Transportation and Highway Engineering
Transportation questions test geometric design, pavement fundamentals and traffic-flow reasoning for Egyptian highways and urban roads. The exam expects you to compute stopping sight distance, relate design speed to curve radius and superelevation, and distinguish pavement structural design from surface condition.
Core rules and concepts
- Stopping sight distance: SSD = vt + v²/(2gf), where v is design speed in m/s, t is perception-reaction time and f is longitudinal friction. Add grade effects when the road is not level.
- Horizontal curves: superelevation and side friction jointly resist centrifugal force. The common relationship is e + f = V²/(127R), with V in km/h and R in metres. Minimum radius is governed by maximum e and allowable f.
- Vertical curves: crest curves are controlled by sight distance; sag curves by headlight sight distance, comfort and drainage.
- Pavements: flexible pavement design uses traffic loading in ESALs and layer properties; rigid pavement design uses slab thickness, joint spacing and subgrade support. CBR is a strength index, not a complete pavement design method.
- Traffic flow: q = kv, where q is flow, k density and v space-mean speed. Level of service depends on density and speed, not flow alone.
Worked example
A highway has a design speed of 80 km/h, perception-reaction time 2.5 s and friction coefficient 0.35. Convert speed: v = 80/3.6 = 22.22 m/s. The reaction distance is vt = 22.22 × 2.5 = 55.6 m. The braking distance is v²/(2gf) = 22.22²/(2 × 9.81 × 0.35) = 493.8/6.867 = 71.9 m. SSD = 55.6 + 71.9 = 127.5 m. On a sustained upgrade, the braking component would be reduced; on a downgrade it would increase, so the design must use the governing direction.
Traps that cost marks
- Using km/h directly in formulas that require m/s.
- Omitting perception-reaction distance and using braking distance alone.
- Applying a minimum radius without checking superelevation runoff and transition length.
- Treating CBR as the sole pavement-design input.
- Confusing flow with level of service; high flow can occur at low speed and poor service.
- Ignoring drainage in sag curves and pavement design.