[talk-au] What gives with roundabouts?

[Darrin Smith]

On Mon, 15 Dec 2008 12:08:54 +1100
Ian Sergeant <isergean at hih.com.au> wrote:

> + When you cross this kind of roundabout when cycling, or with a
> learner driver, you don't have to worry about the characteristics of
> the road you are crossing (since you never turn into the traffic of
> the cross road, you just cross the roundabout).  This isn't just
> about cyclists and learners. Its about the nature of the intersection.

I'm having quite a bit of trouble understanding your point here, what
difference does whether it's a node or a loop-way have on the
characteristics of the side road? What difference does the size of the
roundabout have to do with this? 

Surely you *do* turn into the traffic of the cross road, twice in fact,
once for each direction, once yielding, once with right of way? Again
this happens for all sizes of the roundabout.

(In fact I thought that was the point of roundabouts, to reduce the
points of contact to a minimum and make the laws of yielding
right-of-way very clear - and to slow you down whilst doing it of
course)

> + It represents what is on the ground accurately.  Often there is
> less of an actual diversion than many other traffic calming devices,
> which are not mapped.  To draw it as a deviation in the road, just
> isn't what is there.

The only other traffic calming device I can think of that this applies
to is a 'chicane', perhaps I missed a few options? Humps and their
variants cause no change in the traffic flow, neither do Chokers (all
listed in map features), what others are there? 

Given nearly all small roundabouts occupy close if not the entire road
width, using the intersecting roads carriageway as part of the loop,
this means the average deviation is about 1/2 a road width out and 1/2
a road width back or nearly 1 whole road width. I'm pretty sure I
haven't seen a chicane deviate more than a road width (that would put
you on the footpath at some point), so it's really a close call here as
to whether there is '*less* of an actual diversion than *many* other
traffic calming devices'.

> + These have a very standardised appearance, and should be
> represented in a standardised way, like a template.  The benefit
> isn't just in time-saving, but in identifying that all these
> roundabouts are very much the same.

What standard things about a suburban roundabout are there that don't
equally apply to a large roundabout? The only standard things I can
think of about all suburban roundabouts are:

(1) They go clockwise around a central raised island
(2) All approaches to the road are divided by at least a smaller
splitter island
(3) They have one lane
(4) They have a roundabout sign displayed on all approaches

Beyond this they are as varied as everything else on the planet. 
The only one of those that doesn't also apply to much larger
roundabouts is (3) but it applies to some much larger roundabouts, so
it's a bit of a non-starter.

If we look at the things that vary between suburban roundabouts we find
the following list:

(1) The structure of the centre varies wildly beyond being raised:
- some have a garden bed, some are paved, some are dirt
- some are a single tier, some a 2-tiered, some are 2-tiered with the
lower tier being traffic-able.
(2) Some have extensive street furniture in the centre, some don't.
(3) The splitter islands may or may not have signage.
(4) The outer edges of the approach roads may or may not has extra
curbing added to tighten the road approaches
(5) The roundabouts may occupy just the existing road surfaces or may
extend out beyond them, and given the variety in road widths different
approaches to the same road can yield radically different judgements
by people. (the extended area is often quite large on the opposite side
of a 'side' road in a T-junction)
(6) Not all of these are even circular, some are elliptical in shape,
and of course there's the classic egg-timer shapes. 
(7) Quite a number of circular ones have funny offsets on opposite
roads which mean your have to adjust the centre lines of the adjoining
ways to make it line up (suddenly map doesn't match ground any more).

6 and 7 at least can be handled by actually drawing the roundabout as a
way I guess, but then it defeats the purpose of the standardisation
idea.

I'd suggest in fact that suburban roundabouts don't fit any kind of
template at all.

<snip> the rest for now because you raise a good point about the
decision order, in fact I'd even go so far as to suggest (given jackb's
disagreement with me on what it means) we need to make an explicit list
of what exactly defines a roundabout, I think Liz's list at the top of
her PDF was a good start, plus a no-parking addition).

-- 

=b