"Andrew P Smith" wrote in message
...
In article m, Martin
Underwood writes

"Robert Woolley" wrote in message
.. .
On Sun, 19 Oct 2003 17:18:43 +0100, Andrew P Smith
wrote:

In article , Robert
Woolley
writes

Speed does kill. You don;t have to be a genius to understand that
the
faster the speed of a vehicle, the longer it takes to stop. And the
faster it hits something else the greater the damage.

No. Bad driving kills. The driver selects what speed they drive at. If
that speed is inappropriate then it's bad driving. Nothing else.

I'm a former member of the Institute of Advanced Motorists. Are you?

Nope.

But I hold a PSV licence, gained after comprehensive training. I also
hold a RoSPA road safety engineering certficate, a BSc in Transport
Management and Planning, Chartered Membership of the Chartered
Institute of Logistics and Transport, plus Corporate Membership of the
Institution of Highways and Transportation.

OK. Maybe you can answer this question: what are the circumstances under
which a single 4-way roundabout should be replaced by two linked 3-way
mini
roundabouts? There's a sod of a junction near me which always gets
snarled
up with traffic (junction of Drayton Road, Spring Lane and the two halves
of
Ock Street in Abingdon) and it seems to me that it would have a much
greater
throughput of traffic if it was converted back to a single larger
roundabout
because it would save drivers having to check twice for vehicles from
their
right - once on the first roundabout and then again on the second.


Martin

I know the junction you mean.

We have an even worse one here in High Wycombe and as for the Magic
Roundabout in Swindon.......

Yes, the Magic Roundabout in Swindon is a pain in the bum: it's as if the
road designers decided to make it as tortuous as possible - being cynical, I
wonder if they decided to make it hazardous so as to keep the traffic speed
down: which is silly because the deliberate hazards distract the drivers'
attention from the hazards that they should be looking for - other road
users!

And then there's the roundabout in Hemel Hempstead. This started out as one
big 6-way roundabout. It worked fairly well. Then the traffic planners took
it into their silly meddlesome heads to place a mini roundabout where each
road joins the big roundabout (which they reduced in diameter). To negotiate
the roundabout, you now have to go round several mini roundabouts. Moreover,
if you are turning right, you go clockwise round each mini roundabout but
*anti-clockwise* round the central roundabout, which feels very wrong:

http://www.martinunderwood.f9.co.uk/hemel.gif

There are now three places (marked with an X) where a driver must check for
traffic on his right, whereas on a normal roundabout there is only one -
normally once you are on a roundabout you do not have to give way to anyone.


Strangely the roundabouts in High Wycombe (I presume you mean the ones near
the Fire Station and at the bottom of Marlow Hill) don't seem too bad -
maybe because they are spaced far enough apart that they don't seem like one
big junction - you have you sufficient time and distance to "recover" from
one before encountering the next.


Another question for our road layout expert: who decides which mini
roundabouts are raised up and which are simply white discs painted on the
road? You'd think it would be determined by the amount of circulating space:
if there's plenty, have a raised-up one and make traffic go round it; if
there's no enough maneouvring space, have a painted disc and let traffic go
over the top of it - it's there purely to establish fair play. But no, the
"experts" in Didcot have one on the Broadway (the high street) which it is
impossible to drive round even in a small car - there's just not enough
space between the roundabaout and the kerb. And this one is raised - it's
like going over a one-sided speed hump. If anyone were to misjudge its
severity and negotiate it too fast, they'd lose control and go straight into
the shoppers on the pavement. If only that was a painted roundabout, it
would be so much easier.

In article m, Martin
Underwood writes

And then there's the roundabout in Hemel Hempstead. This started out as one
big 6-way roundabout. It worked fairly well. Then the traffic planners took
it into their silly meddlesome heads to place a mini roundabout where each
road joins the big roundabout (which they reduced in diameter). To negotiate
the roundabout, you now have to go round several mini roundabouts. Moreover,
if you are turning right, you go clockwise round each mini roundabout but
*anti-clockwise* round the central roundabout, which feels very wrong:

http://www.martinunderwood.f9.co.uk/hemel.gif

There are now three places (marked with an X) where a driver must check for
traffic on his right, whereas on a normal roundabout there is only one -
normally once you are on a roundabout you do not have to give way to anyone.


I know this junction well but don't often get held up there despite the
traffic conflicts. The lanes are badly marked and you can have 2 lanes
of right turning traffic both trying to get into the left turn lane for
the next roundabout 15m ahead.


Strangely the roundabouts in High Wycombe (I presume you mean the ones near
the Fire Station and at the bottom of Marlow Hill) don't seem too bad -
maybe because they are spaced far enough apart that they don't seem like one
big junction - you have you sufficient time and distance to "recover" from
one before encountering the next.

Try them at 8.30am tomorrow morning!
--
Andrew
Electronic communications can be altered and therefore the integrity of this
communication can not be guaranteed.
Views expressed in this communication are those of the author and not
associations or companies I am involved with.

On Sun, 19 Oct 2003 18:48:54 GMT, "Martin Underwood"
wrote:

[on Hemel/Swindon magic roundabouts]

I've heard that the reason for the change at Hemel was that there were
too many large, reportable accidents going on (involving injury etc).
The result of the change was apparently to reduce this count
significantly as the traffic was moving more slowly. However, what it
also succeeded in doing was to increase substantially the count of
minor, damage-only, non-reportable accidents instead.

Could be being cynical, but having cycled around it (not driven it) I
can believe it.

Neil

--
Neil Williams
is a valid email address, but is sent to /dev/null.
Try my first name at the above domain instead if you want to e-mail me.

In article m, Martin
Underwood writes
Yes, the Magic Roundabout in Swindon is a pain in the bum: it's as if the
road designers decided to make it as tortuous as possible - being cynical, I
wonder if they decided to make it hazardous so as to keep the traffic speed
down: which is silly because the deliberate hazards distract the drivers'
attention from the hazards that they should be looking for - other road
users!

And then there's the roundabout in Hemel Hempstead. This started out as one
big 6-way roundabout. It worked fairly well.
[...]

Actually, if you study both these junctions and other places where ring
junctions (to give them the proper name) are installed (e.g. the
A13/A130 intersection), you'll see that the junction was trying to
handle too much traffic and snarling up.

The basic point behind a ring junction is to reduce the average
proportion of the roundabout that traffic has to go on, thus increasing
the throughput of the junction.

Consider a ring junction with 5 exits and assume that all 20 possible
flows see equal traffic (this is to aid the explanation; at a real site
you would of course take measurements). If you build it as a normal
roundabout, the average distance that a car travels around the
roundabout is half its circumference [1]. Thus the flow on the
roundabout has to be 2.5 times the flow coming in from each road [2].

If you replace it with a ring junction, the average distance a car
travels drops to 30% of the circumference [3] and the flow only has to
be 1.5 times the incoming flows [4]. Put another way, you gain 66%
capacity (though of course you then lose some because of the additional
needs to give way, but it's still a net win).

[1] 25% of the traffic goes 20% of the circumference, 25% goes 40%, 25%
goes 60%, and 25% goes 80%. That works out as an average of 50%.
[2] The section from road 4 to road 5 carries all the traffic entering
at road 4, 75% of that entering at road 3, 50% of that entering at road
2, and 25% of that entering at road 1. That's 100+75+50+25 = 250% of the
traffic entering at any one road.
[3] Now 25% goes 20% of the circumference clockwise, 25% goes 40% of the
circumference clockwise, 25% goes 20% anticlockwise, and 25% goes 40%
anticlockwise. That's an average of 30%.
[4] The section from road 4 to road 5 now carries (clockwise) 50% of the
traffic from road 4 and 25% of that from road 3, plus (anticlockwise)
50% of that from road 5 and 25% of that from road 1. Total 150%.

Moreover,
if you are turning right, you go clockwise round each mini roundabout but
*anti-clockwise* round the central roundabout, which feels very wrong:

Live with it. Any unfamiliar layout feels wrong; what do you think about
slip roads on the right instead of the left?

--
Clive D.W. Feather, writing for himself | Home:
Tel: +44 20 8371 1138 (work) | Web: http://www.davros.org
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