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At the risk of looking stupid, can anyone come up with a reason why traffic lights are fitted on the entrance sliproad onto a motorway at the beginning of the acceleration lane. Although not in operation yet, some have been fitted at Jnct 22 M6 and the Birchwood entrance onto the M62. I cannot see any reason why they should ever be on red.

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At the risk of looking stupid, can anyone come up with a reason why traffic lights are fitted on the entrance sliproad onto a motorway at the beginning of the acceleration lane. Although not in operation yet, some have been fitted at Jnct 22 M6 and the Birchwood entrance onto the M62. I cannot see any reason why they should ever be on red.

They are for the ramp metering scheme being rolled out across the country, the idea is that if you measure the flow on the motorway and that comming from the slip road you can "meter" the traffic on to the motorway to prevent (or in most cases) delay the break down of traffic flow (flow breakdown). It is something that actualy works!, trust me... :)

here a little bit of blurb... http://www.highways.gov.uk/knowledge/8877.aspx

here's a FAQ from the USA where a significiant amount of work has been done on RM...

http://www.dot.ca.gov/dist4/rmfaq.html

Personally I have worked on both the M6 and the M27/M3 schemes but I am not working on the current one, though some of my staff are :)

Press release:

http://www.highways.gov.uk/news/pressrelea...eleaseid=127593

Press release:

http://www.highways.gov.uk/news/pressrelea...eleaseid=127593

*EDIT*

press release: http://www.highways.gov.uk/news/pressrelea...eleaseid=127593

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The only problem is that it doesn't really eliminate congestion, it just moves it somewhere else.

Take M1 south junctions 13, 12, 11, 10, and 9 on weekday mornings, as an example.

All of these junctions have traffic lights operating off the exit slip roads to a roundabout over the carriageway... the same traffic light system, of course, also controls traffic trying to join the motorway from the interconnecting junction roads, but does this before they join roundabout. During peak times (and usually extending a little before and after), the traffic is overly congested along those trunk roads long before ever reaching the junction.

There is a risk that by using traffic lights as well on the entrance slip road that this would incur additional congestion around the junction roundabout as well causing a complete gridlock of the entire junction.

There are times when the junction is so congested that the motorway itself becomes blocked because the EXIT slip-road traffic cannot get past the end of the sliproad because the junction is gridlocked.. this exiting traffic tailing back up the motorway.

or am I missing the plot here ?

L.

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In a word... yes :) (EDIT: as your a networking IT bod... If it helps, think about it as QoS with the priority being given to the main motorway flow ;) )

The lights at the top of the junction are not controlled with the knowledge of the flow of the motorway. They are just controlled for the locality they are employed in and have no intelegence relating to the motorway.

The new system is also going to interface possibly with the traffic lights that feed in to the on-slip, this will allow for extra queing capacity on the approaches in to the on-slip.

The equation is simple:

Total flow capacity through junction on the motorway = flow in to junction + flow down on slip

The idea is to control (meter) the "flow down on slip" so that the sum does not become greater than capacity through the junction.

Sometimes, it is not the capacity at teh junction, but can be to meter before a certain point (large gradient, place effected by low angle sun etc)... anywhere where the traffic flow can break down...

If you know the point of flow break down and the capacity at which the flow breaks down, then you can attempt to contol the flow such that you stop / reduce / delay the flow break down (every one comes to a halt!).

That the theory any way. It is quite often only suitable for certain types of locations.

Also once the flow break down has occured, the recovery has a hystorisis such that the capacity has to drop a very significant way down before the recovery of flow can start... if you plot a graph of speed/flow, it looks like a wave breaking over into a curl, the tip of the curl being where the flow starts to recover.

On the M6, the ramp metering can make the difference such that without it, the flow breaks down at the beginning of the morning peak then never recovers untill about 19:00! well after the evening peak!

With the ramp metering turned on, the flow break down is delayed in to the morning peak, but recovers before lunch time, so journeys are improved during the mid-day off-peak times.

The stuff they are putting in in a culmination of the lessons learnt from the M27 and M6 schemes and a new control system to boot!... should be interesting (there about 100 sites going to be done over the next couple of years, so it will be a very common view eventualy).

Anyway... enough work stuff... I'm on my hols today (he says in the middle of doing stuff for work!) :)

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This is the graph of traffic flow vrs capacity (occupancy)...

You can see from the graph the effect and amount the traffic flow has to drop by to allow the flow to recover, in this case around 500 vehicles per hour which is a lot!

fbd.jpg

Reproduced from the following document: http://www.ce.umn.edu/~levinson/Papers/Breakdown.pdf

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In a word... yes :) (EDIT: as your a networking IT bod... If it helps, think about it as QoS with the priority being given to the main motorway flow ;) )

heh.. therein lies the problem.. we (networking IT bods) tend to give priority to the less-prevelant traffic (i.e. voice or certain minority priority application) not the "main motorway flow"... I guess that explains why it seemed counter-intuitive on first glance.

I'll stick to my technical and business experience... civil engineering, roadways, and such like make me feel like the former Iraqi Information Minister... :iraqi-info-minister:

Will design international MPLS backbones for food. Will align your IT/ICT strategy to applicable technology, processes, and systems which will reduce costs, increase margin, and EBIT performance for a bit of luvvin' ... you can have your roads ;)

:duh:

L.

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In a word... yes :) (EDIT: as your a networking IT bod... If it helps, think about it as QoS with the priority being given to the main motorway flow ;) )

heh.. therein lies the problem.. we (networking IT bods) tend to give priority to the less-prevelant traffic (i.e. voice or certain minority priority application) not the "main motorway flow"... I guess that explains why it seemed counter-intuitive on first glance.

I'll stick to my technical and business experience... civil engineering, roadways, and such like make me feel like the former Iraqi Information Minister... :iraqi-info-minister:

Will design international MPLS backbones for food. Will align your IT/ICT strategy to applicable technology, processes, and systems which will reduce costs, increase margin, and EBIT performance for a bit of luvvin' ... you can have your roads ;)

:duh:

L.

LOL

:offtopic: Can you convince one of my clients IT/ICT lot to allow SDSL into MPLS for me ;) :lol:

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LOL

:offtopic: Can you convince one of my clients IT/ICT lot to allow SDSL into MPLS for me ;) :lol:

Sure... it'll cost ya though :)

Then again, it took ages for the company I work for to get around to supporting ADSL into MPLS... without even thinking about SDSL... but that's another story!

Nevertheless, as long as you control the LNS-end of the circuit, terminating the PPP session into an MPLS VRF is fairly trivial... what reasons do they give for not allowing it ? ;)

Leland

My Life History for what it's worth...

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LOL

:offtopic: Can you convince one of my clients IT/ICT lot to allow SDSL into MPLS for me ;) :lol:

Sure... it'll cost ya though :)

Then again, it took ages for the company I work for to get around to supporting ADSL into MPLS... without even thinking about SDSL... but that's another story!

Nevertheless, as long as you control the LNS-end of the circuit, terminating the PPP session into an MPLS VRF is fairly trivial... what reasons do they give for not allowing it ? ;)

Leland

My Life History for what it's worth...

The state that it is not a package currently supported (but will be towards teh end of the year)... in my eyes it is just another way of getting in to the cloud, so I think it is the "managed" part of the contractual chain where the sticking point it, that and the ofcom rules not allowing them over the chinese wall within the company!

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