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## 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
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
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