Traffic Engineering (Civ-B10) - Solutions
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Next Technical Exam Sitting: May 6-10, 2019
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About: The Traffic Engineering exam is written nationally for aspiring civil engineers. The code for this exam is:
- Civil - 16-Civ-B10 (or you may see it like YY-Civ-B10, 98-Civ-B10, or Civ-B10)
Format: 3-hour long, open book exam. Answer any 5 of 7 questions.
Dates written: The exam is offered twice every year in the months of May and December.
Approved calculator: Any non-communicating calculator is permitted.
Questions in this paper are asked to test concepts mainly in the following areas:
General Theoretical Questions
You may be asked the general theoretical questions on various concepts such as circular and spiral curves, peak hour factor, pedestrian clearance time, gap acceptance behavior, effective green and displayed green.
You will be asked to analyze booth processing or a closed lane scenario. Queuing system input parameters will be given, such as mean arrival rate (l), mean service rate (m), number of servers (N) and queue discipline. You are expected to calculate the probability of booth being idle (or % of time booth idle), average number of cars in line and average wait time in line. You should have strong knowledge in the analysis of M/M/1 or M/M/N model.
Calculate utilization factor ρ = λ/μ
Probability of n cars in queue Pn = ρ n (1-ρ)
Average number of cars in the system:
Average number of cars in the queue:
Expected waiting time in the system:
Expected waiting time in the queue:
Vertical Crest Curves
You are expected to understand the fundamentals of breaking distance, perception – reaction time and stopping sight distance on vertical curves. You will be asked to determine the minimum length of vertical crest curve or design speed to provide ample stopping sight distance. Find the value of sight distance from the AASHTO chart based on design speed. Use following equations:
A question may be asked to calculate breaking distance and total stopping sight distance for a vehicle travelling uphill or downhill for given speed, perception-reaction time and gradients. These can be calculated using above formula.
Traffic system design / signal progression
You are expected to understand single-alternate, double-alternate and triple-alternate systems.
You will be given traffic parameters such as block lengths and required speed of progression in each direction. You will be asked to determine the appropriate alternate system and calculate cycle length and the actual speeds of progression.
Compute time required to travel one block T = Block length / vehicle speed
Select signal system and cycle time based on round trip time:
- Single alternative – round trip to 2nd intersection
- Double alternative – round trip to 3rd intersection
- Triple alternative – round trip to 4th intersection
Traffic Intersection control
You are asked to analyze four-legged intersection with two lanes in each direction and two -phase cycle. You are given design flow rate and saturation flow rates in each direction. You may be asked to determine Critical flow ratio, critical lane, Saturation flow rate, saturation headway, Capacity of approach and Green time. Use following formula:
Calculate flow ratio = design flow rate/saturation flow rate
The critical lane is the lane that requires the most time to service its queue. It can be found by locating the lane with the highest flow ratio. Calculate flow ratio for each lane, then find the lane with the largest flow ratio (critical lane).
Saturation flow rate - Saturation flow rate is the number of vehicles served by a lane for one hour of green time. In order to determine saturation flow rate, you must know the headway and saturation headway.
Saturation flow rate s = 3600 / h (veh/hr)
Where h is saturation headway
Saturation headway - Saturation headway is the headway of the vehicles in a "stable moving platoon" passing through a green light. A stable moving platoon is a group of vehicles that are traveling but with no relative movement
Capacity of approach c = (g/C) · s
c = capacity (pcu/hour)
g = Effective green time for the phase in question (sec)
C = Cycle length (sec)
s = Saturation flow rate (pcu/hour)
Gi = Vci/ Vc (C-L)
gi = Effective green time (sec)
Vci = Critical volume for phase (veh/ln·hr)
Vc = Total critical volume (veh/ln·hr)
C = Cycle Length (sec)
L = Total lost time (tL · N) (sec)
Traffic measurement procedures
In this task, you are required to estimate traffic volume from traffic data collected by Moving vehicle method. You will be asked to calculate Traffic volume and average traffic time.
Traffic volume is given by: q = (mw + ma) / (tw + ta)
Average traffic time: t = tw – mw / q
tw = time taken by test vehicle along traffic
ta = time taken by test vehicle against traffic
mw = mo-mp
mo = number of vehicle overtaken the test vehicle
mp = number of vehicle overtaken by the test vehicle
ma = number of vehicles moving against the stream
Level of Service
You will be given input data for freeway such as the width of lanes and shoulders, grade, length, peak hour factors and occupancy. You are asked to determine service volumes at level A, B, C & D and level of service provided for given service volume.
Level of Service (LOS) relates the quality of traffic service to a given flow rate. A service volume or service flow rate is the maximum number of vehicles which can be accommodated by a given facility or system under given conditions at a given LOS.
To determine the level of service for a basic freeway section followings steps are followed determine free-flow speed:
The 15-min flow rate in pc/h/ln is calculated from the hourly volume of mixed traffic by
The density D is then calculated as
Density D is compared with limiting values of each level of service to determine the LOS.
Peak Hour Factor (PHF)
Peak hour factor is the measurement of traffic demand fluctuations within the peak hour. It is defined by the ratio of hourly volume during the maximum volume hour during the day and the peak 15-minutes flow rate within the peak hour
Peak hourly factor (PHF) = Hourly Volume / Maximum rate of flow
= V/ (4*V15)
V = peak-hour volume (vph)
V15 = volume during the peak 15 minutes of flow (veh/15 minutes)
Gagan Verma has more than 10 years of experience in civil and structural engineering for medium to large capital heavy industrial projects for the global clients of energy, oil & gas and infrastructure sectors. He has hands-on experience in all phases of project execution from conceptual design and feasibility studies to detail engineering, construction support and commissioning. He holds a bachelor’s degree in Civil Engineering from Indian Institute of Technology, Roorkee.
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