A new approach to appraisal for ITS David Carrignon, Colin Buchanan
Today, most urban traffic management schemes do not include an assessment of the impact of ITS. Dynamic network management systems are commonplace in the UK, but their operational assessment is not frequent. In many cases, the
benefits of such systems are obvious but are those benefits always present? What happens when the infrastructure moves on? Are all road users equally taken into account? This article focuses on the need to assess the impact of
ITS BENEFITS: A LOCAL ISSUE When looking at the international literature on the benefits of ITS, little is said about how differently ITS for traffic management has developed between countries. For example, dynamic network management systems are highly developed in the UK, but it does not have to be the only method of tackling urban traffic congestion. Not many countries in the world have developed similar systems and they are not that widely used. The Paris region for example, developed an extensive network of urban motorways and its traffic management focuses on travellers’ information (radio station announcements and Variable Message Signs), limited ramp metering, motorway toll systems, accident identification and information exchange between the state and local highway authorities (public and private). Greater London on the other hand has a less developed motorway system and a single organisation in charge of traffic signals. Its infrastructure focuses on dynamic network management systems, management of the impact of accidents and bus priority systems. These two cities have approximately 10 million inhabitants each, are both located in Western Europe, followed historically very similar economic and technological changes, but traffic management using ITS has developed following very different paths. These differences could be explained by looking at the road network structure and institutional administrative structure, but it shows that ITS infrastructure could be and indeed has been tailored to suit local circumstances. If sister cities like Paris and London develop such different traffic management strategies, it is fair to conclude that an appraisal method should be focused on local standards agreed between stakeholders on a local basis.
traffic management tools, during the initial scheme appraisal, but also post-implementation. The modelling analysis methods to appraise these systems exist, so why not build an appraisal method and use them?
sponse to this situation and most modes of transport are now benefitting from a more balanced approach. However, the impact of dynamic network management systems and other UTC tools often relies on the assumption that they will improve traffic conditions. The definition of traffic has changed recently however and it now includes cyclists, pedestrians and so on. If there are no clear level of service definitions for each road user group, it becomes almost impossible to analyse how much better or worse a proposed scheme will be for each of the stakeholders.
WE ARE NOT EQUAL IN THE DESIGN PROCESS Road users are not equal in the initial stage of traffic signal timing calculations. Most design tools operate in PCU’s (Passenger Car Units) and do not differentiate very well between the various users. Pedestrian crossing time analyses through complex junctions start to be available, but in most cases, they are not used as a timing calculation input. We still rely on a set of base timings calculated for the best progression of general traffic through a net-
Cyclists, taxis and delivery vehicles and onstreet parking activity are frequently not taken into account at the design stage of the signal timings.
THE NEED FOR AGREED SERVICE LEVELS One observation made from working on traffic management schemes is that stakeholders only have a voice if there is an organisation to defend their interests. This organisation could be part of the public sector, such as bus coordinators, or outside it, such as a local residents association. Without representation, it is difficult to be taken into consideration in a way which can significantly influence a scheme. The Traffic Management Act 2004 is a rewww.tecmagazine.com
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work. From this base, the timings are often manually amended to suit other vehicle requirements such as buses, or pedestrians. Cyclists, taxis and delivery vehicles and on-street parking activity are frequently not taken into account at the design stage of the signal timings. In most cases, the analysis for other stakeholders is done without clear guidelines and only once the scheme has been setup limited to key stakeholders such as traffic signal operation organisations and bus organisations. If a stakeholder is not consulted in the initial stage of the process, it is very unlikely that it will be taken into consideration beyond statutory safety requirements. The ITS for traffic management tools installed on site however, are increasingly more flexible and complex than the modelling used at the initial scheme appraisal. These tools are mostly used to reduce the overall queues and delays in the network, and assuming a benefit was probably a fair assumption. Nowadays gating strategies and vehicles priority tools are affecting the balance between modes of transport on the road network. The level of priority and gating is not detailed or discussed in advance and is not included as part of most appraisals. At best, proxy settings are used. One could argue that the increase in performance and complexity of the available traffic management tools should trigger the development of minimum agreed level of service per mode of transport. These levels of service should ideally be monitored and audited at a regular interval to ensure consistency with the stakeholders’ consultation which took place at the design stage.
AUTHOR’S DETAILS David Carrignon is Principal traffic engineer at Colin Buchanan and can be contacted by e-mail at [email protected]
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If the junction has a road safety function beyond traffic management concerns, such as providing a formal pedestrian crossing, nothing prevents this facility from being in operation only when needed. In this situation, a service level definition would make it possible to identify that some users might be experiencing a disbenefit from ITS without anybody benefiting from it.
MORE ORGANISATIONS IN THE FUTURE This shift to a service level of ITS for traffic management operation should take place before too many organisations take an active role in urban traffic management. Today, existing users are not treated equally when looking at the network operation. Bus operators/coordinators collect data, express level of service concerns and identify targets while private motorists rely on the network not being gridlocked. Their interest is taken into account at the initial assessment of the scheme, but post-implementation the traffic management systems setup are likely to change through the standard maintenance process. Cyclists for example, do not collect data which could lead to an assessment of the service level at any stage of the design and operation process of the infrastructure. Other transport modes such as pedestrian or powered twowheelers are under-represented in the scheme appraisal process too. These network users should ideally specify service level The definition of the service level would need to be a result of negotiations between organisations on a local basis.
AN INFRASTRUCTURE FOR A PURPOSE
THE TOOLS ARE THERE
Historically, the traffic signal standards focused on implementation and infrastructure upgrade. Most traffic signals have been implemented to address road safety concerns (eg pedestrian crossings), improve network resilience and limit traffic delays (eg UTC, SCOOT, MOVA), and service level requirements (eg bus priorities). There is no real systematic record of the reason leading to the equipment of a junction with traffic signals and therefore no real ability to assess whether this infrastructure is still fulfilling its initial purpose. With the expansion of the infrastructure over time, it is possible that the purpose of this particular equipment has either changed or disappeared. For example, heavy traffic in the 1980s could have required a signalised pedestrian crossing, but since then the stream of traffic has been either suppressed (through a road user charging scheme) or redirected onto a bypass. In this example, the initial purpose of the crossing is no more, but there are no records of this purpose and consequently no de-commissioning process. The existence of an agreed service level in this situation would have lead to a purpose definition as well as a mechanism through which organisations representing stakeholders could initiate a review process.
Assessing the service level on a live urban traffic management system is currently difficult. The system is operating in real time and it is not possible to change the system operating parameters to answer ‘What if?’. If the service level for a particular user is not met, it is not easy to identify the source of the non-compliance. Is the problem caused by an ITS fault? Is it the result of seasonal traffic variation? Or perhaps of a network disruption which occurred somewhere else on the network? Traffic microsimulation tools are now able to model the impact of dynamic network management systems and it would be possible to start to perform such analysis. Most microsimulation software packages have a SCOOT or MOVA interface and Transport for London has developed, with the support of TRL, an offline UTC - VISSIM tool which operates on laptops. The range of transport modes being modelled with a calibrated behaviour model is increasing and most transport modes can now be incorporated accurately. There is little stopping the development of an appraisal framework for all transport modes which includes ITS traffic management systems. The tools are there, so why not use them?
THE IMPACT PER TIME OF THE DAY
Similar to the previous example, a standard engineering design process would take the worst case scenario as a reference point; a traffic signal would be implemented if it is required at any time of the day. If a junction is fitted with traffic signals for network resilience purpose for example, this may not be required at night. Recent studies indicate that it would be economically beneficial to turn such junctions to a flashing amber type arrangement for some part of the night, returning the junction to a state close to its original mode of operation.
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