|Idealised drawings of four-arm midi-roundabouts
drawings of idealised layouts to illustrate
|Here is a series of four drawings
of symmetrical idealised crossroads. Each
illustrates central island sizes required to
provide deflection at 60m radius for approx. 2m
wide vehicle paths. The island size is determined
by the deflection required which in turn is
determined by ICD and other
factors such as road widths on entry and exit,
||In this series of drawings with
reducing ICDs the first is the only one with a
solid centre. In the following three layouts it
is only the truck apron which remains, and it is
always designed to be large enough to deflect
light vehicles, and that will often mean a
diameter larger than 4m.
Roundabout with solid central island
What we are seeing in effect here
are two roundabouts! One designed for trucks,
coaches etc. and one designed for light vehicles.
In the UK
this would be legal.
The size of the
truck apron here is determined mainly by the road
widths. Reducing these locally would help pedestrians, but
remember refuge (Splitter) islands are very effective.
(See Millennium Vision page)
There are many
cross-roads like this in the UK and thousands in
traffic by the central island alone requires
quite a large island.
would be assisted here by the use of
build-outs on the approaches, but avoid
significant reduction of the ICD. In the absence of vertical
central island is the only deterrent against
"straight-lining" by drivers.
In my seminars
we look at a possible way of dealing with one of
the sites at Hove which is remarkably similar.
the UK this might be illegal!
or require special authorisation.
In the U.S. this could usefully replace the
thousands of 4-way STOP intersections
that perform so badly.
This is a relatively
extreme case where a midi-roundabout might be
tried on a crossroads with tight corner radii and
wide approach roads. As with the previous layout
narrowing the approaches will make a better
layout altogether but which may still require
over 4m for the central island.
this might be illegal in the UK!
Medium-size flared crossroads
7.3m is the typical width of many
UK roads so this illustrates well the need
for a central island larger than 4m. Many highway
authorities will not now install mini-roundabouts
in these circumstances which is a pity because
they can work well provided there is
sufficient deflection, achievable using a
larger central island. In addition I would
recommend raised splitter islands at a site like this, but these
alone might be illegal although not if headed by a solid part
with a bollard included.
technically illegal in the UK!
UK designers: TD16/93 (now superceded) requires deflection to be provided
on the approaches where possible. On mini-roundabouts
this is nearly impossible, but the deflection must
be provided somewhere on all crossing paths unless
physical vertical deflection is provided to a sufficient
standard. The above three drawings of the MIDI-roundabout
illustrate well the method of design. TD54/07 replaces TD16 but remains
a poor document - See my comments on this HERE.
* * * *
rather large drawing illustrates to approximate
scale my suggestions regarding the central
island. The island is a conic sectional volume or
frustum which is essentially
flat on the top. The side angle must not (UK)
exceed 15º or 1:4 (Traffic calming Regulations).
I have shown the side slope grey in plan but it
should remain white "on the ground".
that a serious problem has developed with
mini-roundabouts generally and that is drivers'
lack of respect for the central island. Many
drivers choose to ignore the island although it
is unlawful to pass to the wrong side of it.
Designers bear a lot of the responsibility for
this by designing poor layouts. So for these
larger junction layouts I strongly recommend this
form of island to deter drivers from running
straight across the intersection.
has it that designers will lay this island on the
existing carriageway profile which may be flat,
level or quite variable. I favour a general
resurfacing of the intersection and the
opportunity will then be taken to have a
relatively even profile with a steady outward
drainage fall of around 2%. The island will fall
into place on top of that and the central island
height will be dictated by the type of vehicle
likely to pass over it; grounding must be
avoided. The centre of the island should perhaps
be marginally raised to drain over the edge.
Overall height must not exceed 125mm and I
generally recommend no more than 100mm. If the
carriageway slopes away at more than 2% take care
that a low-loader or low-floor bus for example does not ground.
recommend too that the central island must be
wide enough to deflect (as per drawings above)
but that there should be room for most buses and
coaches to circumnavigate such islands even if
slowly. It can be very uncomfortable for passengers
when buses and coaches lurch if they have to climb vertical
deflections such as this.
Links to other pages:
Home | Crossroads | Drawings | Millennium Vision
© Penntraff - January 2012
Pages designed by: