Transportation and Infrastructure Planning Flashcards

What perception-reaction time does AASHTO assume when computing stopping sight distance?

Answer: 2.5 s

AASHTO's Green Book uses 2.5 s of brake reaction time for stopping sight distance.

In US customary units, which equation gives the minimum horizontal curve radius (ft) for design speed V (mph)?

Answer: R = V² / (15(e + f))

Point-mass curve design gives R = V²/[15(0.01e + f)], with e entered in percent.

What does the rate of vertical curvature K represent?

Answer: Horizontal length of curve per percent of algebraic grade difference

K = L/A, the length of curve needed for each 1% change in grade.

Where snow and ice are common, AASHTO generally limits maximum superelevation to about what value?

Answer: 8%

Where snow and ice are common, emax is typically 8% so slow-moving vehicles do not slide toward the inside of the curve.

For a sag vertical curve, which criterion usually controls the minimum length?

Answer: Headlight sight distance

At night, the headlight beam must light enough roadway ahead to provide stopping sight distance on a sag curve.

What deceleration rate does AASHTO use to compute braking distance for stopping sight distance?

Answer: 11.2 ft/s²

AASHTO uses 11.2 ft/s², which most drivers can achieve comfortably on wet pavement.

A 1,000-ft crest vertical curve joins a +3% grade to a −2% grade. How far from the BVC is the high point?

Answer: 600 ft

x = g1·L/(g1 − g2) = 3 × 1,000 / 5 = 600 ft.