Transportation engineering: traffic and highway planning and design
The exam probes whether you can size a road or junction, check a geometric standard and explain the planning hierarchy behind it. Hong Kong practice draws on the Transport Department's Transport Planning and Design Manual (TPDM), the Hong Kong Planning Standards and Guidelines (HKPSG) and Highway Capacity Manual concepts.
Flow, speed and density
The identity q = k·v links flow (veh/h), density (veh/km) and space-mean speed (km/h). As density rises, speed falls and flow peaks at capacity; beyond saturation, queues rather than flow measure performance. Level of service bands A–F describe operating quality; LOS D is a common target for urban arterials, with LOS E tolerated at peak.
Capacity and saturation flow
Lane capacity ≈ saturation flow × green ratio. Saturation flow is quoted in passenger car units per hour of green (typically 1,800–2,000 pc/h/lane for through lanes). Apply heavy-vehicle and turning adjustments, then divide by the peak-hour factor. Never mix veh/h and pc/h in one table.
Signal timing
Webster's optimum cycle: C₀ = (1.5L + 5)/(1 − Y), with L the total lost time per cycle (s) and Y the sum of critical flow ratios yi = qi/si. Effective green for a phase: gi = (C − L)·yi/Y.
Worked example: a four-arm signal has L = 12 s and critical ratios 0.30, 0.25, 0.17, so Y = 0.72. C₀ = (1.5×12 + 5)/(1 − 0.72) = 23/0.28 ≈ 82 s. The critical phase receives (82 − 12) × 0.30/0.72 ≈ 29 s effective green. Check reconciliation: sum of greens plus lost time equals the cycle; check that each yi ≤ the green ratio allocated.
Geometric checks
Stopping sight distance SSD = v·t + v²/(2g(f ± G)), with v in m/s, perception–reaction time t usually 2.0 s, f the longitudinal friction factor and G the grade. Check SSD against horizontal curve length, crest vertical curve K-value and superelevation; verify lane and shoulder widths and vertical headroom. Design speed sets geometry; the 85th-percentile operating speed governs safety checks.
Traps that cost marks
- Using design speed where operating speed is required for sight-distance checks.
- Omitting the peak-hour factor or heavy-vehicle adjustment.
- Computing Y from non-critical movements, inflating cycle length.
- Ignoring pedestrian stages, intergreen and minimum green constraints.
- Quoting a cycle length without verifying that it serves the demand ratios.