Comments on: Topologically Non-Trivial Highway

By: John Armstrong

John Armstrong — Sun, 27 Jul 2008 16:55:40 +0000

People always hate on what’s new to them. You should know that — you’re an expert in hating on things.

By: Jacob Freeze

Jacob Freeze — Sun, 27 Jul 2008 12:58:34 +0000

Although I am reluctant to discourage the child-like enthusiasm that Dr. Armstrong and Dr. McBurney express for the Magic Roundabout, it may interest them to know that Auto Express Magazine selected it as one of the World’s Worst Junctions in 2005, and likewise a survey of motorists conducted by Saga Motor Insurance distinguished it as the worst roundabout in the UK.

http://en.wikipedia.org/wiki/Magic_Roundabout_(Swindon)

By: Peter

Peter — Sat, 26 Jul 2008 19:23:10 +0000

As I understand it, the relationship between changes in green-light and red-light durations (as inputs) and traffic throughput (as output) was not continuous and, moreover, differed from one access-point light to another, and differed according to the time of day and dominant direction of traffic flow. So, simply by changing the standard durations of red- and green-lights, the engineers could obtain significantly greater traffic throughput. In some cases, greater throughput was achieved with shorter durations for certain green lights.

In my earlier comment, I mis-named this roundabout: It is officially the Hanger Lane Gyratory System:

http://en.wikipedia.org/wiki/Hanger_Lane_Gyratory_System

By: John Armstrong

John Armstrong — Sat, 26 Jul 2008 11:26:13 +0000

slight changes to the durations for which different access traffic lights remained green, and remained red, improved traffic throughput disproportionately

You mean the system exhibited sensitive dependence? Very interesting…

By: Peter

Peter — Sat, 26 Jul 2008 09:31:31 +0000

Also, the web-page on the Swindon roundabout states the previous single roundabout could not cope with the level of traffic, especially at peak times. A roundabout in London, the North London Gyratory System, had a similar problem, and access to the roundabout was regulated by traffic lights at each access point. Using simulations, traffic analysts were able to show that slight changes to the durations for which different access traffic lights remained green, and remained red, improved traffic throughput disproportionately.

In particular, the duration of green lights for traffic coming onto the roundabout from different directions was no longer made constant across all the access roads. It turns out that the the rate of traffic throughput in this roundabout was not continuously dependant on the durations of access lights, so that a small change in durations (from memory, just 3 seconds difference between two lights) could lead to a large change in throughput. Such a relationship is one that non-mathematicians often struggle to understand.

By: Peter

Peter — Sat, 26 Jul 2008 09:22:06 +0000

The Swindon roundabout is a 2-roundabout, surely! And it would seem an obvious result that traffic must flow in alternative directions (clockwise and counter-clockwise) in each adjacent component of an n-roundabout.

BTW, this structure is not unique to Swindon. There are similar double roundabouts in the north of England.

By: John Armstrong

John Armstrong — Wed, 23 Jul 2008 01:51:46 +0000

Oh, putting it on only one roundabout isn’t what’s brilliant. Of course they should put it on many more!

By: Jacob Freeze

Jacob Freeze — Tue, 22 Jul 2008 23:44:46 +0000

And while we’re talking about Shropshire…

“With rue my heart is laden
For golden friends I had,
For many a rose-lipt maiden
And many a lightfoot lad.

By brooks too broad for leaping
The lightfoot boys are laid;
The rose-lipt girls are sleeping
In fields where roses fade.”

A.E. Housman, A Shropshire Lad

By: Jacob Freeze

Jacob Freeze — Tue, 22 Jul 2008 23:35:28 +0000

I have to disagree with John Armstrong about the brilliance of having drivers revolve counter-clockwise around exactly one roundabout in England, although I appreciate the design as an example of “Shropshire whimsy.”

“Shropshire, aye, that’s where they put a pig on the wall to watch the parade.” In this case the pigs are probably cheering while tourists bang head-on into each other on Swindon’s two-way roundabout, and there’s little enough for a pig to cheer these days in Shropshire, since Madcap Harry Monmouth slew “Hotspur” Percy on Shrewsbury field, and Owen Glendower ran back to Wales.

By: John Armstrong

John Armstrong — Tue, 22 Jul 2008 20:43:39 +0000

I think it’s a brilliant solution. I wish I had a chance to drive there, or on an American analogue.