The text ONLY of the Paper is given below; but it will probably be more useful to read the .pdf file that includes illustrations and can be found in the Library. The text is given below simply so that Google and other "crawl engines" can find the text.
Gyratory Gridlock, Roundabout Resolution
Robin B. Clay
M.Sc. (Transportation Planning & Engineering)
Chartered Civil Engineer
For those readers unaware of the term, http://en.wiktionary.org/wiki/gyratory defines “Gyratory” as:- “(UK) A large traffic roundabout with non-standard lane markings or priorities”, i.e. it’s just a posh term for what the layman calls a roundabout. It is a term also sometimes used to describe an urban arrangement of one-way streets forming a circular route with entries and exits, or the very large “roundabout” on a different level at a motorway junction.
There is just such a feature where the M25 crosses (at J 28) the A12, the main trunk road that runs from London to Chelmsford, Colchester, Ipswich, Felixstowe (and Harwich) and Lowestoft. This junction obviously carries a great deal of traffic, and is on three levels. At the lowest level, the A12 runs uninterrupted as a dual carriageway with two lanes in each direction. Above this is the gyratory / roundabout, with slip roads in each direction from and to the A12 and the M25. Above this again flows the M25. The circular path is about 200m in radius, so about 600 m in circumference, and three or four lanes wide, so can contain at least 300 vehicles.
At the roundabout level, the person
responsible to blame for the design of this junction saw fit to install traffic lights at every entry, in pairs, one facing the incoming traffic the other facing the traffic already circulating or trying to escape, the idea being that each might have a turn. In practice, though, the design is fundamentally flawed, and causes un-necessary delay at the best of times, and gridlock at the worst. It is also twice the price it need be.
The incident, 6:45 p.m., Friday, 31/5/2013
It was the Author’s misfortune to arrive from Kent at this junction en route to Chelmsford at about six p.m. on Friday evening, 31st of May, 2012 – rush-hour at the start of a Public Holiday weekend. The queue to get off the M25 started about half a mile before the start of the slip road (perhaps 200 vehicles on each entry), and it took about 45 minutes to reach the stop line, where the traffic was stationary. We had gridlock.
Gridlock is a term used to describe a phenomenon whereby traffic entering the roundabout blocks traffic trying to leave, and everything grinds to a halt.
After being stationary at the stop line for about five minutes and suspecting that this was the problem, the Author got out of his car and walked anti-clockwise round the entire roundabout. Every exit was clear, but traffic trying to use each exit was blocked by traffic entering the roundabout. The phasing of the lights meant that if there was any movement of traffic, the incoming traffic inched forward, and the traffic trying to escape could not move when the lights changed.
On the second circuit, the author told each driver on the stop line of each entry what was happening, and told each driver to ignore the lights and remain exactly where he was until the author’s return. By the time the author reached the start point, the traffic was flowing fast, and his car and driver was a third of the way round the roundabout. But gridlock set in again.
On the third circuit, the author again explained the situation briefly to each driver on each stop line, and told them not to move until they had counted up to 200.
By the time the author had walked to the second exit – half-way round, and his intended exit – his car was waiting for him well clear of the roundabout. He climbed in and went on his way. Meanwhile, gridlock had returned to the roundabout. It would be interesting to know how long it lasted; the author’s guess would be - probably until after midnight.
It is noteworthy how many drivers asked what was happening, and noteworthy, too, how co-operative and uncomplaining drivers were. But still un-thinking enough to block themselves in.
The philosophy behind the use of roundabouts is (or should be) that traffic coming from different directions can merge and cross paths without having to stop – otherwise, an ordinary cross-roads will do. So here are the basic Principles of Roundabout design and use:-
- Traffic entering a roundabout must merge with traffic already on the roundabout;
- Traffic cannot merge if it is not moving;
- Ergo, traffic must enter the roundabout at the same speed as traffic already on it; and
- Traffic on the roundabout must not be stopped;
- Traffic entering a roundabout gives way to traffic already on the roundabout, i.e. gives way to traffic coming from its right;
- Traffic turning left (-90°) should (ideally) have its own filter lane;
- Traffic going straight ahead (0°) can be blocked by traffic entering from the left;
- Traffic turning right (+90°) can be blocked by traffic entering from the left or from straight ahead.
- Incoming traffic doing a U-turn (180°), or going round the roundabout by more than 180° is so rare it can be ignored.
Cures for the present situation
First, we accept that, for most of the time, there is no problem at this particular Junction – but this is due more to luck than design, for the design is intended to create gridlock by actually stopping traffic on the roundabout, and preventing escape. Gridlock can and does happen, as we have seen. There are three alternative solutions to the present problem. These vary from the instant and money-saving through the slightly more expensive and not money-saving to the more expensive but more effective, and still money-saving :-
- Cheap and quick. Remove all light bulbs from the traffic lights actually on the roundabout, in order to comply with principle 3 above. These traffic lights stop traffic on a roundabout. This must never be allowed to happen - it prevents traffic from escaping from the roundabout. Also, remove all light bulbs from the green phase of the incoming traffic. At the moment, incoming traffic with a green signal has the right of way, and proceeds across the path of exiting traffic, thus blocking the exit. Remove the green phase, and “normal rules” (i.e. give way to traffic on the roundabout) will apply, thus allowing traffic to escape. This solution involves nothing more than removing light bulbs, which saves electricity, yet is an exercise that would cost virtually nothing. One man could do it in ten minutes with an air-gun.
- Not quite so cheap, not quite so quick, not quite so effective. Paint a “yellow box” across the exit lane immediately next to the stop line on each entry, to prevent (?) this lane from being blocked. This requires getting “permission”, buying the paint, getting “possession” of the road, painting the lines, and then waiting for the paint to dry. The only cost is the paint and the workmen’s time. But it still conflicts with the basic principles outlined above.
- The correct solution. Remove all traffic lights from the roundabout. Install a set on each entry some 50 – 100 metres “upstream” of the stop line. Phase these lights such that all but one are red at any given time; at the instant that green one changes to red, the next anti-clockwise entry turns green. This enables that traffic to enter the roundabout at the same speed as the traffic on the roundabout (Principle 2 above). At that moment, though, there will be no opposing traffic on the roundabout (work that out for yourself). This solution means installing only half the number of traffic lights (just one set on each entry), saving both capital and running costs; but it does involve more work in re-locating traffic lights..
Prevention of Gridlock
In accordance with Principle No 1, traffic must enter the roundabout at the same speed as the traffic already on it.
In accordance with Principle No 2, traffic must not stop nor be stopped while on the roundabout.
So what is described above as “The correct solution” should be applied throughout the country, and particularly on “new build”. It is cheaper, and more effective than what is currently in existence up and down the land.
It would be very easy to prove this philosophy – all it needs is to hire four sets of temporary traffic lights and install them some 50 m back from each entry. Re-phase all traffic lights on the roundabout to red, and all entries to green, to create gridlock (a minute or two). Then re-phase the entry lights as suggested above, and turn off the lights on the roundabout. Gridlock will clear by itself within a minute or two of the first red phase on an entry.
What you can do
Place one of these stickers in your rear windscreen. Carry some spares in your car to hand out or show to other drivers “next time”.
If you are ever involved in gridlock:-
- Do not yourself obstruct the exit when you enter the roundabout;
- Tell all drivers near you not to obstruct the exit when they enter the roundabout.