This thesis addresses the challenge of developing and implementing sustainable transport policies to mitigate negative environmental impacts caused by rapid growth of urbanised city areas within Southeast Asia. The recent trend towards urbanisation in Southeast Asia has seen many cities grow at a phenomenal rate as people have moved away from rural areas in search of employment. Resulting pressure on city transport systems has increased traffic congestion, air pollution, and the number of traffic accidents, with correspondingly detrimental effects on the quality of life. Acknowledging the challenging nature of attaining a sustainable transport system, this research investigates the potential of the motorcycle and the shared-taxi to improve sustainability within an urban transport system. Chiang Mai was chosen as the case study; it is the biggest city in Northern Thailand and, in recent years, has experienced rapid population growth, which has resulted in worsening transport problems. Motorcycles currently outnumber cars and are the preferred means of transport for local city residents. In addition, a form of shared-taxi service, which uses converted pick-up trucks, has operated within the city and the surrounding locality for over 50 years. The study begins with (a) a review of relevant past literature on these two modes of transport, (b) an analysis of previous studies of the city's transport system, and (c) a comparison of MARS (Metropolitan Activity Relocation Simulator) with five other sustainable transport planning models. The comparison indicated MARS was suitable for analysing the city's transport system; it operates at an urban level and has a track record of transferability for use in cities across different continents. Modifications to MARS were made to reflect the dominance of motorcycle transport and further changes accommodated analysis of the shared-taxi. The result is the Chiang Mai Metropolitan Activity Relocation Simulator (CNX-MARS). The System Dynamics (SD) model in CNX-MARS and Systems Thinking are the primary analytical tools used in this study. By tracing the development timeline of the city's transport system, the current context of its inherent problems was established. The timeline demonstrates the speed at which Chiang Mai has transformed from a "walking" city to a highly motorized city, plagued by traffic congestion and near-gridlock conditions in some central areas during peak travel times. A survey of 2,319 households was carried out to gather empirical data about the mobility behaviour of the city's residents. This data is used, in combination with data from other secondary sources, to uncover the underlying trends and attributes of the city's transport system. For example, an in-depth trip distribution analysis of the primary data revealed the dominance of motorcycle usage whereas shared-taxis are used for only 1% of all trips made by local residents. The complexity of the transport system and the difficulties faced in formulating suitable measures to resolve the issues were examined next, using a tailored Causal Loop Diagram (CLD) to simulate the interactions between different entities, such as the effect on the environment of preferred extensive use of privately owned vehicles when compared with public transport. This qualitative model is also used to review the effects of solutions offered by local decision makers responsible for Chiang Mai's transport policy and its implementation. The assessment shows a wide gap between the perceived best fit solutions preferred by decision makers and the forecast results of these measures when implemented within the transport system. The solutions suggested by the decision makers are combined with other proposals to form 16 alternative transport policies for the city. An assessment framework is created to evaluate these policies quantitatively. The framework is based on the policy makers' vision for the sustainability of the city and the results accessed from CNX-MARS. The qualitative and quantitative policy assessments reveal a number of interesting points. They show that the promotion of motorcycle ownership and usage will yield localised enhancements, but worsen the overall condition of the system in the long term. However, if a form of parking management is implemented in tandem with this measure, such as restricting on-road car parking and increasing motorcycle parking space, an overall improvement of the system can be achieved. The assessment also shows that improvement to the shared-taxi service, without changing its organisational structure, brings limited benefit. Moreover, the assessments revealed that despite the high commitment to solve the city's transport problems, the decision makers are likely to deploy measures that improve one aspect of transportation but detrimentally affect the system as a whole. This finding points toward the need for a transport planning tool that aids decision makers' understanding of the complex interaction between different entities within the transport system. In summary, the results of this research contribute towards solving the persistent transport problems of Chiang Mai by: 1. Demonstrating the application of System Dynamics in CNX-MARS, assessing its transferability in the context of Asian cities and making incremental improvements to the functionality of the model 2. Showing the importance of understanding the context of the transport system and how its component parts interact. 3. Collecting empirical mobility data from 2,319 households, which, for the first time in transport-related research studies about Chiang Mai, included walking time to and from embarkation/disembarkation points. Understanding the effect of walking time on trip mode choice provides city transport planners with additional information to be considered when formulating sustainable transport policy. 4. Analysing the attributes of motorcycle and shared-taxi trips within the context of the transport system. Contrary to expectations, the analysis reveals that promotion of the motorcycle provides only short-term benefits and, despite its high visibility in the city, the shared taxi is currently the least used transport mode by local residents. The study also shows that without radically changing its organisational structure, suggested improvements to the shared taxi service will bring limited benefits to the system. 5. Providing a suite of qualitative and quantitative tools, including the creation of an assessment framework, to support future city transport decisions. With modifications, these tools are transferable for use in other Southeast Asian cities which have similar characteristics to Chiang Mai's transport system. Although a definitive solution to the city's transport problems has not been reached, the study provides a robust and transferable methodology to test and analyse the sustainability of proposed transport policies. The study also demonstrates that it is possible for the city's transport system to improve its sustainability by significantly lowering its fuel consumption and CO2 emissions. This can only be achieved, however, by resolutely shifting gear; there needs to be significant change at the strategic level of formulating transport policy and at the personal level of persuading residents to move away from their dependence on privately owned vehicles. The linchpin is strong government and political commitment to implement measures that may initially prove unpopular, such as road closures and parking management, but which will eventually improve the overall condition of the transport system. The methodologies used and the results obtained within this study are evidence that their adoption can contribute to changing the current status quo to bring about much needed improvements to the system.