## Role of a traffic engineer

Description:

As a traffic engineer, you are assigned to evaluate the level of service of one intersection

near George Mason University Fairfax campus. The intersection is of Braddock Rd and the

Roberts Rd, and you can see the configuration of the intersection through Google Street Map

(https://www.google.com/maps/@38.8276404,-77.3008148,16.25z). One of your colleagues

went to the field and has collected the data using Jamar Counter (see Figure 2 at the end). The

data is summarized in Figure 1 below. Each arrow represents a unique turning movement (left,

through, and right) from each of the four approaches. For example, the south bound right has a

traffic volume of 97 veh/h.

Table 1 summarizes the current phase plan your colleague observed. It includes four phases

(E/W Left, E/W Through, N/S Left, and N/S Through) labeled in different colors. Although this

intersection is fully actuated and coordinated, for the practice, let us assume the current plan is a

fixed time plan with four phases.

Now, using this set of field data, you will first evaluate the level of service based on the current

phase plan observed by your colleague, and then resign it based on the Optimal Cycle length.

You will reevaluate the LOS under the new phase plan you design, and compare it with the

existing phase plan.

Figure 1: Traffic Data Collected at the Intersection.

Table 1: Current Phase Plan for the Intersection of Braddock Rd and Robert Rd.

Approach EB WB NB SB

Lane group LT T/R LT T/R LT T/R LT T/R

Effective Green (g) 12 133 12 133 15 34 15 34

Lost Time/phase 4

Total Lost Time 16

Cycle Length 210

Lab Tasks:

Field Observation

1. Assuming all traffic are passenger vehicles and the driving population are commuters,

convert the observed flow rate V into analysis flow rate v for different lane groups.

2. Following the procedure of the textbook, evaluate the LOS of this intersection using

collected data. Assume the saturation rate is 1800 veh/h/lane for through and right turning

lanes and 1750 veh/h/lane for left turning lanes (or the lane shared by the through and left

turning movements). In this problem, we assume that there is no standing queue at the

beginning of the red phase and all traffic can go through the intersection within one cycle.

You may apply the formula on page 255 for calculating d1 and you also need to consider

d2 in this analysis. The best way to approach this problem is to organize all information

in a table similar to Table 7.5.

a) What are the effective green time (g) and red time (r) for each phase?

b) What are the analysis flow rate for each lane group (from step 5)?

c) What are the g/C ratio, lane group capacity c, and volume capacity ratio X (please do

not confuse with flow ratio v/s (Y).

d) What are the average delay per vehicle for each lane group (d1)?

e) What are the average incremental delay per vehicle due to random arrival and

occasional oversaturation in seconds (d2)? In this study, we analyze LOS by

considering PHF. So T=0.25. We assume the intersection is pre-timed and take 0.5

for delay adjustment factor k. We assume the arrival pattern following Poisson

distribution and I=1.0. X is the v/c ratio for this lane group.

f) What are the lane group LOSs?

g) What are the average approach delays and approach LOSs?

h) What is the average intersection delay and intersection LOS?

Redesign the Intersection

3. As a traffic engineering, you are going to re-design the intersection using the procedure

in the textbook. First, apply the rules in the textbook to evaluate if a protected left turning

phase is warranted for the left turn movement for both the North-South and the East-West

directions.

4. You would follow the approach in Table 7.5. Assuming the saturation rate is 1800

veh/h/lane for through and right turning lanes and 1750 veh/h/lane for left turning lanes,

determine the flow ratio for all movements.

5. Assuming you do not change the phase plan, determine the critical lane groups and the

corresponding flow ratio. Calculate the optical cycle length with necessary assumptions

(ROUNDED UP to the nearest 5 seconds). Is it the same as the current plan?

6. Determine the effective green length allocation. Is it the same as the current plan?

7. Following the same approach, evaluate LOS if the intersection was using your phase

plan. Summarize your analysis using a table similar to Table 7.5. Does it change the

LOS?

Discussions

8. What are the factors that may help to explain the difference in LOS by using the existing

plan and your phase plan?

Template for Lab Report

A standard lab report should include a brief discussion about the objective of your study and a

detailed description of your approach. In this study, you need to describe both the data collection

approach and the analysis method. The list of tasks serves as a template for analysis method. At

the end, you need to summarize your findings and discuss the significance of your study. In this

study, you are expected to answer the question in task 13.

To make the report more succinct, please use graphs and tables whenever it is possible.

Figure 2: Example of Observation Location