Tteng 441 traffic engineering fall 2021 part3

‫ﺮﳼ‬
‫ﯾﺔ‬‫ر‬‫اﳌﺮو‬ ‫ﻠﺴﻼﻣﺔ‬ ‫اﻟﺴﻌﻮدﯾﺔ‬ ‫اﻣﻜﻮ‬‫ر‬ٔ
Aramco Chair for Traffic Safety Research
Fall 2021/ ElDessouki 126
. TTENG 441 Traffic Engineering

Fall 2021/ ElDessouki . TTENG 441 Traffic Engineering 127

Speed Studies: Spot Speed Studies
Spot Speed Studies:
 Is defined as the average speed of vehicles passing a point on a
highway. This is also known as the time mean speed.
 Usually conducted in free flow condition and not during
congestion, where the flow rate is:
750-1000 veh/hr/ln  for freeway
<500 veh/hr/ln  for other types
Speed Definition of Interest:
- Average or time mean speed
- Standard Deviation
- 85th % speed
- Median speed

Uses of Spot Speed Data:
 To determine speed limit for applications
 To assess speed limit enforcement
 Specific Applications:
 For Level of Service (LOS) Assessment
 For Signal timing: Estimation of Yellow/All Red times.
 To determine appropriate sight distance
 For safety and accidents analysis

Speed Studies: Measurement Techniques
Manual Method: The Simple Stopwatch Method :
By using stopwatch and defining two reference points with
known distance (d) between the two points.
Then, Speed = d / t (m/s)
Advantages: Simple
Disadvantages:
High error due to stopwatch
depressing time variations.
Class Example
40 m

Doppler Radar (Speed Gun):
It uses Doppler’s effect for speed measurements.
How Does it work?
 The radar transmit a pack of waves with initial frequency fini and initial wave
length lini ,
 Due to the motion of the target vehicle, the wave length of the reflected waves
lref will be longer or shorter than the initial wave length lini
direction
s
target'
the
on
depnding
V
f
target
ini
ini
ref 

 *
1
l
l
lini
lref
Transmitted wave
Reflected wave
Target
Radar

Doppler Radar (Speed Gun):
Advantages:
High Accuracy, but the readings must be corrected for aiming
angle.
Disadvantages:
Difficult to conceal, drivers associate Radar
with police which may cause them to
slow their speeds down and yielding
inaccurate results.

Doppler Radar (Speed Gun): Readings Correction
q

cos
Speed
TrueSpeed 

Spot Speed Studies:
Data Reduction & Analysis
The speed data is analyzed and reported as following:
A- Graphical: Frequency Histogram & Accumulative %
0
5
10
15
20
25
30
35
50-60 60-70 70-80 80-90 90-100 100-110 110-120 120-130 130-140 140-150 150-160
Frequencey
Speed (km/hr)
MODE = 105 km/hr
The shown speed data has
a Bimodal Distribution

Spot Speed Studies:
A- Graphical: Accumulative %
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
20 40 60 80 100 120 140 160 180
Accumulative
%
Speed (km/hr)
Median
85%
85th% Speed
15%
15th% Speed

Spot Speed Studies:
B- Quantitative:
1- Mode
2- Median
3- Mean
4- Standard Deviation (SD)
5-85th% & 15th% Speeds
6- Pace(15 km/hr band)

Spot Speed Studies:
C- Precision and Confidence Intervals:
Note: Most spot speed data tend to be normally distributed (however, this
might not applicable in the shown example histogram), then:
Standard Error:
True Mean:
size
sample
the
is
-
N
sample
the
for
deviation
standard
the
is
-
SD
where
N
SD
E 
mean
sample
the
is
-
X
sample
the
of
error
standard
the
is
-
E
where
95%)
Confidence
of
Degree
(at
E
X 96
.
1




Spot Speed Studies:
D- Sample Size for Prescribed Precision with Confidence Intervals:
If the Prescribed precision was set to be +/- e, the needed sample should
not be less than the following number of observations N @ a degree of
confidence 95%:
mean
true
the
in
error
the
i.e.
precision
needed
the
-
e
sample
the
of
deviation
standard
the
is
-
SD
where
e
SD
N
2








 )
96
.
1
(

Speed Studies: Before and After Analysis
Before and After Analysis:
 Usually carried out to evaluate the effectiveness of applying a specific
measure on the prevailing speed in an area or a segment.
 The before and after is basically a comparison testing between two samples
, with the objective of finding that the difference between the two samples is
significant or not.
 Hypothesis Testing
Any hypothesis test, has 4 possible outcomes:
1- Test Result: True, and Reality: True
2- Test Result: False, and Reality: False
3- Test Result: False, and Reality: True Error Type II
4- Test Result: True, and Reality: False Error Type I
Error Type I - must be avoided at all expenses
True/True False/False
False/True True/False

The Statistical Testing (Z-test):
First, calculate the Pooled Standard Deviation for before & after samples:
Second: Calculate Zd the Standard Normal distribution approximation for the
Observed difference between the before & after samples:
resp.
after
&
before
size
Sample
N
N
resp.
after
&
before
Deviation
Standard
S
S
Deviation
Standard
Pooled
S
where
N
S
N
S
S
2
1
2
1





&
&
2
2
2
1
2
1


 
difference
mean
population
ed
hypothesiz
d
Deviation
Standard
pooled
S
resp.
after
&
before
speed
Mean
X
X
after
&
before
between
Diffirence
Normalized
Z
S
d
X
X
Z
o
2
1
d
o
d









&
2
1

The Statistical Testing (Z-test) cont.:
Third, we use the normal distribution curve to find the probability that a value
equal to or less than Zd , assuming that both samples are normally
distributed , then:
A) If Prob.( Z<= Zd ) > 0.95 , that means the observed reduction in speed is
statistically significant.
B) If Prob.( Z<= Zd ) < 0.95 , that means the observed reduction in speed is
statistically insignificant.
For case A, that implies also that there is a 5% chance that the observed
difference in mean speed will be exceeded.

Example:
The following is the before and after summary for speed enforcement project
that was deployed with a target of reducing average speed to 60 mph.
Before After
Mean Speed: 65.3 63 (mph)
SD: 5 6 (mph)
N: 50 60 observation
Solution
P(Z<2.19) = 0.9857 = 98.57 % >95%
Then: The observed reduction in speed was statistically significant
mph
N
S
N
S
S 05
.
1
60
6
50
5 2
2
2
2
2
1
2
1






  19
.
2
05
.
1
0
63
3
.
65




d
Z

Example (cont.):
Now, the question did we reach the target?
That means the true mean is between: (61.48 - 64.52) mph
The Answer is NO, the reduction is not sufficient and we did not
achieve the 60 mph target
mph
63.0
E
X 52
.
1
0
.
63
60
6
*
96
.
1
96
.
1 







Before and After Analysis:
Example: Evaluating the impact of speed enforcement
project such as SAHER.

Traffic Volume Studies:
Volume studies:
 Traffic counts are the most basic of traffic studies and
are the primary measure of demand; virtually all aspects
of traffic engineering require volume as an input,
including highway planning and design, decisions on
traffic control and operations, detailed signal timing, and
others.

Automated and Manual counting techniques are used to produce
estimates of the following:
1. Volume: is the number of vehicles (or persons) passing a point
during a specified time period, which is usually one hour, but need
not be it can be a day, month, year…etc.
2. Rate of flow: is the rate at which vehicles (or persons) pass a
point during a specified time period less than one hour,
expressed as an equivalent hourly rate.
3. Demand is the number of vehicles (or persons) that desire to travel
past a point during a specified period (also usually one hour).
Demand is frequently higher than actual volumes where congestion
exists. Some trips divert to alternative routes, while other trips are
simply not made.
4. Capacity is the maximum rate at which vehicles can traverse a
point or short segment during a specified time period. It is a
characteristic of the roadway.

Example for Volume, Demand & Capacity:

Example: When demand exceeds capacity. What happens?

Traffic Volume Studies: Manual Counting 1
Manual counting is typically used at intersections, however,
it can be used for highways for 1-2 hours, for LOS
assessment.
Tally Sheets:
Recording data onto tally sheets is the simplest means of
conducting manual counts. The data can be recorded with
a tick mark on a pre-prepared field form. A watch or
stopwatch is necessary to measure the desired count
interval. A blank traffic volume count intersection tally
sheet is provided in Appendix B.

Mechanical Counter:

Mechanical Counting Board (for Intersections):

Electronic Manual Counting Board (for Intersections):

Traffic Volume Studies: Automated Counting 1
Portable Counters:
 Portable counters serve the same purpose as manual
counts but with automatic counting equipment.
 The period of data collection using this method is usually
longer than when using manual counts.
 The portable counter method is mainly used for 24-hour
counts. Pneumatic road tubes are used to conduct this
method of automatic counts

Portable Counters: Pneumatic Tube Counters

Permanent Counters:
 Permanent counters are used when long-term counts are to
be conducted. The counts could be
 performed every day for a year or more. The data
collected may be used to monitor and evaluate
 traffic volumes and trends over a long period of time.
Permanent counters are not a cost-effective
 option in most situations. Few jurisdictions have access to
this equipment

Permanent Counters: Inductive Loop Detectors

Video Imaging:
 Observers can record count data by videotaping traffic.
 Traffic volumes can be counted by viewing videotapes
recorded with a camera at a collection site.
 A digital clock in the video image can prove useful in
noting time intervals.
 Videotaping is not a cost-effective option in most
situations.

Video Imaging:

Travel Time Studies:
Travel Time is defined as:
The time required o traverse a segment of specific distance
in the network at specific time.
Travel-time is a Popular Performance Measure, why?:
Because:
 Easy to understand by decision makers and the general
public.
 Easy to work with.
 It can be used in mode choice models and comparing
between different modes.

Travel Time Studies:
How do we use Travel-time information? :
 To identify problem locations on facilities by virtue of high
travel times and or delay.
 To measure arterial level of service, based on average travel
speeds and travel times.
 To provide necessary input to traffic assignment models, which
focus on link travel time as a key determinant of route
selection.
 To provide travel-time data for economic evaluation of
transportation improvements.
 To develop time contour maps and other depictions of traffic
congestion in an area or region

Travel Time: Field Study Techniques:
Driving Test Cars: The test car driver MUST use only one of the
following Techniques
 1. Floating-car technique.
In this technique, the test-car driver is asked to pass as many vehicles
as pass the test car. In this way, the vehicle’s relative position in the
traffic stream remains unchanged, and the test car approximates the
behavior of an average vehicle in the traffic stream. (Mean Speed)
 2. Maximum-car technique.
In this procedure, the driver is asked to drive as fast as is safely
practical in the traffic stream without ever exceeding the design
speed of the facility. (85% Speed)
 3. Average-car technique.
The driver is instructed to drive at the approximate average speed of
the traffic stream. (Mean Speed)

Travel Times Indications of the Driving Techniques
 The floating-car and average-car techniques result in estimates of the
average travel time through the section.
 The floating-car technique is generally applied only on two-lane highways,
where passing is rare, and the number of passing cars can be counted and
balanced relatively easily.
 On a multilane freeway, such a driving technique would be difficult at best,
and might cause dangerous situations to arise as a test vehicle attempts to
“keep up” with the number of vehicles that have passed it.
 The average-car technique yields similar results with less stress experienced
by the driver of the test vehicle.
 The maximum-car technique does not result in measurement of average
conditions in the traffic stream. Rather, the measured travel times represent
the lower range of the distribution of travel times. Travel times are more
indicative of a 15th percentile than an average. Speeds computed from these
travel times are approximately indicative of the 85th percentile speed.

Travel Time Studies Results: Contour Map

Travel Times Indications of the Driving Techniques
 The floating-car and average-car techniques result in estimates of the
average travel time through the section.
 The floating-car technique is generally applied only on two-lane highways,
where passing is rare, and the number of passing cars can be counted and
balanced relatively easily.
 On a multilane freeway, such a driving technique would be difficult at best,
and might cause dangerous situations to arise as a test vehicle attempts to
“keep up” with the number of vehicles that have passed it.
 The average-car technique yields similar results with less stress experienced
by the driver of the test vehicle.
 The maximum-car technique does not result in measurement of average
conditions in the traffic stream. Rather, the measured travel times represent
the lower range of the distribution of travel times. Travel times are more
indicative of a 15th percentile than an average. Speeds computed from these
travel times are approximately indicative of the 85th percentile speed.

Some Considerations for Test Car Driving Technique
 For practicality, too many test cars released into the traffic stream
over a short period of time will affect its operation, in effect altering
the observed results.
 For most common applications, the number of test-car runs that will
yield travel-time measurements with reasonable confidence and
precision ranges from a low of 6 to 10 to a high of 50, depending
upon the type of facility and the amount of traffic.

Travel Time: Other Techniques
GPS Data loggers:
 It is basically a GPS data recorder that record the location of the vehicle in
terms of lat/long coordinates and the time & speed. It is very accurate and
does not distract the driver.
 But the disadvantage is that details regarding delay causes are lost.
 Smart Phone Applications .(myTracks)
License Plate Matching:
 Roadside observers can record license plate numbers as vehicles pass
designated points along the route. The time of passage is noted along with
the license plate number.
 The detail of delay information at intermediate points is lost with this
technique.

Travel Time: GPS Data Sample
Record Time Lat Long Speed Heading Date
GPRMC 184441 A 2620.391 N 5007.252 E 50.65 300.71 10317 A*5C
GPRMC 184441 A 2620.391 N 5007.252 E 50.65 300.71 10317 A*5C
GPRMC 184441 A 2620.391 N 5007.252 E 50.65 300.71 10317 A*5C
GPRMC 184441 A 2620.391 N 5007.252 E 50.65 300.71 10317 A*5C
GPRMC 184441 A 2620.391 N 5007.252 E 50.65 300.71 10317 A*5C
GPRMC 184441 A 2620.395 N 5007.225 E 50.07 298.18 10317 A*55
GPRMC 184443 A 2620.405 N 5007.225 E 50.07 298.18 10317 A*55
GPRMC 184443 A 2620.405 N 5007.225 E 50.07 298.18 10317 A*55
GPRMC 184443 A 2620.405 N 5007.225 E 50.07 298.18 10317 A*55
0
20
40
60
80
100
120
2620.35
2620.4
2620.45
2620.5
2620.55
2620.6
5006.35 5006.4 5006.45 5006.5 5006.55 5006.6 5006.65 5006.7 5006.75 5006.8 5006.85 5006.9 5006.95 5007 5007.05 5007.1 5007.15 5007.2 5007.25
Location
Speed (km/hr)

Delay Studies at Signalized Intersections:
Delay components due to traffic control device at an Intersection.

Delay Studies at Signalized Intersections:
The Highway Capacity Manual Method(HCM2000 ):
This method is based on direct observation of vehicles-in-queue at frequent
intervals. This method requires at least two observers.
To apply this method, the following conditions must be taken into
consideration:
1. The method is intended for undersaturated flow conditions, and for cases
where the maximum queue is about 20 to 25 vehicles.
2. The method does not directly measure acceleration/deceleration delay but
uses an adjustment factor to estimate this component.
3. The method also uses an adjustment to correct for errors that are likely to
occur in the sampling process.
4. Observers must make an estimate of free-flow speed before beginning a
detailed survey. This is done by driving a vehicle through the intersection
during periods when the light is green and there are no queues and/or by
measuring approach speeds at a position where they axe unaffected by the
signal.

Delay Studies: The HCM2000 Method
Observer 1:
 Keeps track of the end of standing queues for each cycle by observing the
last vehicle in each lane that stops due to the signal. This count includes
vehicles that arrive on green but stop or approach within one car length of
queued vehicles that have not yet started to move.
 At intervals between 10 s and 20 s, the number of vehicles in queue are
recorded on the field sheet. The regular intervals for these observations
should be an integral divisor of the cycle length. Vehicles in queue are
those that are included in the queue of stopping vehicles (as defined
above) and have not yet exited the intersection. For through vehicles,
“exiting the intersection” occurs when the rear wheels cross the STOP
line; for turning vehicles, “exiting” occurs when the vehicle clears the
opposing vehicular or pedestrian flow to which it must yield and begins to
accelerate.

Observer 2:
 During the entire study period, separate counts are maintained of vehicles
arriving during the survey period and of vehicles that stop one or more
times during the survey period. Stopping vehicles are counted only once,
regardless of how many times they stop.

HCM2000 Worksheet (see Attachment):

Calculations:
1. Sum each column of vehicle-in-queue counts, then sum the column totals
for the entire survey period.
2. Estimate the average time-in-queue per vehicle arriving during the survey
period is estimated using the following equation:
3. Calculate number of vehicles stopped per lane per cycle :
Vstop- Number of stopped vehicles, Nc- Number of cycles, N- Number of lanes.

Calculations (cont.) :
4. Calculate the fraction of the stopped vehicles, FVS = Vstop/VT
5- Then, estimate the Acceleration /Deceleration correction delay:
CF is a correction factor that can be obtained from EXHIBIT A16-2 using number of
vehicles stopped per lane per cycle calculated in Step 3
6- Then, the Average Total Control Delay (Sec/Veh) will be:

Example:
For the shown delay study
worksheet, estimate the average
delay per vehicle.

Tteng 441 traffic engineering fall 2021 part3

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