M25 Managed Motorways J16 - Highways Agency

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4.2 Option 1 (Preferred): Controlled MotorwayThe existing MIDAS system described above is the simplest application of motorwaycontrol technology. It is solely a safety feature designed to protect queues by providing awarning of their presence to upstream traffic. The next level of control is a system calledControlled Motorway (CM). This system includes MIDAS to protect against queues, butalso uses Variable Mandatory Speed Limits (VMSL) to assist in preventing thedevelopment of queues. Controlled Motorway is sometimes implemented on existingcarriageways as a standalone measure to improve journey time reliability. Alternatively,if the level of congestion is high enough to warrant it, CM can be introduced inconjunction with measures to increase the capacity of the carriageway. In the case ofthe M25 J16-23, traffic flow levels are such that there is substantial traffic congestionand an increase in traffic capacity is required.The two alternative means of increasing traffic capacity are widening of the carriageway,or introduction of the next and highest level of motorway control technology known asthe Managed Motorway (MM) system. Both alternatives include MIDAS and CMtechnology, the essential difference being that MM relies on temporary use of the hardshoulder rather than physical enlargement to provide additional traffic capacity at busytimes.In the case of the M25 J16-23, the preferred option is widening rather than MM. Thewidening element of the scheme has in fact been largely completed and the proposedintroduction of CM technology is required to complete the scheme. The addition of CM isintended to secure a greater degree of improvement to journey time reliability thanwould be obtained from the widening alone.The operation of the MIDAS component of CM is described above in paragraph 4.1. LikeMIDAS, CM uses the same carriageway loops to detect vehicles and also sets speedlimits on variable message signs. The difference is that CM also sets speed limits athigher speeds when information on traffic density from the loops indicates that‘bunching’ may be occurring. It does not therefore wait until a queue develops. Instead,CM sets variable mandatory speed limits of 60mph and 50mph to reduce bunching andthereby reduce the likelihood of a queue occurring. However, if traffic still becomes slowmoving or stationary then, like MIDAS, it will set a 40mph limit. The only difference inthese circumstances is that the 40mph limit is a mandatory limit rather than the advisorylimit used by MIDAS.In more detail, the CM system uses VMSL to slow down upstream traffic. This reducesthe likelihood of it ‘catching up’ with a pocket of slower moving traffic and causing trafficdensity to reach a level at which flow breakdown occurs. Whilst the reduction in speedlimit increases journey times upstream of the high density region, these are cancelledout by journey time savings arising from a reduced incidence of flow breakdown andassociated queuing. Indeed, the evaluation of existing CM operation elsewhere on theM25 has shown that the net effect on average journey times is neutral, but that therange or variation in journey times is reduced, thereby improving reliability. This ismeasured in the assessment process by predicting changes in the standard deviation ofjourney times of trips using the Controlled Motorway as part of their route.24
It should be noted that CM is already installed on the adjacent section of the M25between Junctions 10 and 16. There are also proposals for its introduction betweenJunctions 7 and 10, thereby providing continuous coverage from Junction 7 to Junction23 should all the proposals be implemented.A secondary benefit of CM is a reduction in accidents and the associated queues,thereby reducing queuing delays and further improving reliability. The reduction inaccidents which has been observed in conjuction with CM is believed to be the result ofimposing lower mandatory speed limits and requiring drivers to stay in lane.In order for CM to be successful, it is essential that the variable speed limits which formpart of the system are complied with. This requires the speed limits to be mandatory andsecondary legislation is required to allow mandatory variable speed limits to operate.It should be noted that the mandatory speed limit signs used as part of CM are matrixsigns which can display either 40, 50, 60 or the national speed limit sign. Being amandatory sign, they are required to have a red outer ring in order to comply with thetraffic signs regulations. They are also required to be displayed over each lane. Advisorysigns used for MIDAS are also matrix signs, but do not have the red ring, nor is it arequirement to display them over every lane (though HA standards require this forcarriageways of four or more lanes, making gantries a necessity).Enforcement of the VMSL is planned to be carried out using a combination of gantrymountedspeed enforcement cameras and traditional enforcement by the Police. TheHighways Agency Digital Enforcement Camera System (HADECS) will be used toautomatically monitor compliance with VMSL.25
Page 1: M25 Controlled Motorway January 201
Page 4 and 5: Getting to thisstage:In November 20
Page 6 and 7: Executive SummaryThis consultation
Page 8 and 9: Annex AConsultation Paper - Control
Page 14: Annex AConsultation Paper - Control
Page 21 and 22: Title:M25 J16-23 Controlled Motorwa
Page 23 and 24: Evidence Base17
Page 25 and 26: 2. Rationale for InterventionThe Hi
Page 28 and 29: 4. Description of Options22
Page 32 and 33: 5. Details of Costs and Benefits26
Page 34 and 35: 5.2 Option 1 (Preferred): Controlle
Page 36 and 37: allows more accurate quantification
Page 38 and 39: RECURRING: A reduction in incident
Page 40 and 41: 6. Rationale and Evidence for Propo
Page 42 and 43: 8. Direct Costs and Benefits to Bus
Page 44 and 45: 9.1 Justice SystemIn Controlled Mot
Page 46 and 47: 10. Recommendation, Implementation
Page 48 and 49: 10.2 Implementation PlanThe optimum
Page 51 and 52: Annex 1: Post Implementation Review
Page 53 and 54: Member of the publicOther (please d
Page 55 and 56: Company Name Recipient Address Emai

M25 Managed Motorways J16 - Highways Agency

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