Chapter 9 1

Chapter 9: Speed, travel time, and delay studies

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

Explain the types of delays experienced at signalized intersections Collect and reduce intersection delay data

Chapter objectives: By the end of these chapters the student will be able to (we spend 2 lecture periods for this chapter):

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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

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.

(These concepts appeared in chapter 7.)

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Average speed

Speed data Grouped Not grouped

Standard deviation

Speed data Grouped Not grouped

Variance s2

u = uj/N

s = f(ui – u)2

N - 1

9.2.1 and 9.2.4 Speed definitions of interest

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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

50%

85%

See table 9.1.

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9.2.3 Measurement Techniques

Wire switches Microwave sensor

Radar GunRead p. 207 for measurement issues (parallax, accuracy, etc.)

Radar Guns and Cosine Error

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Chapter 3, Manual of Transportation Engineering Studies, ITE, 2000.

Speed Sample Size

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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; off-peak, TWTh mid blocks of streets, straight,level sections of highways

Specific traffic engineering problems

All other specific purposes Conduct 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

Problem location identification

Evaluation of performance before and after improvement

Collection of rating data

Model calibration

Collect data for economic analysis (user costs)

(like ramp metering vs. no metering)

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9.3.1 Field study techniques

Methods requiring a test vehicle:

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)

2

1,2/

d

tN N

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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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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:

• A travel time contour as an example

Relation between ideal flow and actual flow

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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. Meant for undersaturated flow conditions (max queue is about 20 to 25 vehicles.

2. Does not directly measure acceleration-deceleration delay. Use an adjustment factor to estimate this component

3. Uses an adjustment to correct for errors that are likely to occur in the sampling process

4. Must make an estimate of free-flow speed before beginning a detailed survey (drive through the approach before collecting data).

5. 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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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.

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.

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.

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)

See Fig. 9-11 for a sample survey form and Tab 9-7 for a sample data.

Observer 1 Observer 2

HCM 2000: Intersection delay study method (by lane group)

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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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Manual Method

-40%

-20%

0%

20%

40%

60%

80%

100%

0 5 10 15 20

Time Increment

Pe

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nt

De

via

tio

n

731-733

733-735

735-737

737-739

739-741

741-743

743-745

730-732

732-734

734-736

736-738

738-740

740-742

742-744

Ave

Ave+std

Ave-STD

10 second interval seems to be the best compromise according to my study with Jay Walker.