Civil Engineering PE Transportation and Traffic Engineering Questions and Answers — Questions and Answers
Question 1: According to the fundamental relationship of traffic flow, what is the traffic flow rate when the traffic density reaches its maximum possible value (jam density)?
- Maximum flow (capacity)
- Zero (Correct answer)
- Equal to the free-flow speed
- Equal to the critical density
Correct answer: Zero
The fundamental equation of traffic flow is Flow = Speed × Density. At jam density, vehicles are stopped or nearly stopped (bumper-to-bumper), so their speed is zero. Therefore, the flow rate, being the product of speed and density, is also zero.
Question 2: A horizontal curve on a two-lane rural highway has a radius of 1,200 ft and a design speed of 55 mph. According to AASHTO guidelines, which of the following is the primary purpose of applying superelevation to this curve?
- To improve roadway drainage during storm events.
- To increase the required stopping sight distance around the curve.
- To counteract the centrifugal force experienced by a vehicle. (Correct answer)
- To reduce the required length of the approach tangent.
Correct answer: To counteract the centrifugal force experienced by a vehicle.
Superelevation, or banking a curve, tilts the roadway to help vehicles navigate the turn. A component of the vehicle's weight is directed toward the center of the curve, which counteracts the outward centrifugal force. This reduces the reliance on side friction between the tires and the pavement, improving vehicle stability, safety, and driver comfort.
Question 3: An engineer is calculating the required yellow interval for a signalized intersection approach. The design speed is 40 mph, the driver perception-reaction time is 1.0 second, and the comfortable deceleration rate is 10 ft/s². The approach is on a +2% (uphill) grade. Using the standard kinematic equation, what is the minimum required yellow interval?
- 2.8 s
- 4.5 s
- 5.1 s
- 3.8 s (Correct answer)
Correct answer: 3.8 s
The formula for the yellow interval (Y) is: Y = t + V / (2a + 2Gg), where t = perception-reaction time, V = approach speed, a = deceleration rate, G = grade, and g = acceleration due to gravity (32.2 ft/s²). First, convert speed: V = 40 mph * 1.467 (ft/s)/mph ≈ 58.7 ft/s. Then, plug in the values: Y = 1.0 s + 58.7 ft/s / (2 * 10 ft/s² + 2 * 0.02 * 32.2 ft/s²). Y = 1.0 + 58.7 / (20 + 1.288) = 1.0 + 58.7 / 21.288 = 1.0 + 2.76 s ≈ 3.76 s. The closest answer is 3.8 s.
Question 4: According to the Highway Capacity Manual (HCM), which performance measure is primarily used to determine the Level of Service (LOS) for a basic freeway segment?
- Density (passenger cars per mile per lane) (Correct answer)
- Average control delay (seconds per vehicle)
- Volume-to-capacity (v/c) ratio
- Average travel speed (mph)
Correct answer: Density (passenger cars per mile per lane)
For basic freeway segments, Level of Service is defined by density, measured in passenger cars per mile per lane (pc/mi/ln). Density is a measure of vehicle proximity and is a better indicator of operational quality on uninterrupted flow facilities (like freeways) than speed, as speed can remain relatively high even as conditions worsen from LOS C to D.
Question 5: A traffic engineer needs to determine the required stopping sight distance (SSD) for a vehicle traveling at 60 mph on a roadway with a -3% downgrade. Assuming a standard perception-reaction time of 2.5 seconds and a coefficient of friction (f) of 0.30, what is the approximate SSD?
- 485 ft
- 525 ft
- 615 ft (Correct answer)
- 425 ft
Correct answer: 615 ft
The AASHTO formula for Stopping Sight Distance is SSD = (Perception-Reaction Distance) + (Braking Distance). Perception-Reaction Distance = 1.47 * V * t = 1.47 * 60 * 2.5 = 220.5 ft. Braking Distance = V² / (30 * (f + G)) = 60² / (30 * (0.30 - 0.03)) = 3600 / (30 * 0.27) = 3600 / 8.1 ≈ 395.6 ft. SSD = 220.5 ft + 395.6 ft = 616.1 ft. The closest answer is 615 ft.
Question 6: An engineer is performing a flexible pavement design using the 1993 AASHTO Design Guide. Which of the following parameters is the required direct input to characterize the structural support of the subgrade soil?
- California Bearing Ratio (CBR)
- Resilient Modulus (Mr) (Correct answer)
- Plasticity Index (PI)
- Unconfined Compressive Strength (qu)
Correct answer: Resilient Modulus (Mr)
The 1993 AASHTO Design Guide's empirical equation for flexible pavements requires the subgrade Resilient Modulus (Mr) as the direct input to characterize its stiffness and support. While other properties like CBR can be used to estimate Mr through correlations, Mr is the fundamental property used in the design equation itself.
Civil Engineering PE Transportation and Traffic Engineering Questions and Answers
According to the fundamental relationship of traffic flow, what is the traffic flow rate when the traffic density reaches its maximum possible value (jam density)?