Implementing Differential Bus Priority for Timetabled Service
N B Hounsell, B P Shrestha, University of SouthamptonSouthampton, UK
Differential priority is considered as one important measure to help buses. The paper will describe various aspects of differential priority including its advantages, limitations and implementation issues.
The need for sustainable transport operations in cities is focusing more attention on the needs of buses to provide fast, frequent and reliable services. One favoured measure is bus priority at traffic signals, particularly where roadspace is limited and traffic signal density is high. As bus fleets are increasingly equipped with satellite-based Automatic Vehicle Location (AVL) systems, it is now possible to provide ?differential? priority, where different levels of priority can be awarded to buses at traffic signals according to their adherence to schedule/frequency. The objective of this type of priority method is usually to produce greater punctuality in the service (in the case of timetabled service).This type of priority gives a higher level of priority to late buses and a lower level or no priority to the buses which are early or on time.
Although differential priority can help make buses more punctual and reduce passenger waiting time, it may not always be the preferred strategy. For example, two main limitations of differential priority are (i) its inability to help buses when bunched (i.e. in groups of 2 or more) and (ii) the lower journey time savings compared to the policy of giving priority to all buses. Clearly passengers waiting for a bus gain from improved punctuality, whilst those on board benefit from reduced running times and these two objectives may require different priority strategies.
Important implementation issues needing consideration are: passenger arrival patterns; junction characteristics (e.g. degree of saturation) and bus timetable. Passenger arrival patterns need to be considered when deciding the priority strategy. For example, if passengers tend to arrive randomly at bus stops (as happens in higher frequency services even if timetabled), the priority strategy might be better based on regularity (not punctuality) to reduce waiting time. Differential priority can be good in constraining bus priority (by giving priority to late buses only) and avoiding unacceptable delays to non-priority traffic, especially at junctions with higher degrees of saturation. However, in the case of under-saturated junctions, priority can be given to all buses with little disbenefit to the non-priority traffic, perhaps allowing bus drivers to do their own time keeping. Good timetabling can also play a crucial role in differential priority; the timetable should be properly designed, regularly updated and should incorporate the benefits obtained from bus priority. Differential priority may then operate to best effect.
This paper will discuss and analyse these various aspects of differential priority with reference to case studies in London and simulation modelling, to illustrate the concepts and provide quantification of alternative strategies. Conclusions will include recommendations for implementation related to alternative performance criteria which may be adopted.
Association for European Transport