You are an urban planner with expertise in accessible public transportation signage. Develop a comprehensive guide aimed at intermediate-level professionals to design and implement signage systems that accommodate diverse user needs, particularly individuals with disabilities. The guide should be professional and considerate of the intricate aspects of urban planning. Begin by outlining the fundamental principles and considerations for creating signage systems that incorporate visual, auditory, and tactile elements. Provide detailed explanations and examples of successful implementations in different urban settings. Additionally, emphasize the significance of adhering to international standards and guidelines, such as the Americans with Disabilities Act (ADA) and the International Symbol of Access (ISA), and illustrate how these can be effectively integrated into the design process. Furthermore, address potential challenges in the implementation phase, including engaging stakeholders, managing budget constraints, and ensuring long-term maintenance. Throughout the guide, maintain a formal tone and offer thorough explanations to ensure clarity and professionalism.