Highway facilities are designed for

Answer: Option D

In Highway Engineering, annual average daily traffic (AADT) is a key parameter used for designing highway facilities.

AADT refers to the total volume of vehicle traffic passing through a specific point or segment of a highway over the course of a year, averaged over each day.

Option A: annual average hourly volume is incorrect because it focuses on the average traffic volume per hour, not per day, which is less relevant for overall highway design.

Option C: thirtieth highest hourly volume is incorrect because it refers to a specific hourly volume ranking, not the overall average traffic flow used for design purposes.

Option D: peak hourly volume of the year is incorrect because it refers to the highest hourly traffic volume observed during the year, which is not used for designing highway facilities that need to accommodate typical traffic patterns over time.

Therefore, the correct answer is annual average daily traffic.