Value of Travel Time Reliability in Transportation Decision Making: Proof of Concept—Portland, Oregon, Metro (2014) / Chapter Skim
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14 C h a p t e r 3 3.1 Definition of travel time reliability According to a recent survey (Cambridge Systematics 2013b) , there are two fundamental definitions of travel time reliability: 1.
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15 independent, and then both mean and variance of travel time are additive, as shown in Equations 3.1 and 3.2: ∑µ = µ ∈ (3.1)
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16 The user costs now possess two terms. One term is the usual cost of travel time measured by VOTT multiplied by the mean value of travel time µ, and the other term is the cost of travel time variability, measured by VTTR multiplied by the standard deviation of travel time s.
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17 where FFT is free-flow time and v/c is the ratio of volume divided by capacity. • Eliasson (2009)
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18 RR can then be calculated as 1 2 α α because RR is calculated as the ratio of VOTT to VTTR, and both VOTT and VTTR are calculated with respect to a cost variable, which is intentionally left out in this utility function. Mathematically, the coefficient of the cost term is canceled out when calculating RR.
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19 After the workshop, the research team exported and processed the collected data into the format needed for the model estimation software. The study initially used the easy logit model developed by Frank Koppelman, an emeritus Northwestern University professor in transportation econometrics.
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20 3. Dispersion is consistent and measurable using the proposed SHRP 2 L03 approach (Cambridge Systematics 2013a)
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21 (work, peak hour; nonwork, peak hour; and off-peak hour) were chosen to simplify the model estimation and avoid a long questionnaire.
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22 3.5.2 Model Estimation Results The online survey was active from September 20 to 27, 2013. In total, 36 members of the Metro staff responded to the questionnaire; 34 responses to the automobile survey and 24 responses to the transit survey were completed.
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23 within the major range of 0.3 to 0.9 reported by that study, as shown in Table 3.6. The RR value for transit in the Dutch study (Significance et al.
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24 variable. TTI is defined as the ratio of the actual travel time to the travel time under free-flow conditions, as shown by Equation 3.9: =TTI actual travel time free flow travel time (3.9)
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25 If incident management programs have been added as a strategy or if a strategy lowers the incident rate (i.e., frequency of occurrence) , then the "after" delay is calculated as shown by Equation 3.20: p p1 1 (3.20)
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26 value of travel time over the average that accounts for how travelers value reliability. How the TDM, which was calibrated using average travel time, will behave with this inflated travel time value is unknown and was the subject of testing in this project.
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