$6\mathrm{.}9$ Consider a scenario where a city is planning to construct a new bridge to connect two major neighborhoods across a river. As a civil engineer tasked with overseeing the project, outline the comprehensive steps you would take in the planning, design, construction, and maintenance phases of this bridge project. Additionally, discuss the key factors and considerations that need to be taken into account at each phase to ensure the safety, efficiency, and sustainability of the bridge structure over its intended lifespan.