Innovative Modelling and Design of Integrated Light Rail Transit Priority Systems ? Case Study of LRT in Nottingham, UK
S Ahuja, F Dreher, T van Vuren, J Smith, Mott MacDonald, UK
Experiences from iterative processes of review & design of Nottingham LRT scheme are presented which includes fulfilling multiple and conflicting objectives of road users which are addressed by intelligent, adaptive traffic signals.
The design of integrated transport systems is a muti-dimensional problem where users often compete for resources and priorities. For modern light rapid transit systems the interaction of trams with various users for example, pedestrians, buses and emergency vehicles leads to a multiple objective problem where there can be multiple optimal solutions for designing an efficient system. An effective and successful LRT system is one where public transport gets (best possible) priority while satisfying the objectives of other road users as well.
We present in this paper our experiences from the iterative processes of review and design needed to fulfil the multiple objectives of all road users affected by the Nottingham LRT extension scheme, using a microsimulation model. Intelligent, adaptive and demand dependent signalling systems were proposed to provide the primary objective of provision of public transport priority where the tram was interacting with other traffic and pedestrians.
An innovative traffic signal control algorithm is proposed in this paper that includes the process of mutual learning where the traffic signals aim to avoided congestion problem and ?blocking back? not only to themselves but also to the adjoining intersections, while providing public transport and pedestrian priority. In addition the signals can adapt to rapidly changing traffic conditions. The signal control algorithms have been designed in a flexible way so that they can adapt to proposed changes in surrounding land use without major changes to the core algorithms. The design methodology included initial signal design and appraisal using TRANSYT and LINSIG programs and then designing an adaptive control algorithm based on results of the conventional studies.
The developed VISSIM micro-simulation model simulates 1.5 kilometres of an on street tram corridor of with 4 proposed LRT stops. Different levels public transport priority for trams and buses are provided at 6 highly congested intersections and 4 exclusive pedestrian crossings. During the design process different policy interventions were tested which included varying levels of traffic growth, the impact of future land-use developments including a large commercial supermarket, testing different layouts of infrastructure, design of a Bus-LRT interchange, evaluating the effect of various traffic management and restraint measures on the tram corridor.
The results of the study indicated that using the adaptive signal control algorithms and the innovative infrastructure design options that came out of the simulation study, the travel times for cars would reduce by 19% in the AM peak hour and by 67% in the PM peak hour compared to conventional signal control strategy. Similarly for buses there was a reduction of travel time by 5% in the AM peak hour and 59% in the PM peak hour.
The tram on the other hand had had more critical target travel times which meant that in order to provide a reliable time table service, the tram the journey times in the corridor must be less than 8 minutes. The simulation results showed that with minimal infrastructure and conventional traffic signals, the tram run time on the corridor would be about 14 minutes in the AM and 14 minutes and 30 seconds in the PM peak hour. However, using the proposed adaptive priority signalling system and enhanced road layout the tram journey times would reduce by 51% (to 6 minutes and 47 seconds) in the AM peak and 53% (to 6 minutes and 46 seconds) in the PM peak hour respectively.
The study concludes that without the flexible microsimulation model it would have been impossible to design an effective integrated light rail transit priority system. The highly visual VISSIM model plays a key role in the planning process. The adaptive signal control algorithm is an innovative feature of the system that provides public transport priority while trading off between conflicting objectives of various road users. These include reducing tram journey times while increasing the level of pedestrian safety and simultaneously maintaining high levels of throughput for other vehicles. The high transparency of the microsimulation model makes the complicated LRT design process easier to appraise and understand so that ineffective solutions can be eliminated. It has helped in fighting litigation and objections to the scheme, effectively saving time and money. It provides greater confidence to the client, the designer and public in the scheme and helps develop a solution that WILL WORK!
Association for European Transport