What is the assumed deceleration rate according to the AASHTO Green Book?

• A. 15 ft/s^2
• B. 11.2 ft/s^2
• C. 9.8 ft/s^2
• D. 7.5 ft/s^2

Answer: (B)

The assumed deceleration rate according to the AASHTO Green Book is 11.2 ft/s². This value is particularly relevant for highway design, as it represents the rate at which vehicles are expected to reduce their speed during an emergency braking situation.

This rate considers typical vehicle capabilities and conditions, allowing for the formation of proper sight distances, stopping distances, and decision-making distances in design standards. By employing this deceleration rate, engineers can ensure that roadways are designed to accommodate safe stopping in various conditions. The other values listed, while relevant in different contexts (for example, gravitational acceleration or different roadway scenarios), do not reflect the accepted standard used in highway design as indicated in the AASHTO guidelines.