What physical obstacle did many of the roads going from east to west have to overcome?

Many roads going from east to west had to overcome significant physical obstacles such as mountain ranges, particularly the Rocky Mountains in North America. These rugged terrains required extensive engineering solutions, including tunnels, bridges, and switchbacks, to facilitate travel and transport. Additionally, rivers and valleys posed challenges, necessitating the construction of crossings and embankments. Overall, the diverse geography significantly influenced the planning and construction of these roads.