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									Evaluation of Freeway Travel Time Estimates


          INTERIM REPORT


      Experimental Design & Final Work Plan




                    Draft


                May 6, 2005
Evaluation of Travel Time Estimates



Table of Contents

1.0      INTRODUCTION .......................................................................................................................... 3
2.0      PRELIMINARY ANALYSIS........................................................................................................ 3
3.0      EXPERIMENTAL DESIGN ......................................................................................................... 6
   3.1       SAMPLE SIZE ANALYSIS ............................................................................................................... 6
   3.2       REQUIRED NUMBER OF RUNS....................................................................................................... 8
   3.3       DATA COLLECTION PLAN............................................................................................................. 8
4.0      WORK PLAN ................................................................................................................................. 9
   4.1       REVISED SCHEDULE ....................................................................................................................11




Experimental Design and Final Work Plan                                 2
Evaluation of Travel Time Estimates




1.0 INTRODUCTION

This document serves as an interim project report for the ODOT evaluation of freeway
travel time project, and includes the following project deliverables:

          Task 7.2.3 Deliverable – statistically valid design; and
          Task 7.2.4 Deliverable – Final Work Plan.

Portland State University (PSU) has performed preliminary data collection and
subsequently designed a statistically valid experiment whereby probe vehicles will travel
along the freeway corridors collecting GPS location data at defined intervals. The
experiment will compare the observed travel times for probe vehicles to travel times
predicted by ODOT’s Traffic Management Operations Center (TMOC) in Region 1.

Following this brief introduction, the remainder of this document will summarize the
statistically valid design and define the schedule for completing the remainder of the
project tasks.


2.0 PRELIMINARY ANALYSIS

The preliminary analysis portion of this project was intended to collect and analyze an
initial set of data to enable the project team to develop a statistically valid Work Plan for
the remainder of the project. This project has identified 18 directional freeway links for
analysis, although it is understood that maintenance or construction activities may create
a situation where one or more links is not a valid source for data within this project
timeline. These freeway links are shown in Table 1 (presented later in this document).

The TransPort PORTAL data archive developed at PSU ingests and archives the 20-
second loop detector data provided by ODOT. As part of a significant effort earlier in this
project, PSU customized an area of PORTAL to compute the travel time on each of the
18 identified freeway segments using the exact travel time algorithm that is used within
the ODOT TMOC. This application can produced travel time estimates used in the
TMOC either using a traditional midpoint segment method or using the modified
midpoint method currently used by ODOT. A screen shot of this function is shown
below in Figure 1. Note that this feature is currently only available in the testing area of
PORTAL.




Experimental Design and Final Work Plan        3
Evaluation of Travel Time Estimates




                       Figure 1PORTAL Travel Time Measurement Interface

As a next step in the research project, PSU collected one probe vehicle run on each of the
18 directional freeway links for analysis. The data from one probe vehicle run was
analyzed on the section of I-5 NB from Wilsonville to I-405. Figure 1 shows the probe
vehicle trajectory as well as the travel times computed by PORTAL at the same time. In
Figure 2, the probe vehicle trajectory is shown as the dark solid line and the predicted
travel time for PORTAL at the time the vehicle entered the link is shown for directional
freeway links. Note that the scale on the x-axis represents the time of day and is
presented in minutes. In Figure 3, the total estimated travel time, calculated by summing
the estimated PORTAL travel times for each directional link at the time the probe vehicle
enters the first link is shown. In addition, travel time estimates for 3 minutes before and 3
minutes after are shown as dashed lines. In later reports, similar graphs will be prepared
for each of the probe vehicle runs in order to demonstrate the comparison of the probe
vehicles to the PORTAL or TMOC travel time estimates.




Experimental Design and Final Work Plan       4
Evaluation of Travel Time Estimates




                                             I-5 Northbound 02/18/2005: Trial Run Versus Detector Data


             300



                               Trial
             298
                               PORTAL: Link 1
                               PORTAL: Link 2
                               PORTAL: Link 3
             296
                                                                                                                     217 Interchange to I-405 Interchange



             294
  Milepost




             292



             290
                                                                                                                     I-205 Interchange to 217 Interchange



             288

                                                                                                                        Wilsonville to I-205 Interchange

             286
               14:59   15:00    15:01    15:02   15:03   15:04   15:05   15:06   15:07   15:08       15:09   15:10   15:11   15:12   15:13    15:14   15:15
                                                                                 Time (PST)


Figure 2 Sample Probe Vehicle Trajectory and PORTAL Travel Time Estimates

                                        I-5 Northbound 02/18/2005: Trial Run Versus Hypothetical Trajectories


             300

                                Trial
             298
                                PORTAL



             296



             294
  Milepost




                                                                                                                      217 Interchange to I-405 Interchange

             292



             290
                                                                                                                      I-205 Interchange to 217 Interchange



             288

                                                                                                                        Wilsonville to I-205 Interchange

             286
               14:56 14:57 14:58 14:59 15:00 15:01 15:02 15:03 15:04 15:05 15:06 15:07 15:08 15:09 15:10 15:11 15:12 15:13 15:14 15:15
                                                                                 Time (PST)


                       Figure 3Probe Vehicle Trajectory and PORTAL Estimated Travel Times

As background for developing the data collection plan and sample size analysis, PSU
prepared scatter plots of one week (April 4- April 8) of predicted travel times from the
PORTAL data for each of 18 freeway links to identify travel time trends. These data were
used to estimate sample size and develop a data collection plan and times that will be able
to capture the travel conditions required. A sample of these plots is shown in Figure 4.



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Evaluation of Travel Time Estimates




                                    217 Southbound: US26 Interchange to I-5 Interchange [Link 9]

                 60




                 50




                                                                                                                    Cumulative Travel Time
                 40
  Travel Times




                 30




                 20




                 10




                     0
                      0:00   2:00   4:00   6:00    8:00   10:00   12:00   14:00   16:00   18:00   20:00   22:00   0:00
                                                                  Time

                              Figure 4 Scatter Plot of Travel Times, PORTAL April 4-8, 2005


3.0 EXPERIMENTAL DESIGN

The intent of the experimental design task was to define the number of data collection
runs, the number of days of data collection, and probe vehicle headways required to
complete sufficient data collection for a statistically valid study. The experiment has
been designed to meet these goals:

                       Statistical comparison - using the probe vehicle data as “ground truth”, the
                        experiment will produce estimates of the statistical bounds on the validity of the
                        travel time estimates produced by the TMOC method.
                       Provide recommendations for improving travel time algorithms and estimates.
                       Determine the effect of added instrumentation - a survey of existing loop detector
                        stations will be made along with an assessment of bottleneck locations in order to
                        conduct sensitivity analyses toward estimating the potential for improving travel
                        time estimates.

3.1                   Sample Size Analysis

For a travel time study, a minimum sample size is desired to minimize the data collection
cost in order the fit with the budgetary constraints. However, output from this sample size
determination is also a valuable resource to statistically ensure the level of confidence
and reliability of such data. Therefore, it is important to execute a number of travel time


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Evaluation of Travel Time Estimates



collection runs to determine a statistically permitted level of error from the sample size.
In general, the statistical estimation for the sample size n is based on specifying the
probability statements about level of confidence that the error is most acceptable. The
permitted error E is expressed as
            
 E  Z / 2
             n
where
        n = minimum sample size
        Zα/2 = standard normal curve area to its right equals α/2 for a confidence level of 1
        -α
        σ = standard deviation of population
        E = maximum error of the estimation

Often the estimation is done based on prior information or an initial sample which led to
a random variable having a t-distribution with n – 1 degrees of freedom. At the same
level of confidence of (1 - α)100%, the new equation is written upon solving for n as
     
              2

n   t 
     E
where
        s = estimate standard deviation of random samples
        t α = t distribution statistic (used instead of Z α/2 when dealing with random
        samples or small sample size)
        E = maximum error of the estimation

For this research, estimates of the mean travel speed (equivalent to travel time) were
made and the standard deviation of the travel speed was calculated based on link length
and travel time. This was done for one hour in the AM peak 7:30AM to 8:30AM and one
hour in the PM peak 4:30PM to 5:30PM. These hours were chosen because they were the
most likely times PSU would be collecting travel time information where travel was
transitioning from free-flowing to congestion. This was calculated for both the 20 second
data and the travel times aggregated to 5 minute averages. The allowable error, E,
recommended by ITE for travel time studies is between 3 and 5 mi/h. For example, on
link 3 in AM period

Mean speed = 37.42 mi/h
Standard Deviation = 4.82 mi/h
α = 0.05 (corresponds to 95% level of confidence)
E = ± 3 mi/h

Since the statistic t α is a function of n, an iterative procedure is needed to solve for n.
As a result, n ≈ 5 runs.




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Evaluation of Travel Time Estimates



3.2    Required Number of Runs

Based on the analysis of PORTAL data described above, the number of runs required on
each of the directional links to make a statistically valid comparison of travel times at two
statistical confidence levels (90% and 95%) was calculated. Only the 95% level is
reported in table below. For segments that require more probe runs, PSU will recalculate
the number of needed runs based on the travel time handbook and based on what the ITE
Manual on traffic engineering studies recommends


Freew                Freeway Link Name                 AM/PM   Required Number of   Planned
  ay                                                            Samples @ 95%       Number
 Link                                                                                  of
                                                               +-3 MPH   +-5 MPH
                                                                                    Samples
   1       Wilsonville to I-205 Interchange             PM       15+        10       None
   2       I-205 Interchange to 217 Interchange         AM       10         5          5
   3       217 Interchange to I-405 Interchange         AM        9         5          5
   3       217 Interchange to I-405 Interchange         PM       15+        8          8
   4       I-84 Interchange to Interstate Bridge        PM        5         3          5
   5       Interstate Bridge to I-84 Interchange        PM       11         5          5
   5       Interstate Bridge to I-84 Interchange        AM        6         4          5
   6       I-405 Interchange to 217 Interchange         PM       10         5          5
   6       I-405 Interchange to 217 Interchange         AM       15+        10        10*
   7       217 Interchange to I-205 Interchange         PM       15+        9         10*
   8       I-5 Interchange to US 26 Interchange         AM        4         3          5
   9       US 26 Interchange to I-5 Interchange         PM        7         4          5
  10       I-5 Interchange to Oregon City               PM       15         7          5
  11       Oregon City to I-84 Interchange              PM        9         5          5
  12       I-84 Interchange to Oregon City              PM        4         3          5
  13       I-5 Interchange to I-205 Interchange         PM        6         4          5
  14       I-205 Interchange to I-5 Interchange         AM       15+       15+        10*
  15       Helvetia to 217 Interchange                  AM       14         7         As
                                                                                    needed
  16       217 Interchange to I-405                     AM        4         3          5
  17       I-405 Interchange to 217 Interchange         PM       10         5          5
  18       217 Interchange to Helvetia                  PM       15+       15+        As
                                                                                    needed
       Table 1 Computed Number of Runs Required for the Statistically Valid Sample

3.3    Data Collection Plan

The data collection routes were designed to minimize wasted travel and collect the
required number of samples for each link. The intent will be to collect data during the
transition from freely-flowing to congested conditions, and from congested to freely-



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Evaluation of Travel Time Estimates



flowing conditions, if feasible.           In accordance with the Federal Highway
Administration’s Travel Time Data Collection Handbook,
[http://www.fhwa.dot.gov/ohim/start.pdf] drivers will follow standard probe vehicle
instructions, so that the data will represent a “typical” driver.

Prior to the beginning of extensive data collection, PSU will contact Region 1 staff to
ensure that construction projects are not interfering with the loop detector system.


Route      Start       Path                                          Number of            Headway
                                                                     vehicles             between Probes
   1         AM        I-84 WB from the I-84/I-205 jct. to I-5 jct   5 vehicles / 1 run   5-7
                       then I-5 SB to OR217 NB to US26 EB
   2         AM        I-5 SB from Interstate Bridge to I-405        5 vehicles / 1 run   5-7
                       loop and returning I-5NB to Interstate
                       Bridge
   3          PM       US-26WB from PSU to OR-217 SB to I-5          5 vehicles / 1 run   5-7
                       SB to the Stafford Rd interchange
                       returning on I-5 NB to Interstate Bridge
   4          PM       I-84EB from PSU to I-205SB to I-5 jct to      5 vehicles / 1 run   5-7
                       I-5 SB to the Stafford Road interchange I-
                       5 NB to I-205NB
   5          PM       US26 WB from OR217 to Helvetia exit           2 vehicles / 2 run   10
                                                                     each + 1 run
   6         AM        US26 EB from Helvetia to OR217                2 vehicles / 2 run   10
                                                                     each + 1 run
                                      Table 2 Data Collection Routes


4.0 WORK PLAN

This section describes the remaining project tasks and presents the schedule for
completing the remaining project activities.


4.1      Task 7.2.5: Probe Vehicle Data Collection

Efforts in Task 7.2.5 will collect the probe vehicle data defined in the data collection
plan. The following table (Table 3) presents the data collection plan for collecting the
route information defined in Table 2 above.

        Time Period for Collection                                 Data Collection Routes
         May 2, 2005 – May 6, 2005                                   Route 1 and Route 3
        May 9, 2005 – May 13, 2005                                   Route 2 and Route 4
        May 16, 2005 – May 20, 2005                                  Route 5 and Route 6
        May 16, 2005 – May 20, 2005                             Data Assembly and Presentation


4.2      Task 7.2.6: Loop Detector Data Analysis


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Evaluation of Travel Time Estimates




Using the TransPort Portal data archive developed at PSU, the 20-second loop detector data
will be processed for the same days, times and corridors as the probe vehicle data as
established in the work plan. Using sound techniques for assigning influence areas to each
detector site, travel times will be determined at 20-second level of resolution. Efforts in this
task will compare travel times calculated here with travel times calculated using the same
algorithm as the TMOC.


Data Comparison:
Using the results of the data analyses conducted, a statistical comparison will be developed
in order to reveal the reliability of the travel times calculated from the loop detector data
compared to the actual probe vehicle travel times. Using the probe vehicle data as “ground
truth”, the Contractor will provide statistical bounds on the validity of the travel time
calculations using the probe vehicle method and the TMOC method. The Contractor will
provide recommendations for improving travel time algorithms and estimates.

Effect of Added Instrumentation:
Using the results from previous tasks, a survey of existing loop detector stations will be
made along with an assessment of bottleneck locations. The Contractor will analyze the
potential for increasing data gathering instrumentation, and will conduct sensitivity analyses
toward estimating the potential for improving travel time estimates.


Efforts related to Task 7.2.6 will be conducted over the period of May 16, 2005 to June 13,
2005.


4.3      Task 7.2.7: Final Report Preparation


Efforts in Task 7.2.7 will prepare a Final Report of project findings. This report will be
prepared in draft form for ODOT review, and a Final version of the report will address any
feedback received from ODOT.


Efforts in Task 7.2.7 will be performed over the period of June 13, 2005 to June 30, 2005.




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Evaluation of Travel Time Estimates




4.1    Revised Schedule

The following schedule shows how the Contractor will complete the data gathering,
analysis, and evaluation before June 30th, 2005, as required by the contract.




Experimental Design and Final Work Plan   11

								
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