Traffic contributes nearly 25% of global greenhouse gas emissions. For designing new traffic routes and decision-making processes, it is essential to incorporate integral life cycle assessments (LCAs) to ensure sustainable solutions and to achieve the UN Sustainable Development Goals (SDGs). This study compares two fictitious routes: a typical Austrian mountain pass road (Route A) with a 3% gradient and a new route (Route B) featuring a 1000 m tunnel, reducing distance and inclines. The LCA analyzes Route B’s lifecycle, from material supply to 100-year tunnel usage, comparing it against a traffic LCA of Route A’s operational emissions. The tunnel assessment considers the New Austrian Tunneling Method, local materials, and typical geology. Traffic effects are analyzed using Austrian vehicle stock data, following EN 17472 and EN 15804 standards. The results, based on Global Warming Potential, indicate that Route B’s construction, maintenance, and utilization generate lower environmental impacts than Route A’s traffic emissions. The tunnel offers overall environmental savings, with its construction and maintenance impacts offset within approximately 10 years. Traffic usage is identified as the primary long-term emission source. This research highlights the significance of integral LCAs in creating a sustainable built environment and supporting a decision-making process in transport infrastructure construction.