What is the maximum grade for a design speed of 70 mph recommended for road safety?

For a design speed of 70 mph, the recommended maximum grade is 5%. This guideline is established to ensure safety and operational efficiency on roadways.

When dealing with higher design speeds, such as 70 mph, steeper grades can pose challenges to vehicle performance, particularly for heavy vehicles. A grade that is too steep can increase stopping distances and reduce the ability of some vehicles to maintain speed, thereby affecting the overall flow of traffic. Additionally, steeper grades can create safety hazards such as increased likelihood of skidding, loss of control, and driver fatigue.

The 5% grade is a compromise that allows for sufficient slope while maintaining a balance between safety and roadway efficiency. This standard is supported by design criteria outlined in various transportation engineering guidelines, which emphasize the importance of maintaining safe conditions for all vehicle types, including passenger cars and heavy trucks.

Other choices, such as 3%, while possibly safe for lower speeds, do not account for the needs and dynamics associated with higher speed facilities like those designed for 70 mph. Meanwhile, grades of 7% or 10% exceed the recommended standards, likely leading to increased risks and safety concerns, particularly in adverse weather conditions or involving heavier vehicles. This makes 5% the most appropriate