Paul Blakeman, consultant, Managed Motorways Optimisation, TRL, and Jill Weekley, Transport consultant, TRL, discusses the problem of congestion and how to best deal with it
Two years ago the transport community commemorated half a century of British motorways with 50 years having passed since the opening of the Preston bypass in 1958. Last year a less heralded but perhaps even more notable milestone was reached – the 50 year anniversary of the first motorway traffic jam in Britain, which occurred on a bank holiday in early 1959 as people flocked to try out the newly built road.
This event marked the arrival of a now all-too-familiar problem and the beginning of the ongoing process of how to manage motorway congestion. Providing more capacity through road building and widening has been the traditional response, but from as early on as the 1960s, using technology for motorway management has been considered a supporting weapon. With the Strategic Road Network virtually complete and widening projects few and far between, technology has now taken centre stage in the struggle to keep motorway traffic moving.
For as long as technology has been considered part of the solution, Transport Research Laboratory (TRL) has been evaluating and researching the impact of the technology on motorway traffic, and advising on both operation and design. Work has ranged from the development of the original algorithms, and monitoring and evaluation of the M25 Controlled Motorways Pilot, to driving simulator trials of developing designs and the development of a Basic Controlled Motorway generic design and business case.
Technology began to be used early, primarily as a safety measure; in 1964 the first variable signs were trialled on the M5 in Worcestershire and a year later a buried loop detector system and CCTV system were installed on the M4 between Heston and Chiswick to give an alarm to the police in the event of unexpected gaps in the traffic.
In 1966, the first computer controlled signalling systems were installed on the M4 at Heston and the M18 in South Yorkshire – some of which are still in use today. As an interim measure before matrix signals could be installed, Motorwarn was used, which consisted of flashing amber lanterns advising 30mph.
The 1970s saw the arrival on the motorway network of overhead gantries supporting manually set signals for individual lanes, and in 1986 the first Ramp Metering was launched on M6 J10 to improve traffic flow and safety by controlling the flow of vehicles joining the main carriageway. In 1988 a pilot of Automatic Incident Detection was carried out on the M1 J9-19, to reduce the response time for setting signals, which previously relied on a phone call from the police. The system detects slow moving traffic and set 40mph and 50mph central reservation signals to protect the back of a queue. This system quickly evolved into the MIDAS (Motorway Incident Detection and Automatic Signalling) system, which by the early 1990s became commonplace on busy stretches of the network.
Over the past 20 years, UK traffic levels have almost doubled and, as a result, congestion has become a serious problem for UK motorists and the national economy. The causes of this congestion vary. It is estimated that 34 per cent of congestion on the network is caused by incidents and road works; the remaining 66 per cent of motorway congestion is therefore caused by pure traffic density, i.e. the demand of the road at a given time is exceeding its capacity, where capacity, in this sense, is the number of vehicles that can safely and smoothly traverse the section of road in a given time frame.
Peak time queuing and slow moving traffic is a regular occurrence at key points of the network, none more so than the M25. The M25 circles London and is one of the busiest motorways in Europe carrying in excess of 200,000 vehicles per day. Congestion regularly occurs at several points, delaying thousands of motorists every day.
In the early 1990s plans for extensive widening of the M25 were discussed to deal with this problem. However, there was growing concern over the significant cost and environmental impact of such schemes; additionally advances in technology were offering potential solutions to the congestion problem. As a result, senior Department of Transport officials made a study visit to the Netherlands and Germany to see the variable speed limit applications in use in Utrecht and Frankfurt, leading to approval of the M25 Controlled Motorway trial.
Controlled Motorways (CM) uses automatic detection, mandatory variable speed limits and variable message signs to reduce the risk and minimise the effects of flow breakdown on busy stretches of motorway. A CM Pilot was installed between Junctions 10 and 15 of the M25 in August 1995 by the newly-formed Highways Agency. Seven years later it was extended to Junction 16. For the first time, signs and signals were used not just for safety but to manage congestion. Also this marked the first use of ‘Red rings’ on electronic message signs signifying mandatory speed limits.
TRL undertook an intensive six-month period of tuning, where the operating parameters were modified on a fortnightly basis until the signals being displayed were more consistent, yet still appropriate to the traffic conditions. In April 1996, an optimal set of control parameters was determined. Because traffic conditions change over the years, tuning continues to be performed to improve the operation of the system and the signal settings are regularly monitored to identify any inappropriate settings. The underlying algorithms have also been upgraded over the years to improve the operation of the system. These improvements have included modifications to stop signals switching off in queues, to add a queue protection facility, and to display supporting VMS messages.
The CM pilot’s objectives were to demonstrate improved driver behaviour, speed control, lane discipline, lane utilisation, safety, driver comfort, journey time reliability and environment. If successful it was believed that by controlling speeds and lane use from overhead signals, future use could also assist maintenance operation, incident management and hard shoulder utilisation, i.e. active
Active Traffic Management
A decade later, in December 2005, Active Traffic Management was introduced on the M42 J3a-7 near Birmingham. Following an assessment of the possible operational regimes to be used within this pilot, TRL recommended those that would be most effective in tackling the congestion problems on this stretch of road. For the first nine months, the system operated as 3-Lane Variable Speed Limits (3LVSL), equivalent to the Controlled Motorways system on the M25. In August 2006, Hard Shoulder Running (HSR) was introduced making use of the hard shoulder as a running lane during peak hours. The two systems were designed to work with each other, managing the traffic in stages and switching between 3-lane and 4-lane CM operation. Ramp Metering was also introduced at sites throughout the country to manage the flow of traffic joining the motorway.
The M42 ATM pilot successfully demonstrated the feasibility of hard shoulder utilisation as well as confirming the other benefits from controlled motorways.
The evolution of the Highways Agency from being builders to operators of the Strategic Road Network took another step forward in March 2008 when Ruth Kelly, the then Secretary of State for Transport, publicly launched the Managed Motorways initiative. The success of the pilot schemes, particularly the ATM pilot, and the ever-increasing need to tackle congestion brought this initiative to life. Along with it came announcements and plans for the introduction of similar schemes to be rolled-out across
Managed Motorways is the catch-all term for the toolkit of technologies and operational regimes that the Highways Agency is rolling out across the network to tackle congestion by making best use of the existing roadspace – a package of improvement schemes which could deliver over 520 additional lane miles to the national strategic road network, of which over 340 lane miles are HSR.
Managed Motorways delivery is constantly evolving and improving. With changing economic and political agendas, new uses of technology to manage the network are continuously being considered. In the current economic climate, it is perhaps unsurprising that the question arises – can the same benefits (if not more benefits) be achieved with less infrastructure, and therefore at a reduced cost? In response to this, the second generation of the Managed Motorways concept is being developed. The key requirement for the Highways Agency is to ensure that, with any reduction in signs and signals etc, the current level of safety provision for road users is maintained or improved.
The Active Traffic Management pilot has shown that Hard Shoulder Running is safe and effective for several hours each day – clearly the next question becomes, can the hard shoulder therefore be used 24 hours a day? The challenges facing the future remain. Work needs to be carried out to make sure any changes will be safe, operable and effective, and to support effective and evidence-based decision making – thus shaping a future of Managed Motorways that will bring maximum benefits to both the Highways Agency and the road-using public.
At certain points in the network, and certain times of day, there is more traffic than the capacity provided by existing road space can handle. The four broad approaches to reducing this congestion on the network are: reduce demand, spread demand (temporally), increase or make better use of the existing road space, and improve driver behaviour to increase working capacity.
A Hard Shoulder Running scheme adding an additional lane obviously raises the capacity of the link and reduces the likelihood that demand will exceed capacity; ramp metering controls the amount of traffic that joins the motorway as demand approaches capacity. Variable speed limits, however, neither increase road space nor reduce demand; instead the aim of Controlled Motorways (CM) is the subtler approach of increasing capacity through changing driver behaviour.
CM sets signals for one of two reasons: incidents and congestion. Incident settings are used when slow moving traffic is detected, to protect the back of the queue by slowing the speed of approaching vehicles. These settings are largely a safety measure, dealing with queues that have already formed. Congestion settings, however, attempt to deal with congestion before capacity is reached as well as helping regain smooth traffic flow as demand drops.
The main aim is to reduce the traffic speed just before traffic flow reaches the critical level where flow breakdown is likely to occur. The controlling algorithm relies on the principle that the capacity of a section of motorway is not merely dependent on physical factors such as the incline of the road or the number of lanes, but also on how the drivers themselves use the road space at the particular location and point in time.
That capacity is affected by driver behaviour has been seen in TRL’s work in calculating site specific flow thresholds for every scheme that is active or in the design phase. Two sections of road with exactly the same vehicle mix, geography, at the same time, on the same day can require quite different thresholds because of what the vehicles are doing. When vehicles are changing lane they are in effect taking up two lanes and reducing the capacity of that point. Lane changing also causes sharp braking and creates uneven headways. When there is a consistent stream of vehicles, as there may be when traffic flow levels approach capacity, then any sharp braking ripples back upstream in “shockwaves”.
The principle is that it is not simply the reduction of speed that increases capacity, but the changes to driver behaviour that result from this. The accompanying message sign for CM congestion settings reads “Congestion Stay In Lane” – it is this advice, suggested explicitly in the message sign, which is implied and encouraged in the reduced speed limit. By lowering the speed limit, the proportion of vehicles that can drive near it is increased and hence vehicles will drive at a more uniform speed and the need for lane changing will be reduced. Of course, lane changing will always be required for those leaving or joining the main carriageway, but this approach aims to reduce lane changing for overtaking.
The Transport Research Foundation
Originally established in 1933 as part of the UK Government, TRL privatised in 1996 to become a fully independent private company. TRL is wholly owned by the Transport Research Foundation (TRF), a non-profit-distributing foundation with no shareholders, enabling profits made by TRL to be passed to TRF and re-invested in scientific research. TRF is comprised of over 80 sector members from the transport industry, ensuring TRL continues to undertake the high-quality research it’s renowned for.
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