Transportation: Curves, Sight Distance, and Traffic Flow
Geometric design
Transportation questions emphasize horizontal and vertical curves, sight distance, superelevation, and access management. For a circular horizontal curve, the degree of curve D (arc definition) and radius R are related by R = 5,729.58 / D. Stationing runs along the curve; the tangent length is T = R tan(Δ/2), and the curve length is L = RΔ in radians. A common trap is mixing the intersection angle Δ with its half-angle, or using chord length where arc length is required.
Sight distance and vertical curves
Stopping sight distance depends on perception-reaction time, speed, and friction; passing sight distance is much longer. Vertical curves use K = L/A, where A is the algebraic grade change in percent. Do not confuse crest and sag criteria. Superelevation questions require checking the side-friction assumption and the transition length.
Worked example
A horizontal curve has D = 4° (arc definition). R = 5,729.58/4 = 1,432.4 ft. If Δ = 30°, T = 1,432.4 tan(15°) = 1,432.4(0.2679) = 383.8 ft. L = RΔ(π/180) = 1,432.4(30)(0.01745) = 750.0 ft. Rounding early or using 5,730 instead of 5,729.58 rarely changes the answer, but using the chord definition when arc is specified will.
Traffic and pavements
Know flow-speed-density relationships, level of service, signalized intersection delay, and the AASHTO flexible pavement design inputs. A trap is treating capacity as a fixed number regardless of lane width, grade, or heavy-vehicle percentage. Read whether the question asks for free-flow speed or operating speed.