Miniroundabouts A Definitive Guide

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Mini-roundabouts A Definitive Guide

• Flows Capacity

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Accuracy

• Mini-roundabouts at or near capacity are highly
  interactive
• Dont expect accuracy closer than /- 10
• Rather assume that the roundabout layout will
  work better than whatever else preceded it
• Watch for heavily dominant streams

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Some myths

• Flows have to be in balance
• The first public road site had right turning
  flows in the ratio 1511
• Queues will merely transfer to another arm
• This will depend entirely upon the relative
  turning movements and demand flows but the
  intersection operation is so different that this
  is rare
• Dominant streams will develop
• This can happen in the virtual absence of a
  turning movement, but the operation will still be
  improved - see double mini-rbts

Two roundabouts close together often work like
this, for example in a village in practice they
complement one another. You will rarely get
lock-ups between the two. The disparate turning
proportions are often closely linked to the
degree of turn 180 degrees being the max and
below about 45 degrees expect little turning
movement in most cases
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Rule of thumb formulas

• For a single stream entering and turning left
• QEL 1200 - QC
• where QEL is the flow entering and turning left
  and QC is the stream with priority over QEL

We know that the slope of the graph is not -1 but
nearer -0.6. In time I shall try to develop a
spreadsheet that moves more close to this. The
steeper slope is therefore underestimating
capacity. If your scheme has sufficient capacity
using the simple formula above then you will
almost certainly not need to look any further!
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Rule of thumb formulas

• For a single stream entering and turning right
  
• QER 1500 - QC
• where QER is the flow entering and turning right
  and QC is the stream with priority over QER

Turning right is often easier and if there is a
sufficient lane-split I.e. two separate lanes
allowing good usability on approach then the
capacity of this lane will be higher.
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Rule of thumb formulas

• For a single stream entering turning left and
  right
• QELR 1200 - QC
• where QELR is the flow entering and turning left
  and right, QC is the stream with priority over
  QELR
• Note Right turning traffic on a single lane
  entry will inhibit entering traffic on the next
  arm counter-clockwise.

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Early Capacity Assessment
One of the earliest attempts to correlate entry
flow with circulating flow and its not a bad
line!The green line is my basic rule-of-thumb
for crossing streams.
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Entry Capacity
Clives rule-of-thumb
Again, my rule-of thumb for single lane
operation is close to others but falls too early
with increasing circulating flow. The second line
is for right turning flows where there is good
access.
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This slide was not shown at Kansas08 but may be
useful it illustrates a LHD 3-arm mini-rbt in
the UK with 4 entry lanes demand flows are set
out in the top section and grown to pcu/h in
the second.
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The same illustrating the capacity of each entry
and how the figures are derived. The lower
section represents the r.f.c. as ages. The site
layout was demonstrated in the Saturday seminar
session.
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This site was demonstrated at the seminar first
as if it was a single roundabout and then the
northern mini-roundabout half. Although crude,
this assessment has proved a good starting point
most mini-roundabout installation deals with the
queuing problems anyway and little detailed
assessment is needed.
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Rule of thumb formulas

• Examine right turning flows to see if queues will
  form
• No adjacent (right turning) flows should exceed
  1200 pcus per hour or equivalent
• If expecting overload, do research on 5 min basis
  or less and run ARCADY
• For double roundabouts, ignore queues on
  link-road predicted by ARCADY, they usually wont
  happen but will re-locate to first yield line

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Left turn overload

• If the three right turn flows exceed the
  equivalent of 1800 pcu/h in any period, you may
  wish to design for double left turning in
  practice this usually works best on an ahead
  movement, usually the largest of the three, which
  could be doubled up.

You will need to estimate the distribution
between the two lanes available, in practice the
offside one will saturate first, then drivers
will use the nearside one as an overspill
provided that there is capacity there. Road arrow
striping will be needed to ensure that drivers
understand your intentions.
Make sure that there is room on the exit for
parallel streams to flow through then merge as
necessary.
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Right turn overload
The red arrow will be less than the adjacent flow
but its capacity will 1200 less the flow to which
it yields and less the right turning element.
Take care that the exit will work properly. If
the total flows are high, allow a long
merge-in-turn.
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The two cars in the image came together across
the mini-rbt the exit comprises 2 lanes which
then diverge at a subsequent mini-roundabout a
most satisfactory arrangement.
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Other formulas

• In dealing with larger roundabouts or junction
  systems, it is well worth bearing in mind the
  following formula
• Q N / ? where
• Q is the flow (in or out),
• N is the average number of vehicles in the system
  and
• ? is the average time each vehicle takes in the
  system.
• This basic formula explains why capacity reduces
  with high proportions of left and U-turning
  flows. It can of course be applied to any
  traffic system not just roundabouts and any
  network of any size provided that it is in steady
  state.
• It helps the understanding of Ring Junctions

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For Ring junction conversion

• Before with large roundabout
• N 12 vehicles at any one time (on average)
• ? 9 s (say)
• Q 3600 X 12/9 4800 vehicles per hour

• After with ring junction
• N 30
• ? 20
• Q 3600 X 30/20 5400

Dont be afraid of ring junctions! The driving
task is much easier than the Rotary that it will
replace just take great care over the capacity
of the individual links and the signing through
the system. It will be much cheaper to retrofit a
ring junction than to construct a modern
roundabout within it.
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Networks

• A Ring junction is a small network
• Many towns in the UK have several roundabouts
• Wokingham (Berks)
• Newton Abbot
• Tavistock
• These networks are on the whole very efficient.

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Wokingham
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Tavistock
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Tavistock Centre
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Newton Abbot
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Newton Abbot Centre
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Summary

• Mini-roundabout installation will
  reduce/eliminate most queuing
• If in doubt check using formulas
• Cross-check using ARCADY
• Interpret sensibly ignore silly results
• If doubling up traffic streams, ensure drivers
  fully understand your intentions!
• Make sufficient room on exit
• (Mini-) roundabouts are on the whole good in
  networks.
• Dont be afraid of Ring Junctions for Rotaries