M606 Chain Bar Roundabout, (South of Bradford) West Yorkshire
Slight delays are expected at the M606 Chain Bar roundabout due to work on a high occupancy vehicle lane. One lane of the M62 eastbound entry slip road will be closed over 24 hours with traffic using the hard shoulder and one lane of the roundabout will be closed from 9pm to 5.30am. Work is scheduled to continue until February.

That sounds benign enough, doesn’t it? If only…

The M606 is a very short Motorway (3 miles long from the M62 to the Bradford ring Road) which features on a site called Pathetic Motorways. I use it to get to Bradford from Morley, a trip of two junctions on the M62 then the length of the M606. Getting in isn’t too horrible and getting out was similar until a couple of months ago. If you want to head towards Manchester on the way out there is a Flyover, but those of us who want to head towards Hull have to indulge in the Chain Bar Roundabout, very prone to getting clogged up if the traffic light phasing is a bit off or there is a broken down car. (You sometimes get the feeling that a flickering streetlight is enough to introduce gridlock). The M62 heading Eastwards gets rather crowded at peak times and plans are afoot for widening.

The reason for the deterioration in journey time is the roadworks mentioned at the top. There is a prototype Motorway high occupancy vehicle lane being introduced from the M606 to the M62, continuing for a mile onwards. The idea of this is that if there is more than one person in the car (which is apparently only about 16% of the cars at rush hour) then they can whizz along this short-cut and supposedly save about eight minutes on their journey over the Single Occupancy Drivers who still have to go via the roundabout. It won’t be any use for me of course as I am one of the aforementioned SODs having to stick to the old road. Of course, if some of the traffic is going by a different lane then there will be a slight improvement in time.

Since the roadworks began, the traffic volume has become somewhat unpredictable. The tailback is sometimes only to the end of the sliproad down to Chain Bar, other times it is three miles worth all the way back to the ring road. The trouble is it is a funnel effect, two lanes merge with another two, then further on another two lanes join it, then when it gets to chain bar the two lane slip road only widens to three for a few hundred yards with only the two left ones being able to head to the motorway. You can see this on the satellite image of the sliproad shown (pre-roadworks).

Now, I have noticed a causal relationship between how much of the third lane is bollarded off and how bad the traffic is. If the bollards simply follow the line of the hard shoulder, all is well. However, if any of the left hand lane is not available, the traffic throughput during a lights cycle is much reduced and the cars start to tail back. Yesterday, ONLY TWO CAR LENGTHS were bollarded off, that is only about 30 feet or so, but it was really horrible.

I found this graph on the Highways Agency site and it was quite familiar, because it suffers from exponentiality; phone and data Networks also need to be engineered for traffic volumes and in certain circumstances minor increases in volume on an unstable system can result in catastrophic reductions in throughput. (I sometimes experience this with my Sky Max Broadband service where late night packet loss approaches 100%).

The Guru of all things network traffic related was a Danish Mathematician known as A K Erlang. A household name in Telecommunications circles, he gave us this catchy little formulaic ditty:

Truth be known, we use computer programs to get the answers and when I was younger, we used tables. What do the Highways Agency use? Social Engineering…