Transportation Engineering
Transportation Engineering tests geometric design, stopping and passing sight distance, horizontal and vertical curves, cross sections, pavement design, traffic flow, and MUTCD applications. The exam emphasizes formula selection and consistent units, especially where speed is given in mph.
Sight distance and curves
Stopping sight distance is SSD = 1.47Vt + V²/[30(f + G)], where V is in mph, t is perception-reaction time in seconds, f is friction, and G is grade as a decimal. A downhill grade reduces the denominator and increases SSD. Horizontal curve radius is limited by superelevation and side friction. Vertical curves use K = L/A, where A is the algebraic grade change in percent.
Traffic and pavement
Traffic flow is q = k u, relating flow, density, and speed. Level of service is evaluated with the Highway Capacity Manual methodology. Pavement design uses traffic loading in ESALs and structural-number or mechanistic-empirical procedures. MUTCD controls sign, signal, and marking warrants.
Worked example
Find SSD for V = 50 mph, t = 2.5 s, f = 0.35, and level grade. The first term is 1.47(50)(2.5) = 183.75 ft. The second term is 50²/[30(0.35 + 0)] = 2,500/10.5 = 238.1 ft. SSD = 421.9 ft, about 422 ft. If the grade were −3%, the denominator would become 30(0.35 − 0.03) = 9.6, increasing braking distance and SSD to about 444 ft.
Traps
The 1.47 factor converts mph to ft/s; using V directly in the braking term without the 30 denominator is wrong. Downhill grades increase stopping distance, so the grade sign matters. Do not substitute passing sight distance for stopping sight distance. For vertical curves, A is the absolute algebraic difference in percent grades, not a length.
Key points
- SSD combines perception-reaction distance and braking distance.
- The 1.47 factor converts mph to ft/s.
- Downhill grades increase required stopping sight distance.
- Vertical curve K equals length divided by algebraic grade change.
- Traffic flow equals density times speed.