| 1. |
- Batley, Richard, et al.
(författare)
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New appraisal values of travel time saving and reliability in Great Britain
- 2019
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Ingår i: Transportation. - : Springer. - 0049-4488 .- 1572-9435. ; 46:3, s. 583-621
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Tidskriftsartikel (refereegranskat)abstract
- This paper provides an overview of the study Provision of market research for value of time savings and reliability' undertaken by the Arup/ITS Leeds/Accent consortium for the UK Department for Transport (DfT). The paper summarises recommendations for revised national average values of in-vehicle travel time savings, reliability and time-related quality (e.g. crowding and congestion), which were developed using willingness-to-pay (WTP) methods, for a range of modes, and covering both business and non-work travel purposes. The paper examines variation in these values by characteristics of the traveller and trip, and offers insights into the uncertainties around the values, especially through the calculation of confidence intervals. With regards to non-work, our recommendations entail an increase of around 50% in values for commute, but a reduction of around 25% for other non-workrelative to previous DfT WebTAG' guidance. With regards to business, our recommendations are based on WTP, and thus represent a methodological shift away from the cost saving approach (CSA) traditionally used in WebTAG. These WTP-based business values show marked variation by distance; for trips of less than 20miles, values are around 75% lower than previous WebTAG values; for trips of around 100miles, WTP-based values are comparable to previous WebTAG; and for longer trips still, WTP-based values exceed those previously in WebTAG.
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| 2. |
- Daly, Andrew, et al.
(författare)
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Calculating errors for measures derived from choice modelling estimates
- 2012
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Ingår i: Transportation Research Part B. - : Elsevier BV. - 0191-2615 .- 1879-2367. ; 46:2, s. 333-341
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Tidskriftsartikel (refereegranskat)abstract
- The calibration of choice models produces a set of parameter estimates and an associated covariance matrix, usually based on maximum likelihood estimation. However, in many cases, the values of interest to analysts are in fact functions of these parameters rather than the parameters themselves. It is thus also crucial to have a measure of variance for these derived quantities and it is preferable that this can be guaranteed to have the maximum likelihood properties, such as minimum variance. While the calculation of standard errors using the Delta method has been described for a number of such measures in the literature, including the ratio of two parameters, these results are often seen to be approximate calculations and do not claim maximum likelihood properties. In this paper, we show that many measures commonly used in transport studies and elsewhere are themselves maximum likelihood estimates and that the standard errors are thus exact, a point we illustrate for a substantial number of commonly used functions. We also discuss less appropriate methods, notably highlighting the issues with using simulation for obtaining the variance of a function of estimates.
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| 3. |
- Hess, Stephane, et al.
(författare)
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Understanding attitudes towards congestion pricing : a latent variable investigation with data from four cities
- 2019
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Ingår i: Transportation letters. - : TAYLOR & FRANCIS LTD. - 1942-7867 .- 1942-7875. ; 11:2, s. 63-77
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Tidskriftsartikel (refereegranskat)abstract
- Numerous cities around the world are considering the implementation of road pricing to ease urban traffic congestion, following on from the success in cities such as London and Singapore. However, policy-makers are also all too aware of the generally negative public opinion toward such measures. This study makes use of data collected in four cities (two in Sweden, one in Finland, and one in France) using a very consistent survey probing for citizens' attitudes toward pricing. We find very strong similarities across the four cities in terms of a number of underlying attitudinal constructs that help explain people's answers in a hypothetical referendum on congestion pricing. The similarities across cities indicate that the increase in the opinion toward congestion pricing once they are introduced is not primarily an effect of changes in underlying attitudes, changes in how the underlying attitudes influence the support for congestion pricing, or differences in anticipated versus experienced or perceived self-interest. Instead, this effect seems to be caused by a status quo acceptance, tending to increase the support for the current situation.
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| 4. |
- Lācis, Uǧis, 1988-, et al.
(författare)
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Nanoscale sheared droplet : volume-of-fluid, phase-field and no-slip molecular dynamics
- 2022
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 940
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Tidskriftsartikel (refereegranskat)abstract
- The motion of the three-phase contact line between two immiscible fluids and a solid surface arises in a variety of wetting phenomena and technological applications. One challenge in continuum theory is the effective representation of molecular motion close to the contact line. Here, we characterize the molecular processes of the moving contact line to assess the accuracy of two different continuum two-phase models. Specifically, molecular dynamics simulations of a two-dimensional droplet between two moving plates are used to create reference data for different capillary numbers and contact angles. We use a simple-point-charge/extended water model. This model provides a very small slip and a more realistic representation of the molecular physics than Lennard-Jones models. The Cahn-Hilliard phase-field model and the volume-of-fluid model are calibrated against the drop displacement from molecular dynamics reference data. It is shown that the calibrated continuum models can accurately capture droplet displacement and droplet break-up for different capillary numbers and contact angles. However, we also observe differences between continuum and atomistic simulations in describing the transient and unsteady droplet behaviour, in particular, close to dynamical wetting transitions. The molecular dynamics of the sheared droplet provide insight into the line friction experienced by the advancing and receding contact lines. The presented results will serve as a stepping stone towards developing accurate continuum models for nanoscale hydrodynamics.
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| 5. |
- Lacis, Ugis, 1988-, et al.
(författare)
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Steady moving contact line of water over a no-slip substrate
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The movement of the triple contact line plays a crucial role inmany applications such as ink-jet printing, liquid coating and drainage(imbibition) in porous media. To design accurate computational toolsfor these applications, predictive models of the moving contact line areneeded. However, the basic mechanisms responsible for movement ofthe triple contact line are not well understood but still debated. We investigatethe movement of the contact line between water, vapour anda silica-like solid surface under steady conditions in low capillary numberregime. We use molecular dynamics (MD) with an atomistic watermodel to simulate a nanoscopic drop between two moving plates. Weinclude hydrogen bonding between the water molecules and the solidsubstrate, which leads to a sub-molecular slip length. We benchmarktwo continuum methods, the Cahn{Hilliard phase-eld (PF) model anda volume-of-uid (VOF) model, against MD results.We show that bothcontinuum models reproduce the statistical measures obtained fromMD reasonably well, with a trade-o in accuracy. We demonstrate theimportance of the phase-eld mobility parameter and the local sliplength in accurately modelling the moving contact line.
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| 6. |
- Lacis, Ugis, 1988-, et al.
(författare)
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Steady moving contact line of water over a no-slip substrate Challenges in benchmarking phase-field and volume-of-fluid methods against molecular dynamics simulations
- 2020
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Ingår i: The European Physical Journal Special Topics. - : Springer Nature. - 1951-6355 .- 1951-6401. ; 229:10, s. 1897-1921
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Tidskriftsartikel (refereegranskat)abstract
- The movement of the triple contact line plays a crucial role in many applications such as ink-jet printing, liquid coating and drainage (imbibition) in porous media. To design accurate computational tools for these applications, predictive models of the moving contact line are needed. However, the basic mechanisms responsible for movement of the triple contact line are not well understood but still debated. We investigate the movement of the contact line between water, vapour and a silica-like solid surface under steady conditions in low capillary number regime. We use molecular dynamics (MD) with an atomistic water model to simulate a nanoscopic drop between two moving plates. We include hydrogen bonding between the water molecules and the solid substrate, which leads to a sub-molecular slip length. We benchmark two continuum methods, the Cahn-Hilliard phase-field (PF) model and a volume-of-fluid (VOF) model, against MD results. We show that both continuum models reproduce the statistical measures obtained from MD reasonably well, with a trade-off in accuracy. We demonstrate the importance of the phase-field mobility parameter and the local slip length in accurately modelling the moving contact line.
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| 7. |
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