CE 115
Transportation Engineering Exercise
Introduction
Your purpose in this laboratory is to time the signal offsets so that you can drive your car
through the course in a way that closely reflects your plan to get through the course. You will
meet in room 110 on the basement floor of the Engineering Physics Building with your team,
where you will fine tune your offset design. Then when your design is complete, you will run
the course in room 113.
The map in Figure 1 shows the course layout for the project that is to scale. Table 1 defines the
distances between the signals and the average speeds for the sections.
Table 1 Signal Distances.
Distances (ft)
Average Speed (fps)
1 to 2
10.3
3.4
2 to 3
10.0
3.3
3 to 4
9.5
3.1
4 to 5
5.5
3.6
5 to 1
7.4
3.7
The information in Table 1 will play an important role in the design of your signal offsets.
CE 115
Transportation Engineering Exercise
4
5
1
2
3
SCALE: 1 inch = 1 foot
Figure 1 Course Layout.
CE 115
Transportation Engineering Exercise
50
45
40
35
Time
30
25
20
15
10
5
0
0
50
100
150
200
Distance
Figure 2 Example of Your Designed and Actual Vehicle Trajectory.
You can draw a two-dimensional graph to represent the position of your car on the course as
time passes, with two lines. One line would be for your plan, let’s call this your designed vehicle
trajectory. The other line would be for what you were actually able to do, let’s call this your
actual vehicle trajectory. For an example, you should look at Figure 2, above. Your designed
vehicle trajectory is the blue-dotted line in Figure 2 and your actual vehicle trajectory is the
green-solid line. In this example, you should see that your actual trajectory does not match your
designed one very well. You start driving, but you over estimated your speed so the light turns
red at the second signal before you get to it. So, you have to stop and this was not part of your
design. According to the green line, you experience similar setbacks later on in the course at the
third and fifth signals.
Design Specifications
The cycle length of the signals (time from the start of green until the start of the next green) is 6
seconds. The cycle will be as follows:
 2 seconds of green time
 1 second of yellow time
 3 seconds of red time.
Design Evaluation
Your designed signal offset should have two characteristics. First, the vehicle that you drive
through the course should have a trajectory similar to your designed trajectory. Referring to
Figure 2, this means that the blue-dotted line needs to be similar to the green-solid line. Second,
your signal offsets design should minimize the possibility that you will arrive at a signal while it
is red. The quality of your signal offsets design depends on these two characteristics.
To judge your signal timing plan, you measure the quality of your design according to these two
characteristics. To measure the similarity of the actual vehicle trajectory to your designed
trajectory, you will record the time that it takes your vehicle to arrive at pre-specified locations
on the course. For this laboratory, these locations are signals 2, 4, and 1 (end). Then you will
compare these recorded times to those of your designed vehicle trajectory, when you pass a
signal on red, record this event in Table 1. If a signal shows red you should not pass it, but if it
CE 115
Transportation Engineering Exercise
shows yellow or green you may proceed. You should record all of the design evaluation
information on the form shown in Table 1, below.
Table 2 Design Evaluation.
Signal
Actual Time
(seconds)
1
Design Time
(seconds)
Time Difference
(seconds)
Passed on Red
(Y or N)
2
3
4
5
End
Average
Space-Time Diagram
50
45
40
35
Time (sec)
30
Begin R
Begin G+Y
25
Vehicle
20
15
10
5
0
0
5
10
15
20
25
Location (ft)
Figure 3 Course Space-Time Diagram.
30
35
40
45
CE 115
Transportation Engineering Exercise
Design Instructions
The space-time diagram in Figure 3 has lines at each of the signals. Using this diagram, you can
draw a vehicle trajectory line representing the position of the vehicle for a given time. Whenever
that line crosses a signal, you want that signal to be green (above a green dot and below the next
red dot). Also, you want the vehicle to progress through all of the stop bars as quickly as
possible. To accomplish this, you need to coordinate the signal green times for all of the signals.
This is accomplished by defining when a signal turns green relative to the beginning of green at
signal 1, which is the offset for that signal.
Remember, if your travel time from signal 1 to signal 3 is 12 seconds then your offset should be 0
seconds. If your travel time from signal 1 to signal 4 is 15 seconds then your offset should be 3
seconds.
To setup a good signal coordination scheme, follow the steps given below, using the Traffic
Signal Coordination Design Worksheet:
1. Determine how fast you can drive the vehicle through the course. Remember to consider
the effects of corners and driving skill. Use the design
2. Adjust your vehicle trajectory line by changing the speeds so that it reflects the speeds
you have chosen.
3. Define the offsets that will most closely fit the vehicle trajectory line that you have
established. Remember, you want the offsets to be such that your vehicle can proceed
through green at all of the stop-bars of the course with the minimum time possible. Also,
remember that the first signal is also the last signal, which means that you will be passing
it twice. Signals that you pass twice should be shown on your space-time diagram twice
with green time occurring at the same time. The cycles on the first and last lines for
example have to be the same (zero offset) because they are controlled by the same signal.
D
Hint: You may initially define the offsets using the offset equation, which is t i  ,
S
where ti is the offset, D is the distance between signals, and S is the speed of the vehicle.
4. Adjust the vehicle speeds so that your vehicle will arrive at the signals when the light is
green, as defined in step 3.
5. Repeat steps 2, 3, and 4 until you have found the offsets that look like they work best.