be noted that the inscribed diameter
values of roundabouts in the UK research database are in the range 15 -
100 m (49 - 328 ft) and cover the mini-roundabout size.
It may also be of interest that, in addition to the delay time gains,
the above analyses indicated fuel consumption and CO2 emission
reductions with the mini roundabout compared with AWSC. The differences
in values between the analyses as a roundabout and AWYC were very small.
This quick analysis indicates that it is not too bad to analyse mini
roundabouts as roundabouts although they may operate as AWYC, at least
partially. Further research would be interesting.
Best wishes
Rahmi Akcelik
Thanks to Edmund for clarifying much about minis. The UK/international sign needs to be used with care. In parts of Europe
it signifies a roundabout, but not necessarily a modern one. In most cases, Greece for example, the yield rule is the wrong way round. In the UK the rule of a roundabout is to circulate in the correct direction; the yield rule has been in place since 1966 but was in practice at many sites long before that. The yield rule is advisory in most instances suggesting that we should design our roundabouts such that all drivers go slowly, carefully and allowing for others' mistakes. Drivers should be prepared to slow down even when they have priority.Miniroundabout HistoryMiniroundabout History
Subject: Re: Is Roundabout an Alternative at This Intersection?
Left turns from N to NE and from SE to S will NEVER hook. So a single roundabout of whatever size is probably ruled out. The correct scheme I believe will be a double mini-roundabout. However, in an area where drivers may not understand roundabouts, let alone mini-roundabouts and what the hell - double mini-roundabouts, you need to begin the education process at the more obvious sites working towards this in a year or two.
Here are views of two double mini-roundabouts in the UK. I designed and installed the first in 1986.
http://pixplot.com/?Album=Roundabouts#+051.377085-000.856011_19_S
The second is one I first went to yesterday as it needs adjustment. It works quite well and has been in place for many years. Just the detailing is wrong. I shall be setting the yield lines back and making the two mini-roundabout centres quite a lot larger as well as lots of additional pedestrian crossing facilities in the area, refuges etc. and the signing and lighting needs working on.
http://pixplot.com/?Album=Roundabouts#+051.465620-000.036215_19_S
Look out for opportunities like this. Such a scheme can be retrofitted very cheaply.
Clive Sawers MA MICE CEng
_www.mini-roundabout.com_ (http://www.mini-roundabout.com) www.midi-roundabout.co.uk www.penntraff.co.uk
Ed
Please see limited comments in your text.
On 9/11/2010 2:35 AM, Edmund Waddell wrote:
> Hi Andrew:
>
> I gather from this response that you've never actually built a mini roundabout. Is that correct? Has anyone in Australia any experience at all with minis? If not, it might lead one to conclude that your commentary is based entirely on supposition and irrelevant safety studies of other types of roundabouts.
Your response appears to be to an email from Mike Wallwork. However, if
one can be flexible enough to call a small (4-8m) fully-traffickable
island a "mini", then I have designed numerous ones - bit still using
deflection as a basis.
> One obvious question is, if you've never built a mini, why would FHWA have you write their guideline, and why would you accept the job? It would be as absurd as me writing a book on scuba diving or mountain climbing. (I've never done either one and would not presume to do so.)
I am not aware that I had/have a job with FHWA
> I agree regarding basics, but the key is to understand principles - i.e. not WHAT to do, but WHY we do things. The purpose of deflection is to control entry speed. If speed is already low, deflection is not critical and other site objectives and design principles come to the fore. Watch out for cookie cutter designs without understanding the underlying principles.
If you provide the ability to speed through an intersection (including
rbts) enough drivers will do it - enough to create a safety problem -
refer to Mike and Clive Sawers writings. Why do you think
cross-intersections have major safety problems?
> Australia is a great and proud country of 20 million people, but it is a mistake to let national pride cloud one's judgment. I recommend studying the UK research and data with an open mind, as I have done. The depth and breadth of knowledge derived from decades of analyses and field experience are astonishing, and I've learned to respect that. It is a mistake to underestimate it.
I have read the UK research - and it suggests the problems I am
discussing. If the crash rates are 2-4 times higher in the UK, then it
prompts one to try to find out why.
I will take the other gratuitous advice on board.
> Best wishes always,
>
>
> Ed Waddell
> AFINPREPA
> Cheyenne, Wyoming USA
> FREEDOM IS NOT FREE. REMEMBER AMERICA'S VETERANS.
>> It seems that people are forgetting the basics of roundabout design. Roundabouts work mainly because they forcibly slow vehicles. Even the FHWA mini-roundabout guidelines that I helped to write and design stress the need to limit vehicle speeds, while allowing large vehicles to drive over the central island while turning. They are not meant to be "squeezed" into small intersections with little deflection. As part of the FHWa mini-roundabout project we inspected a number of mini-roundabouts and found that the good ones had the proper deflection. The poor ones lacked deflection.
>>
>> So please remember the basics of roundabout design.
>>
>> We also need to be careful of importing designs from the UK which has a much lower speed environment.
>>
>> Some if the minis in the US have poor safety records.
>>
>> Michael Wallwork
>> Alternate Street Design, P.A.
>> (904) 710-2150
>
Date: Fri, 18 Feb 2011 06:42:06 -0800
From: "tonyrvt@ACEWEB.COM" <tonyrvt@ACEWEB.COM>
Subject: Re: Great concern about roundabouts--Call Montpelier Housing Director
Clive:
The "before" condition on Main Street roundie you viewed one early evening in Montpelier (VT) was TWSC with Main Street past the Middle School to downtown the free flow with
speeds of 25 mph or even higher. Installing the roundabout traffic calmed and improved access to side streets/driveways will over 300 feet. The roundabout was installed for
freeing up the third leg, controlled with a stop sign for lefts and yield for rights. (The design is in NCHRP 264). Roundabouts by nature traffic calm, though it is seldom the prime
reason for installation. The So. Golden Road study (speeds measured in the three "between" roundabout links as well as the before three signal/TWSC) with over 1000 feet
between intersections found decrease speeds (48 mph to 32 mph) in spite of an installed median with some breaks--through traffic (modeled) actually time actually decreased
versus a signal configuration.
The point is, roundabouts because of their traffic calming character may well--even in a single installation--reduce speeds significantly at more than 300 feet thereby making it
easier for side road/driveway entry movements, i.e., creating more acceptable gaps for drivers. Finally, why install four way stops when one can just put in a nice residential
traffic calming circle/ or normal/mini roundabout?
Tony Redington
Montreal
New Blog: TonyRVT.xanga.com
Date: Wed, 21 Nov 2012 09:38:12 -0700
From: Scott Ritchie <scott@ROUNDABOUTS.US>
Subject: Re: Mini roundabouts article in Nov/Dec Public Roads magazine
Yes Phil. Depending on the design vehicle (truck needs) I recommend raised splitter islands for minis wherever possible. Although we (FHWA mini roundabout team) showed and studied various options of painted and curbed islands, curbs assist in conspicuity, ped safety, and traffic calming greatly. I think the one we did at the East Atlanta Village Project in Georgia has non-traversable islands (contact Wei Zhang or Joe Bared) and I have two other minis with non-traversable curbs going in this year. All other minis I have been involved with have abrupt but mountable raised curbs with a few legs in paint. If you are interested in details of those, feel free to contact me.
We also made a short presentation on Minis at the 2011 National Roundabout Conference at the following link:
http://teachamerica.com/RAB11/RAB1123Ritchie/player.html
Thanks to Wei and Joe for writing the recent articles and pushing this tool in the U.S.
Scott
Scott Ritchie, p.e., president
Roundabout Specialist
Roundabouts & Traffic Engineering
Direct: (928) 284-0295
Main: (928) 284-0366
scott@roundabouts.us
www.roundabouts.us
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