Traffic Engineering (Civ-B10) - Solutions

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Next Technical Exam Sitting: May 6-10, 2019

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Exam Basics

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. 

Strategy for Exam Questions

Questions in this paper are asked to test concepts mainly in the following areas:

  1. 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.

 

  1. Queuing Theory

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:  

avg # of cars in system

 

 

Average number of cars in the queue:

avg # of cars in que

Expected waiting time in the system: 

waiting time in system equation

Expected waiting time in the queue: 

waiting time in que equation

 

  1. 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:

3 - vertical crest formula

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. 

 

  1. 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

 

  1. 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

Where:

c = capacity (pcu/hour)

g = Effective green time for the phase in question (sec)

C = Cycle length (sec)

s = Saturation flow rate (pcu/hour)

 

Green time

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)

 

  1. 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

 

  1. 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:

free flow speed equation

The 15-min flow rate in pc/h/ln is calculated from the hourly volume of mixed traffic by

hourly volume of mixed traffic equation

The density D is then calculated as

density equation

Density D is compared with limiting values of each level of service to determine the LOS.

 

 

  1. 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)

Your Reviewer & Technical Support Helper

Gagan Verma - Civil (structural)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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