Miniroundabouts

1
Mini-roundabouts

• A Definitive Guide

2
Locked roundabout (Leeds?)
Extracted from The Design of Roundabouts - Mike
Brown HMSORoundabout before the yield rule
3
Flows falling on lock-up
Graph illustrating the decline in flow with
locking roundabouts before the yield rule
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Introducing the yield rule

• o/s priority trials on
• TRL test track
• several public road sites over a number of years
• Accidents fell
• Traffic kept flowing
• General advisory yield rule introduced in UK in
  1966
• Give way/yield to vehicles from immediate right
• Traffic on roundabout can be expected to slow
  down!
• Wardrop roundabout system no longer applied
• Entry width more important than weaving length
• Much reduced central island diameter possible

Wardrop formula was used to calculate capacity of
weaving sections
5
Operational working of 3-arm small/mini
roundabout
A three arm (mini) roundabout cycles as the left
turns proceed. This applies at any size and a
large layout proved a capacity in excess of
12,000 pcu/hr. It is this fact that makes
networks of 3-way roundabouts such as ring
junctions attractive it may be cheaper and more
effective to change a rotary to a ring layout
than to construct a completely new modern
roundabout.
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Mini-roundabouts

• Yield rule removed relevance of weaving length
  which could now become short
• Reduced overall junction sizes possible, led to
  
• Small diameter roundabouts (1966-70)
• Mini-roundabouts! (1970 onwards)
• which in turn lead to double (mini-) roundabouts
• and multiple (mini-) roundabouts
• Double multiple 3-way roundabouts are easier to
  drive through than a large single roundabout

After just a little practice drivers at ring
layouts got well used to the system. It was
indeed easier to drive through
7
Off-side priority roundabouts

• Capacity safety are usually improved
• Flared approaches to improve capacity
• 3-way intersections (theoretically) have infinite
  capacity
• 4-way or more arms limited to max 7000 pcu/h

8
Problems of 4-way operationesp. if small centre
high right turns
At large layouts this is quite a problemthe
driver of the green car stops half way across the
entry area between the yield line and the center
island.
9
Definitions

• A modern roundabout will usually have
• large solid central island
• splitter islands
• a narrow overrun area adjacent to the central
  island
• A smaller roundabout will need
• a solid central island
• a more substantial overrun area (USA truck apron)
• A mini-roundabout occurs
• when the solid central island has to go, leaving
  the overrun only - WHATEVER SIZE THAT MAY BE!!

Do not make the mistake of limiting the center
island to 4m as we did in the UK!
10
One of few single lane roundabouts in the UK with
a large truck apron but note the absence of
curb in the UK the side slope must not exceed
14 this may help with US truck aprons where the
upstand might tip a truck.
11
Definition of a Mini-roundabout

• An intersection which is a modern roundabout
• Yield on each entry to circulating vehicles
• Must pass to correct side of central island
• Central island must be fully overrunnable
• Large vehicles otherwise baulked
• NO size limit unlike currently in UK

I offer this or similar definition of a
mini-roundabout DO NOT prescribe a max. size of
center island as we did in the UK. This is wrong
and I am setting out to change it
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Roundabout or Mini-rbt?

• Roundabout easy(relatively)
• Deflection on entry
• Solid central island
• Easy design rules
• Approach flare

• Mini-rbt (more) difficult
• Deflection on entry difficult to achieve
• Central island inconspicuous
• Approach flare difficult
• Drivers overshoot

Each roundabout must be designed individually
mini-roundabouts even more so
13
So what is the difference

• Scale
• As sites become very small much detail alteration
• At the interface virtually NO difference
• ICD 28-30m (90-100 feet)
• Same yield on entry
• Same circulatory roadway
• Same visibility requirements
• The Differences?
• Signs formerly on central island now on approach
• Cant use central light column

14
Why do (mini-) roundabouts work?

• After all, drivers have to be prepared to
• give-way/yield
• negotiate physical layout
• i.e. to do things which individually they
  sometimes fail to do at
• bends chicanes, (loss of control)
• X-roads, (fail to yield/overshoot)
• L R shoulder T-junctions (loss of control/fail
  to yield)
• Where there will be more than one reason to slow
  down, drivers do not seem to push the limit on
  either...

• Important that both yielding and geometric
  deflection/features work together

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Selection of suitable sites

• Intersections of (usually) 2-way roads
• where some turning space can be added (if needed)
  by
• widening corners or
• narrowing approaches
• 3-way urban intersections
• 4-way urban intersections
• Double and multiple 3-way intersections
• Current control may be signals or priority
• Not normally new intersections

• Side-road flow ?10 of main road flow UK 500
  vehicles per day

I was the first to make this important point
having come across many non functioning
mini-roundabouts with very low side-road flows.
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Site Selection factors Sites exhibiting any of
the following

• Getting knotted up at peak periods
• mainly priority intersections but
• can include signals
• Long queues getting on (or off)
• History of crashes involving
• turning
• speeding
• pedestrians

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T-intersection under pressure!
Mini-roundabouts should be installed to deal with
turning problems and crashes, not just for
traffic calming purposes sites that operate with
difficulty like this are good potential
candidates.
18
T-layout now as a mini-roundabout
As the system cycles so all approaches get a
reasonable chance of entry with no knotted
conditions
19
Scissors intersection getting locked up
This drawing depicts exactly what happened at
California crossroads during the before situation
it was an awful knot!
20
A double (mini-) roundabout would resolve this
This layout deals with all movements and reflects
the actual operation at the site. While queues
can form at the intermediate yield lines, this is
relatively rare as the two yellow streams tend to
have little to yield to compared with the first
yield lines.
21
California Crossroads, where a double
mini-roundabout has been in operation for 22
years.
22
California Crossroads, UK
Installed April 1986 only very minor
modifications since thenA short video was shown
here (not available)
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(Mini-) roundabouts or traffic signals?

• Many highway authorities are wasting millions
  
• installing,
• running and
• maintaining traffic signals
• at intersections where roundabouts would operate
  
• more cheaply,
• more safely,
• with less delay for traffic and pedestrians,
• and with less air pollution.

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Safety (vehicle-vehicle) A. CROSSING movements

• In effect right turners (3-arms)
• Ahead and right turns (4-arms)
• speeds to be controlled by 60m radius paths
• Imposes sideways g-force
• 0.2g at 100m radius and 30mph
• 0.2g at 60m radius and 20mph
• But NOT if circulating carriageway is dished!
• OR use vertical deflections (more about that
  later).

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Safety (vehicle-vehicle)B. Merging conflicts

• (the movement along a (nearly) straight curbline)
• Avoid bulging out the curbline
• Causes accidents (Wilts CC)
• Drivers run into bulge/blister island
• Cycles pinched
• Could make overrunnable
• Use two narrow lanes and/or
• Change surface colour/texture
• Merging accidents normally rare

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Pedestrian Safety

• It is a myth that signal intersections guarantee
  safety for pedestrians. Recent research shows
  that the provision of red/green pedestrian
  crossing stages does not enhance safety in many
  cases such facilities actually make matters
  worse.
• For pedestrians to get the best and safest deal
  they need
• To cross a single traffic stream at a time
• Speeds to be very well controlled
• Not normally to have priority
• Refuges have been the Cinderella of pedestrian
  facilities. Essex County rediscovered refuges!
  With speed tables pedestrians are even better
  off as at Borehamwood, Herts.

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Cycle Powered 2-wheeler Safety

• All drivers MUST recognise presence of (mini-)
  roundabout in good time
• Drivers perception of the need to yield
  important
• Design must incorporate speed reducing features
• Two narrow lanes more cycle-friendly than one
  wide lane
• Lighting must be excellent

28
Conflicts at mini-roundabouts
The basic conflict diagrams these are examined
in more detail later on
29
Source Accidents at mini-roundabouts - TRL
This is vitally important early data. But it is
somewhat different from my experience.
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Source Accidents at mini-roundabouts - TRL
(broad-side)
All crossing accidents represent over 50 of
injury crashes at 4-arm mini-roundabouts
deflection through the layout is essential to
reduce this. My sites tended to suffer only the
crossing crashes particularly where I could not
get the necessary deflection for reasons of site
constraint. Data 1988-1994
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Conflicts at mini-roundabouts
Illustrating the various geometrical conflicts
relatively complex at 4-arm and illustrating
again the need for good deflection to bend
ahead vehicle paths otherwise broad-side
crashes are quite possible at mini-roundabouts as
identified in the previous pie-chart. Make sure
your center islands are larger enough!
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Conflicts at mini-roundabouts
At a scissor layout there is more deflection
built-in
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Geometric Crash Data
Several years of data the crash frequency
relates to the highest numerical conflict of
flows nearly all movements pass through the top
leg so the greatest crossing flow product is at
the bottom as indicated by the 19 crossing
crashes. The highest merging flows occur where
there are 2 merging crashes (upper right)
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Geometric Crash Data
At this site just south of the last one, the
crossing crashes rarely occur, possibly due to a
better layout on the ground. The remaining
crashes are a mixed bunch the road on the right
flows downhill into the intersection hence the
two crashes involving shunts and the brake
failure. ve above alcohol limit
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General note(Additional slide)
These next few slides are highly relevant but
relate particularly to the UK issue of central
island size. Where there is a solid central
island the truck apron and central island may be
any size. Where a mini-roundabout is used the
central island is limited to 4m. But we have some
sites in the UK where we are (rightly) ignoring
this rule so that sufficient deflection can be
achieved. This is an uphill struggle and people
in high places are reluctant to change the rules.
Watch this space
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Mini-roundabouts at X-roads
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Mini-roundabouts at X-roads
A real site based on the need for good
deflection being installed May 2008. The central
island at the site will be around 7m not the 10m
illustrated here
40
UK (oversize) central island
A 7m central island as advised by me, but the low
chevrons are indecipherable! The site failed an
Arcady run but the Sawers formula indicated no
problems expected and the site worked.
41
Design principles

• Clear simple layout of junction - avoid clutter
• Approaches must NOT resemble any other form of
  intersection
• Conspicuous central island
• NEVER sink island in a hollow - drain outwards
• Central island position based on deflected inner
  swept paths
• Yield lines follow outer swept paths NOT normally
  ICC
• Speed of crossing streams must be closely
  controlled
• Speed of merging streams need not be so
  controlled
• Avoid curbline bulges - landscape any such
• Place yield signs (UK dia 611.1) where best seen
• Ensure all visual continuity across layout is
  broken
• Light intersection as a node

42
Features of mini-roundabouts

• Central island
• Approaches
• Crossfalls
• Signs
• Lighting

43
Central Island A fundamental point (UK)

• Install a conspicuous circular domed or conical
  island in the middle of any road junction and the
  vast majority of drivers will automatically
• slow down
• give way to any vehicles from the right
• pass to the left of the island
• And this without
• signs
• OR yield lines

The central island is a very important traffic
sign and must be clearly visible and distinctly
round or domed
44
Central Island (mini-roundabout)

• Must not be sunk in a hollow
• Island size
• probably 2-4m diameter for 3-arm junctions
• up to any diameter for 4-arm junctions
• IN ANY EVENT AS LARGE AS NECESSARY FOR DEFLECTION
• Raised 25mm per 1m dia. to max height 100mm
• Less where buses turn
• Side slope not steeper than 14 (UK - traffic
  calming regulations)
• Construct in HRA/DBM - usually no kerbs or setts
  
• Must be white reflectorised (UK) but recommended
• Will need regular maintenance

45
Binfield Crossroads 1 slight injury accident in
17 years!Typical average at crossroads between 1
2 per annum
46
Approaches

• At a T-junction there is one side-road
• At a crossroads there are two
• At a mini-roundabout EVERY approach is a side
  road!!
• The responsibility for this lies with US
• Do NOT USE / REMOVE bifurcation arrows
• Avoid any arrows unless doubling lane use to the
  same exit (Capacity module)
• Work centre-line to off-side to allow
• Early lane-split line where possible
• Visual break with opposite C/line (T-junction)
• Use surface colour change if you wish

47
It is important to change the approaches to a
(mini-) roundabout and not leave arrowslike this
which suggest that the layout is a priority
T-intersection
48
Lots of deception on this approach to a
mini-roundabout incorrect signs and arrows.
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Approaches (cont.)

• If single lane approach
• Develop
• Curbed or painted island (painted island no
  longer legal in UK)
• Yield line on outer swept path
• Add yield sign if particular problems expected
• Avoid curbline bulges if possible
• Change colour of surfaceUse anti-skid material

50
The slight build-out on the right is NOT a speed
reducing feature drivers can drive past it as
the same speed as if it had not been placed, but
visually it may help and promotes the UK
mini-roundabout sign.
51
This drawing illustrates how little real
curvature there is on the upper green vehicle
path deflected by the build-out
52
Crossfalls - all circular roundabouts

• Drain outwards (now standard practice in France)
  
• Raises central island (by up to 400mm)
• Eliminates crowns - main cause of HGV overturning
  
• Keeps circulating speeds low
• Circulatory roadway into (better) view
• Can improve capacity safety
• May save 000s on UTILITIES
• Avoids drain gullies next to central island
• Sloping sites maintain consistent outward falls
  (France)

Lots of reasons to follow the French method and
they have proven a lower crash rate
53
A site near Boulogne, northern France
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A 985/Queensferry Road, DunfermlineOutward
draining roundabout in Scotland
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Two charts to illustrate G-force against speed
and drainage fall for different radii. Lower
speeds occur with outward falls.
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Highly successful roundabout near Lewes (UK) with
outward falls and environmental art.
59
American design
The truck apron in the US seems to require a curb
to deter drivers of light vehicles. The curb
could however cause rollover in extreme cases It
may be worth developing a rumble area instead
that would remove the need for a curb. In the UK
the slope must not exceed 14.
UK 14 max.
60
Signs

• Make mini-rbt (UK 611.1) sign visible!!
• Up to 15 (not 1.5)m from yield line
• On a splitter island
• Use Roundabout warning sign (UK 510)/ADS
• On small roundabouts in urban areas
• Use 606 (turn left) not 610 (keep left)
• Use bollards but avoid clutter
• Remember - a roundabout breaks up route
  continuity - ensure direction signing retains
  this.
• Locate exit direction signs carefully

61
Place yield signs sensibly not in the hedge or
behind trees!
Galmington Rd/Trull Rd, Taunton
62
Again, identify correct position for yield sign.
63
Poor positioning of signs first the yield sign
then the flag type direction sign
64
The direction sign should be where the green
arrow appears
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Lighting

• Introduce continuity change - link into node
• Light the whole intersection well
• On small roundabouts
• Light the island with post top unit
• Introduce environmental art
• Minimise lighting units on island
• Keep columns away from edge
• Rarely it may be OK not to light a small
  roundabout in certain areas
• Ensure central island lightly coloured
• Good reflectorisation of signs

66
Joining it all up

• Nodes
• (Mini- ) roundabouts at the main intersections
• Minor intersections restricted turns
• Links
• Add central median if sufficient width
• Otherwise refuges where possible
• Speed Tables at frequent intervals

67
Developing your scheme
P
Rec.
P.O.
Ensure correct footway provisionAdd medians if
possibleAdd gatewaysAdd mini-roundabouts if
justifiedAdd speed tables
School
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A common layout in Abuja
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A common layout in Abuja
This intersection does not work well and peds
have to cross two-way flows
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A better layout
The mini- or smallroundabout as illustrated here
is apowerful, cheap and very safe tool.
Turning is easy and U-turning, needed often, is
available. Pedestrians cross with little
difficulty. See Mini-roundabouts A Definitive
Guide.
71
Layout for divided highway intersections

• This roundabout would have
• Very high capacity
• A good safety record
• Pedestrian crossing points
• Minimum maintenance
• A central feature, maybe

Cost N1M (100K)
72
Yarnton Way (SE London)
Roundabouts 1 2

• The Concept
• Reduce 2-lane divided highway to single traffic
  lane retaining median
• Cycle lane/track added separated by hatching
• Roundabouts at key intersections
• Additional crossings where school pupils gather

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Single Lane dualling roundabouts
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Single Lane dualling roundabouts
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A solution to block median crossovers
Using two at-grade roundaboutsnot more than
about 3 miles/5km apart
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A solution to block median crossovers
One roundabout available to redirect left
turnsPartial closures of median gaps
possibleBUT very difficult to enforce
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A solution to block crossovers
Allow right turns ON
One roundabout available to redirect left
turnsFULL closures of median gaps possible.May
help reduce pedestrian conflict with roadside
salesby restricting these to side-roads.
78
A solution to block crossovers

• Safe turning facilities
• U-turning for maintenance/service/emergency
• More land take than diamond interchange
• Longer bridge for accel. Lanes
• Sign routes carefully

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Summary

• Judge option of mini-roundabout objectively
• reduced congestion and air pollution
• reduced local rat-running (short-cutting)
• reduced speed and crashes
• assistance for pedestrians
• NOT
• It wont work
• Prejudice, opinions etc
• Dont experiment with inadequate layout
• e.g. flat central island when raised island
  needed
• Must get ALL drivers out of auto-pilot
• Must hold drivers respect (enough turning
  traffic)
• Overall appearance to be neat, tidy and effective