Transportation: Geometric Design and Traffic Flow
Transportation questions cover geometric design, traffic flow theory, and pavement basics. Know sight distance (stopping sight distance SSD = 1.47Vt + V²/(30(f ± G))), horizontal curve relationships (R, T, L, M, degree of curve), superelevation (e + f = V²/(15R)), and vertical curves (K-value, L = KA for crest/sag). Traffic flow concepts: flow = density × speed (q = k·v), level of service, and peak hour factor. Pavement basics include ESALs and the AASHTO structural number concept.
Traps
- Using the wrong sign on grade G in the SSD formula - positive G (uphill) reduces stopping distance; negative G increases it.
- Confusing stationing (100-ft stations) with actual distance - a station of 10+50 is 1,050 ft.
- Mixing units: V in mph, but SSD in feet - the 1.47 factor converts mph to ft/s.
- Forgetting that superelevation and side friction share the centrifugal demand: e + f = V²/(15R).
Worked example
SSD for V = 60 mph, t = 2.5 s, f = 0.35, level grade: braking = 60²/(30 × 0.35) = 3,600/10.5 ≈ 343 ft; perception-reaction = 1.47 × 60 × 2.5 ≈ 220 ft; SSD ≈ 563 ft. Rule of thumb: on a downgrade, the negative G increases braking distance - always check the sign before plugging in.