ASCE Transportation and Infrastructure Planning 2 — Questions and Answers
Question 1: Under the Greenshields linear speed-density model, maximum flow occurs at what density?
- Jam density
- One-half of jam density (Correct answer)
- One-quarter of jam density
- Two-thirds of jam density
Correct answer: One-half of jam density
Greenshields' parabolic flow-density curve peaks at k = kj/2.
Question 2: A freeway lane has a free-flow speed of 60 mph and a jam density of 200 veh/mi. Using Greenshields' model, what is the lane capacity?
- 3,000 veh/h (Correct answer)
- 6,000 veh/h
- 12,000 veh/h
- 1,500 veh/h
Correct answer: 3,000 veh/h
Capacity = vf × kj / 4 = 60 × 200 / 4 = 3,000 veh/h.
Question 3: An approach carries 1,800 veh/h, with a peak 15-minute volume of 500 vehicles. What is the peak hour factor?
- 0.75
- 0.85
- 0.90 (Correct answer)
- 1.11
Correct answer: 0.90
PHF = 1,800 / (4 × 500) = 0.90.
Question 4: Which statement about space mean speed is correct?
- It is always greater than time mean speed
- It is the arithmetic mean of spot speeds
- It is measured only with radar at a single point
- It is the harmonic mean of spot speeds over a point (Correct answer)
Correct answer: It is the harmonic mean of spot speeds over a point
Space mean speed is the harmonic mean of spot speeds and is less than or equal to time mean speed.
Question 5: Under the Highway Capacity Manual, which measure defines level of service on a basic freeway segment?
- Average travel speed
- Density in pc/mi/ln (Correct answer)
- Volume-to-capacity ratio only
- Percent time spent following
Correct answer: Density in pc/mi/ln
The HCM uses density in passenger cars per mile per lane as the LOS measure for basic freeway segments.
Question 6: Upstream flow is 1,500 veh/h at 30 veh/mi. Downstream flow is 1,000 veh/h at 100 veh/mi. What is the shock wave speed?
- +7.1 mph
- +25 mph
- -7.1 mph (Correct answer)
- -14.3 mph
Correct answer: -7.1 mph
Shock wave speed = (1,000 − 1,500)/(100 − 30) = −7.1 mph, so the wave moves upstream.
Question 7: Which speed statistic is most commonly used as the basis for setting posted speed limits?
- 85th percentile speed (Correct answer)
- Mean free-flow speed
- 15th percentile speed
- Design speed
Correct answer: 85th percentile speed
The 85th percentile operating speed is the traditional basis for speed zoning in the MUTCD.
ASCE Transportation and Infrastructure Planning 2
Under the Greenshields linear speed-density model, maximum flow occurs at what density?