Exploring new bus priority strategies at isolated Vehicle Actuated junctions

Abstract

Bus priority in various forms has become an important application in towns and cities around the world, as Local Authorities seek to improve the efficiency and sustainability of their transport systems by promoting the use of public transport with its high passenger carrying capability. Segregating buses from general traffic, using busways, bus lanes, etc is increasingly being supplemented with priority at traffic signals, where signal timings respond to the approach of a bus to give it priority signalling through the junction. This was first trialled in UK on a significant scale in the 1980’s in an area of south east London containing isolated traffic signals operating under the UK’s ‘D-system’ of Vehicle Actuation (VA). There followed a period of some 20 years where equivalent priority strategies were developed, tested and implemented in the more difficult environment of Urban Traffic Control. These strategies have kept pace with the significant advances in technologies over this period, such as in detection, communications, processing and optimisation. However, this has not been the case with bus priority at isolated VA junctions, where strategies developed in London some 30 years ago still prevail – and even in large cities a significant number of signal controlled junctions operate in this way. This then suggested a research gap which has been taken up in this research – the exploration and development of new strategies for bus priority at isolated VA junctions.

Taking existing strategies as the ‘base case’, their effectiveness was first explored through theoretical and mathematical analysis. This led to the first new output from this research – more comprehensive predictive equations for bus priority benefits (delay savings) than existed, covering a range of operational conditions. The limitations of the mathematical approach were identified, so research then progressed to the development of microscopic simulation modelling (VISSIM) for junction modelling and for exploring new strategies. This involved four variants of junction design and scenarios reflecting differences in levels of congestion, bus flows, signal timings, etc. New and improved strategies were then developed through modelling, including (i) re-optimising parameter values for the existing priority methods, (ii) improved bus detector locations taking advantage of new Automatic Vehicle Location technologies and (iii) new strategies for bus priority, including a ‘stronger’ strategy termed ‘always green bus’ and ‘differential priority’, where the level of priority given to a bus depended on its performance (eg regularity) at that time. Strategies were also developed to minimise the impacts on general traffic through various forms of compensation, with total person delay then being used as an evaluation criterion.

The research has concluded with a series of recommendations for improved implementation of bus priority at isolated VA controlled junctions, taking advantage of the new technologies which are widely available and used within most bus fleets in the UK.

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