Traffic Engineering Question: Evaluate the existing LOS and compare it with the LOS
under the new phase plan that you will design.
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 at that location. 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 a
colleague (who recorded the data), 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
The data is presented in a standard format generated by PetroPro, a software package coming
with the JAMAR counter by default. The image is the standard turning movement graph. The
numbers you saw on this graph are all traffic counts (flow rate if the duration is one hour, which
is the case here).
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?
8. What are the factors that may help to explain the difference in LOS by using the existing plan
and your phase plan?