Chapter 9: Speed, travel time, and
delay studies
Chapter objectives: By the end of these chapters the student will be
able to (we spend 2 lecture periods for this chapter):

Explain when speed, travel time, and delay studies are needed

Determine how many samples are needed
Collect and reduce speed data
Compute descriptive statistics
Apply a Chi-square test to speed data
Conduct a before and after study

Collect and reduce travel time data



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Explain the types of delays experienced at signalized intersections
Collect and reduce intersection delay data
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9.1 Introduction
They are used to evaluate the performance of a traffic
facility, like arterials and signalized intersections. “Speed”
here is a so-called a spot speed measured by a radar gun,
etc., at a point in the facility. If you determine travel time
and compute speed for a relatively long section, then it is
typically a space speed.
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9.2.2. Uses of spot speed data
Spot speed studies are conducted to estimate the
distribution of speeds of vehicles in a stream of traffic
at a particular location on a highway.
Used for:
 Establish the effectiveness of new or existing speed limits and/or
enforcement practices
 Establish trends to assess the effectiveness of national policy on
speed limits and enforcement
Specific design applications (like sight distance)
 Specific control applications (yellow/all red timing – the size of
dilemma zone depends on speed)
 Investigation of high-accident locations at which speed is suspected
to be a causative factor
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9.2 Spot Speed Studies
Once data are collected, the first thing you do is to compute
several descriptive statistics to get some ideas about the
distribution of the speed data. (Note that many statistical analyses
used in traffic engineering assume data are normally distributed.
 So, the goal is to check whether they are really normally
distributed.)
Typical descriptive statistics are:
 Average speed
 Variance and standard deviation
(These concepts
appeared in
chapter 7.)
 Median speed
 Modal speed (or Modal speed range  Needs a histogram)
 The ith-percentile spot speed
 Pace  Usually a 10-mph interval that has the greatest
number of observations.
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9.2.1 and 9.2.4 Speed definitions of interest
Average speed
Speed data
Not grouped
Grouped
u = uj/N
Standard
deviation
Speed data
Not grouped
Grouped
s=
f(ui – u)2
N-1
Variance
s2
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Median speed
The speed at the middle value in a series of spot
speeds. Or, 50th-percentile speed
Modal speed
The speed value that occurs most frequently in a
sample of speeds
ith-percentile
speed
The spot speed below which i percent of the
vehicles travel, e.g. 85th-percentile speed
Pace
The range of speed that has the greatest number
of observations; usually 10-mph range
85%
50%
Chapter 9
See table
9.1.
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9.2.3 Measurement Techniques
Microwave sensor
Wire switches
Read p. 207 for measurement issues
(parallax, accuracy, etc.)
Radar Gun
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Radar Guns and Cosine Error
Chapter 3, Manual of
Transportation Engineering
Studies, ITE, 2000.
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Speed Sample Size
For percentile speed comparisons
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9.2.5 Location, time of day, and duration…
The objective and scope of the study dictate these.
Basic data collection
Like deciding speed limits  Find
locations where system characteristics
change and TWTh
Speed trend analyses
Avoid external influences such as
traffic lights, busy access roads; offpeak, TWTh  mid blocks of streets,
straight,level sections of highways
All other specific purposes  Conduct
Specific traffic
engineering problems it at the location of interest and time
of day
At least 1 hour or at least 30 data (if you want to assume normal
distribution)
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p.216 Before and After Spot Speed Studies

Two questions that must be answered:
 Is the observed reduction in average speeds real? – checked
by comparison of the means test (Was there any statistical
difference?) – Use the Simple t-test
 Is the observed reduction in average speeds the intended 5
mph (or whatever the value expected) – checked by the
confidence interval to see if the targeted speed is within the
confidence interval of the after speed value (Was the goal
achieved? Use only the results of the “after” distribution.)
Review 7.8.2.
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9.3 Travel-time (travel delay) studies
* Determines the amount of time required to travel from one
point to another on a given route. Often, information may
also be collected on the locations, durations, and causes of
delays
•Indicate the level of service
•Identify problem locations
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Applications of travel time and delay data
Many uses…
Efficiency check
Collection of
rating data
Problem location
identification
Model calibration
Evaluation of
performance before
and after improvement
(like ramp metering vs.
no metering)
Chapter 9
Collect data for
economic analysis
(user costs)
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9.3.1 Field study techniques
Methods requiring a test vehicle:
 t / 2, N 1   
N 

d


2
Floating-car
technique
The test car “floats” with the traffic. Attempts to
pass as many vehicles as those that pass the test
vehicle. (More or less average travel time. Meant for
2-lane 2-way highways. Difficult on multilane
highways)
Average-speed
technique
Drive the test car at a speed that, in the opinion of
the driver, is the average speed of the traffic stream
(Get average travel time and less stressful)
Maximum-speed
technique
Drive as fast as is safely practical in the traffic
stream without ever exceeding the design speed of
the facility. (About 85th percentile speed, meaning
15th percentile travel time)
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Methods not requiring a test vehicle:
License-plate
observation
Each observer located at strategic points record last
3 or 4 digits of license plates. Need to synchronize
the observer’s watch.
Interviews
Ask the drivers!
License-plate method  NG for a grid
network – too many routes
Use of ETC
for freeways
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Chapter 9
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Running times may be distributed normally but travel times may not be
distributed normally because of stopped delays that may follow entirely a
different distribution. (Note that this figure 9.5 and table 9.4 are talking
about two different data sets.)
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Use of travel time data:
Relation between ideal flow
and actual flow
• A travel time contour as an
example
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9.4 Intersection Delay Studies
Travel time and delay studies take delays at
signalized intersections but such samples
(observations) are not adequate to evaluate the
delay at a particular signalized intersection. 
Conduct an intersection delay study.
Measure of effectiveness  Delay
(Current HCM2000 uses the “average control delay”, but
previous one used the “average stopped delay.”
The average control delay: the total delay at an
intersection caused by a control device,
including both time-in-queue delay plus delays
due to acceleration and deceleration. (See HCM
method notes in pages 228 and 229.
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Delay Types





Stopped-time delay - completely stopped
Approach delay – Adds delays by acceleration and
deceleration to stopped-time delay
Time-in-queue delay = (time to cross the stop line)
– (time when joined a queue), meaning in between
the vehicle is in stop-and-go state. (Remember how
queued vehicles are released.)
Control delay = (time-in-queue delay) + (accel/decel
delay)
Travel-time delay = (actual travel time) – (desired
travel time)
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4 assumptions made for control
delay field measurements (p.228)
1.
2.
3.
4.
5.
Meant for undersaturated flow conditions (max queue is
about 20 to 25 vehicles.
Does not directly measure acceleration-deceleration
delay. Use an adjustment factor to estimate this
component
Uses an adjustment to correct for errors that are likely to
occur in the sampling process
Must make an estimate of free-flow speed before
beginning a detailed survey (drive through the approach
before collecting data).
Start at the beginning of the red phase of the subject
lane group. No overflow queue should exist from the
previous green phase (ideally).
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HCM 2000: Intersection delay study method (by lane group)
Task 1: 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
includes vehicles that arrive on green but stop or approach within one car
length of queued vehicles that have not yet started to move.
See Fig. 9-11 for a sample
survey form and Tab 9-7
for a sample data.
Observer 1
Observer 2
Task 2: Record the number of vehicles in
queue on the field sheet (at the end of the
interval). Vehicles in queue are those that
are included in the queue of stopping
vehicles (as defined in Task 1) and have
not yet exited the intersection.
Count arriving vehicles & the
number of vehicles that stops
once or more. Stopping
vehicles are counted only
once, regardless of how many
times they stop.
The count interval is typically between 10 sec and 20 sec. It should be an
integral divisor of the cycle length. Task 3: At the end the survey period,
vehicle-in-queue counts continue until all vehicles that entered the queue during
the survey period have exited the intersections. (Observer 1)
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Delay data
collection form
(fig 9.11)
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Average time-in-queue estimation
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Adjustments for accel/decel delay
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731-733
Manual Method
733-735
735-737
100%
737-739
739-741
Percent Deviation
80%
741-743
743-745
60%
730-732
40%
732-734
734-736
20%
736-738
0%
738-740
740-742
-20%
742-744
Ave
-40%
0
5
10
15
20
Time Increment
Ave+std
Ave-STD
10 second interval seems to be the best compromise
according to my study with Jay Walker.
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