3.1 Introduction
Meeting specific objectives is necessary for the success of any scheme. However it is equally important that scheme designers, modellers and traffic engineers consider wider strategic transport objectives.
3.2 Overarching Objectives
All design decisions must be made taking account of the requirements and objectives set out by the following:
Mayoral Policy;
The Network Management Duty as defined in the Traffic Management Act (2004);
and
The strategic and policy requirements of the local highway authority.
3.3 Interested Parties
There should be coordination and cooperation between interested parties in the design of scheme proposals.
All relevant parties should be consulted before undertaking the design of a new junction, or the re-design of an existing junction. Often a scheme sponsor will have a particular focus; however it is the responsibility of the scheme designer(s) to ensure that all junction users are considered. In addition the scheme designer should contact all relevant authorities, who have jurisdiction over the area being impacted by the scheme, to ensure that any concurrent scheme proposals are taken into consideration.
3.4 Scheme Design
The modelling of proposed schemes is assessed by TfL to ensure that it is correct and accurate. In addition to this role TfL must be satisfied that the proposed scheme design makes full consideration of the objectives outlined in section A3.2.
The existence of other proposed schemes could impact on traffic flows, junction layout and signal control. Failure to consider these impacts could result in modelling which would not be fit for purpose.
Before undertaking any design or modelling work it is strongly recommended that TD is contacted. The scheme sponsor or their consultant can discuss, with the appropriate TD team, the scope of the proposals and the area they affect.
As well as strategic objectives there are many local level considerations. The detailed considerations relating to the design of a scheme are outlined below to highlight some of the key areas for discussion.
3.4.1 Junction Layout
The layout of proposed junctions is determined by a wide range of factors. The final design must comply with the appropriate design standards and safety requirements. It must also deliver the best service for all road users that it is possible to achieve within the physical limitations of the site.
Often there will be a number of different junction layouts that comply with design standards and safety requirements. In this situation it is necessary to assess the impact of the different options on network capacity, in order to determine which layout delivers the best performance within any assessment criteria.
This assessment is critical within the design process, and accurate modelling is required in order to give confidence to any design decisions. The type of modelling software to be used depends on variables such as the size of the network being assessed and the level of congestion present within the study area.
3.4.2 Fixed Time and Adaptive Control
Fixed time signal plans are pre-calculated timings, which are usually used to operate a linked group of traffic signals. The timings are commonly derived through offline traffic modelling techniques and applied according to distinct network conditions relative to time of day and day of week. They are implemented either remotely by the Urban Traffic Control (UTC) system, or coded into the controllers using a Cableless Linking Facility (CLF).
UTC SCOOT is a dynamic, demand-responsive traffic management system which controls approximately 40% of the London traffic signal network. UTC SCOOT is an adaptive real time system which continually optimises signal timings to meet traffic demand. The modelling of UTC SCOOT-controlled signals requires a methodology outlined in section B2: Modelling Principles.
3.4.3 Traffic Signal Timing Plans
Modelling for a proposed scheme determines which signal timing plans may be implemented if a scheme was to be built. Hence it is extremely important that the plans which are developed are correct in terms of safety and functionality, and that they deliver the TfL objectives outlined in section A3.2 as much as possible.
For operational groups of traffic signals linked via UTC or CLF control, optimisation techniques (as commonly used by traffic modelling software) generally aim to set signal timings which reduce delays and stops. Care must be taken to ensure that signal timings cater for all road users and remain consistent for all junctions contained within the same operational group. Consideration should also be given to wider strategic objectives including journey time reliability, as explained in section A2.2.2.
For isolated junctions the signal timing plans can be developed, tested and optimised using modelling software for standalone junctions. Where two or more junctions operate within the same linked group, more complex software, able to optimise signal coordination, must be used. In certain circumstances, for example in over-saturated networks, micro-simulation software can be used to fine tune the optimised signal timings in an offline environment. Section A9 gives more detail on criteria which influence the selection of correct software for the modelling purpose.
3.4.4 Contingency Signal Timings
Junctions under UTC control require additional traffic signal plans to cater for unplanned incidents on the network. These contingency plans can also be used to mitigate the impact of planned road closures and traffic diversions needed for large events.
Scheme designers should be mindful of contingency issues and aware of any local requirements during the design process. Designers are therefore encouraged to contact TD NP for advice on necessary contingency measures.
3.4.5 24/7 Operation
Scheme designers and traffic modellers should ensure that any scheme design considers impact at all times and highlights any issues that may arise outside of the traditionally modelled peak periods. Consideration should be given to weekend operation, where traffic demand may be similar to that of a weekday but upon a capacity constrained network, for example through the relaxation of parking, waiting and loading restrictions.
The smooth operation of the network 24 hours a day, 7 days a week is becoming increasingly important as travel demand in London expands beyond weekday peak times.
3.4.6 Scheme Safety
Road safety is an area of key concern for TfL. Overall scheme objectives should always consider improvements to road user safety. Changes to operations of junctions can have significant influence on the safety of road users, including pedestrians, cyclists and general vehicular traffic.
Better Routes and Places Directorate (BR&P) can advise on best practice for modelling road safety factors within a traffic context. The SAFENET modelling tool was useful for networks but is no longer supported by TRL Ltd9. Old versions can, however, still be used and BR&P can discuss specific scenarios with scheme designers.
9 http://www.trlsoftware.co.uk