Issue
Red Text (this column) is compulsory on
trunk roads |
My comment |
Action |
| 1.1
Central islands 4m wide max. |
I suppose I could hardly expect HA to recommend
changes here despite my protests over many years.
This would hang on changing the UK TSRGD, so this error and
all the problems associated with too small central
islands at mini-roundabouts will continue for
another decade or more.
DfT and HA must
realise that deflection at mini-roundabouts can only
be achieved with larger central islands in many
instances. Yet in both TD 54/07 and Mini roundabouts
good practice guidance we are recommended not to use
the central island as a speed reducing device!
Absurd! |
Highway
authorities must learn to use common sense
here and apply real deflection criteria based on
forced vehicle path curvature or vertical
deflections.
DfT and HA must
"get real" on this issue. Watch any
reasonably well installed mini-roundabout with a
raised centre and most drivers will be deflected by
it.
|
| 1.10
Mini-roundabouts with standard advisory give way
markings rely on the "priority to thy
right" rule for traffic (diagram 611.1). |
Not
so. The sole function of sign 611.1 is to replace
the sign to diagram 606 turn left, which cannot be
provided on a mini-roundabout; its legal requirement
is that it is placed with the mini-roundabout
marking and drivers must pass to the left of that
marking unless it is not possible to do so. The
give-way rule remains, correctly, advisory unless a
mandatory form of give-way marking is used. |
|
| 2.1
Mini-roundabouts must only be used on roads with a
speed limit of 30mph... |
Ideally,
yes, but the actual speeds are far more relevant. |
Remove
the speed limit from the mandatory requirement. |
| 2.15
Four arm mini-roundabouts... are not recommended
where the sum of the maximum peak hour entry flows
for all arms exceeds 500 veh/hr. |
Hardly
an issue, but some busy mini-roundabouts at
crossroads are difficult to drive. The larger
central islands that I recommend at such sites can
be very helpful at separating out the conflicts and
controlling speeds.
This is in
preference to reducing the ICD by adding overrun
areas on the outside of the junction areas and
reducing the corner radii that I have seen recently
at a number of sites.
|
The
TD should be recommending trialling these larger
central island mini-roundabouts. |
| 2.16
Mini-roundabouts with five or more arms must not be
used. |
At
such sites, before rejecting a mini-roundabout,
consider one with a central island larger than 4m.
One came into a seminar and was later introduced at
Eldwick in the North. It works fine.
See this location
in PixPlot Roundabout database. (Opens new window.) |
The
TD should be recommending trialling these larger
central island mini-roundabouts. |
| 2.24
The capacity at an intermediate give-way line
between double mini-roundabouts will be reduced by
the effect of the first junction. |
I
do not believe that the capacity will be affected
but the flow will. In most instances the amount of
yielding going on will be far greater at the first
yield lines than the intermediate ones. At the latter, yielding
will be to right-turning (UK & LHD countries) vehicles which will usually
be fewer in number than the "ahead/thru"
movements which take priority at the first yield
lines. |
|
| 2.3
It may be inappropriate to install mini-roundabouts
on approaches to ports, industrial areas etc. |
No
evidence - this statement should not have been made!
The
whole idea was that mini-roundabouts brought
roundabout operation to urban sites by being
overrunnable! |
Ignore
this comment! Ensure construction is strong enough
for the traffic likely to use it! |
| 2.5
Mini-roundabouts shall not be used on dual
carriageways. |
Mini-roundabouts
are extremely successful on single lane dualling
schemes such as Shenley Road, Borehamwood. According
to HA single lane dualling is not a standard
cross-section. |
Amend
TD to allow this exception. |
| 3.3
Safety - TRL accident study ... covers accidents
from 1986-1992. |
The
relevance of this point is that there was virtually
no comprehensive design guidance before 1996 so the
accidents all occurred at sites with hugely variable
geometry and layouts. TD 16/93 was published after this period but had very little aimed
specifically at mini-roundabouts anyway. |
It
might well be appropriate to inaugurate a new safety
study based on similar numbers of sites but with
specific criteria built into the designs. |
| 3.9
Accident Categories |
There
is just one shown for entering/circulating. This is
misleading. TRL report 281 has three categories:
1. Entering
(merging); i.e.
entering vehicle taking the next immediate exit and
striking a circulating vehicle leaving at the same
exit;
2. Entering (crossing); the
two vehicles in conflict were crossing one
another’s paths but one would have been physically
turning so only one vehicle could be moving at
relatively high speed; e.g. at a T-junction
mini-roundabout configuration;
3. Entering (right angle crossing - only at 4-arms) i.e.
both vehicles crossing one another’s path with
potential high speed for both movements – this can
normally only occur at a crossroads layout, but
represented nearly half of the injury accidents
reported at crossroads mini-roundabouts. Right angle
crossing + other crossing accidents represented well
over half of all injury accidents at four-arm
mini-roundabouts in the study.
The techniques for
dealing with these are a little different. |
The
document should identify these three accident types
as they are significantly different and the design
aspects to deal with these crash types also are
different. |
| 6.4
As vehicle speeds should already be low, full
deflection as required for standard roundabouts need
not be provided. |
I have always applied
forced vehicle path curvature to crossing streams
and believe that this remains a most effective way
of reducing the risk and severity of crossing
accidents. "Full" deflection may
include movements running along the kerb which
concern me less at urban mini-roundabouts. Provided
that a good approach layout is used a straight
approach for the merging stream usually does not
matter. But 60m radius vehicle path or less remains
essential for applying to crossing streams.
|
A
thorough revision of this section of the TD is
essential. |
| Fig
6.2 Vehicle path diagrams |
I am
pleased that at last there is reference to designing
mini-roundabouts on the basis of vehicle paths,
which most of us have been doing since Frank
Blackmore first did his "doodles" as he
called those early sketches. But the opportunity has
been missed to add the necessary radius to these
paths to indicate that there is forced curvature;
particularly necessary on the crossing movements.
|
|
| 6.16
Visibility. Road users approaching the
give-way line on any approach to a mini-roundabout
need to be sure that it is safe to enter the
circulating area. |
OK so far as it goes,
but the first requirement is for drivers to be able
to identify early enough that they are approaching a
junction at which they must be expected to
give-way/yield. This message must never fail to
reach the driver and sight of the junction area
and the approach layout are absolutely crucial. If
approach speeds are higher than desirable then it is
necessary for the transition area into the
roundabout to be sufficiently long to allow time for
adjustment. Forward visibility is part of this as
set out in Fig 6.5. But the most important aspect is
the appearance of the layout including the
approach layout.
|
|
| Table
6/1 Visibility to the right on entry |
If the vehicle entering
on the right is forced to 60m radius in accordance
with my standard then the visibility distance
becomes much less critical. Fig 6/5 is weak in that
there is little or nothing done on the approach and
inadequate deflection.
|
DO NOT
REPLICATE THIS LAYOUT |
| 6.19
The give-way marking requires road users to give way
to circulating traffic... |
In the UK the rule has
always been advisory. Our layouts should reflect
this more. Drivers get too upset when absolute
priority is not afforded at a roundabout; but this
was never intended. Shared space methodology
suggests that a lot of give and take should be the
way that roundabouts operate, not absolutes.
|
Ensure
all crossing streams are well constrained. |
| 6.24
Solid or raised areas of markings are not permitted
other than for the white circle. |
I believe that this is
wrong. I am not sure of the legislative basis but I
know of many sites that use raised splitter islands
very successfully. I believe that this slipped in
somewhere without any real consultation. The
Americans are using raised splitters but they design
them somewhat differently. See Dimondale
|
Amend
the legislation on an experimental basis |
| 6.25
A kerbed splitter island must be provided where
without it, vehicles would encounter an easier path
if they were to pass on the wrong side of the white
circle. |
There may not be room
for it and of itself it is not guaranteed to be
effective. But an additional regulation would be
broken by a driver passing to the right of a keep
left sign as has been observed occasionally.
|
|
| 6.33
No more than one lane must be marked as being for a
given exit arm. |
Where is the evidence
for this? I have seen some
mini-roundabouts that have two lanes marked for the
same exit arm, but the exit arm MUST be capable or
receiving the flow; a single lane exit will not
work. The degree of taper with a "merge in
turn" will depend upon the level of demand on
the exit.
|
|
| 6.36
Typically the give-way line is placed on the
circumference of the largest circle that can be
inscribed within the junction kerbs. |
NO! Design in accordance
with realistic vehicle paths. See Drawings
page for an explanation why this can be all wrong
especially at very small sites.
|
This
is so wrong and must be amended. |
| 6.6
Examples of island functions |
These layouts are dangerous!
|
DO NOT
REPLICATE THESE DANGEROUS LAYOUTS |
| 7
Conspicuity |
Note
particularly 7.2 - visual aspects of the approach
layout which must ensure beyond any doubt that
drivers are approaching a mini-roundabout.
|
|
| 7.9
Vertical features on a build-out |
I am concerned about
build-outs but I recommend that vertical features on
them should have a distinctive 3D shape and not just
be vertical. e.g. shrubs and soft planting.
A raised overrunnable area might be effective - I
have seen them used but not at a mini-roundabout.
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