Highway Flashcards

What is the standard superelevation rate limit recommended by AASHTO for highways in areas with frequent ice or snow?

Answer: 0.06

AASHTO limits maximum superelevation to 0.06 (6%) in regions prone to ice and snow to prevent vehicles from sliding on slippery pavement.

The California Bearing Ratio (CBR) test is used primarily to evaluate which pavement design parameter?

Answer: Subgrade soil strength

CBR measures the bearing strength of subgrade soil relative to a standard crushed stone, directly informing pavement layer thickness design.

Which traffic flow condition exists when density reaches its maximum value (jam density) and speed approaches zero?

Answer: Forced flow

Forced (breakdown) flow occurs when density is at jam density, vehicles are nearly stopped, and the roadway is congested beyond capacity.

A highway curve has a radius of 500 ft and a design speed of 60 mph. What is the approximate centrifugal ratio (v²/gR)?

Answer: 0.47

Converting 60 mph to 88 ft/s, v²/gR = 88²/(32.2×500) = 7744/16100 ≈ 0.48, closest to 0.47.

In the AASHTO flexible pavement design method, what does the Structural Number (SN) represent?

Answer: Weighted structural capacity of all pavement layers

SN is the sum of each layer's thickness multiplied by its layer coefficient and drainage coefficient, representing the combined structural capacity.

What is the primary purpose of a transition (spiral) curve between a tangent and a circular curve?

Answer: Provide gradual introduction of centrifugal force

Spiral curves allow superelevation and curvature to change gradually, preventing abrupt lateral forces on vehicles moving from straight to curved alignment.

Which pavement distress type is characterized by longitudinal cracking in the wheel path caused by repeated traffic loading?

Answer: Alligator (fatigue) cracking

Alligator cracking forms in wheel paths as a bottom-up fatigue failure caused by repeated flexural stress under traffic loads.