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Mini-roundabouts
- Getting them Right!
Capacity of (mini-) roundabouts
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| I used a rule of thumb in my book and I
continue to promote this. Because mini-roundabouts are so
interactive, drivers may be forgiven when occasionally
they all look at one another wondering who is going to make the first
move. This three-way stand-off is relatively common
and it has to be said that different drivers will accept
very different conditions before entering a busy
mini-roundabout. |
Frank Blackmore invented a capacity rule
based on the area of the junction. He argued that the
capacity was in direct proportion to the linear size of
the junction expressed by a formula that summed the entry
widths and added a value for the square root of the
remaining area. This was based as much on his research on
the TRL test track as anything else; it suggests the
possible capacity if the area is used most efficiently.
It relates to a fundamental formula that I illustrate in
my seminars: |
Q = N/t
where Q is the flow,
N is the average number of vehicles in the
system at any one moment and
t is the average time taken by any vehicle to
pass through the system.
In my capacity work I assume a reasonable
degree of one-to-one; if one driver cannot proceed then
another usually can. This works reasonably well for small
junctions. and seems to relate closely to the formulas
that the Americans have used.
For a stream entering a mini-roundabout and
turning right:
QR = 1200 - QC
where QC is the circulating
traffic in vehicles per hour equivelent.
For a stream entering a mini-roundabout and
turning left:
QL = 1500 - QC
For a stream entering a mini-roundabout and
turning left and right:
QL&R = 1200 - QC
This formula results in a straight line
graph at 45°, which is close to the graphs produced by
others. There are slight end effects that we can ignore
here.
The French have identified that when trying to enter a
roundabout the traffic turning off at the first exit
(left in the UK) does have an inhibiting effect on the
next entering stream around. This arises particularly
with a single lane approach where drivers are turning
left and right from the same lane. Entering drivers
cannot be sure exactly what the drivers from the previous
approach are doing.
[under construction]
© Penntraff - May 2007
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