Publications by authors named "William J Frith"

20 Publications

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Rheology of protein-stabilised emulsion gels envisioned as composite networks. 2 - Framework for the study of emulsion gels.

J Colloid Interface Sci 2021 Jul 9;594:92-100. Epub 2021 Mar 9.

Unilever R& D Colworth, Sharnbrook, Bedford MK44 1LQ, UK. Electronic address:

Hypothesis: The aggregation of protein-stabilised emulsions leads to the formation of emulsion gels. These soft solids may be envisioned as droplet-filled matrices. Here however, it is assumed that protein-coated sub-micron droplets contribute to the network formation in a similar way to proteins. Emulsion gels are thus envisioned as composite networks made of proteins and droplets.

Experiments: Emulsion gels with a wide range of composition are prepared and their viscoelasticity and frequency dependence are measured. Their rheological behaviours are then analysed and compared with the properties of pure gels presented in the first part of this study.

Findings: When the concentrations of droplets and protein are expressed as an effective volume fraction, the rheological behaviour of emulsion gels is shown to depend mostly on the total volume fraction, while the composition of the gel indicates its level of similarity with either pure droplet gels or pure protein gels. These results help to form an emerging picture of protein-stabilised emulsion gel as intermediate between droplet and protein gels. This justifies a posteriori the hypothesis of composite networks, and opens the road for the formulation of emulsion gels with fine-tuned rheology.
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http://dx.doi.org/10.1016/j.jcis.2021.02.088DOI Listing
July 2021

Rheology of protein-stabilised emulsion gels envisioned as composite networks 1- Comparison of pure droplet gels and protein gels.

J Colloid Interface Sci 2020 Nov 5;579:878-887. Epub 2020 Jul 5.

Unilever R&D Colworth, Sharnbrook, Bedford MK44 1LQ, UK. Electronic address:

Hypothesis: Protein-stabilised emulsion gels can be studied in the theoretical framework of colloidal gels, because both protein assemblies and droplets may be considered as soft colloids. These particles differ in their nature, size and softness, and these differences may have an influence on the rheological properties of the gels they form.

Experiments: Pure gels made of milk proteins (sodium caseinate), or of sub-micron protein-stabilised droplets, were prepared by slow acidification of suspensions at various concentrations. Their microstructure was characterised, their viscoelasticity, both in the linear and non-linear regime, and their frequency dependence were measured, and the behaviour of the two types of gels was compared.

Findings: Protein gels and droplet gels were found to have broadly similar microstructure and rheological properties when compared at fixed volume fraction, a parameter derived from the study of the viscosity of the suspensions formed by proteins and by droplets. The viscoelasticity displayed a power law behaviour in concentration, as did the storage modulus in frequency. Additionally, strain hardening was found to occur at low concentration. These behaviours differed slightly between protein gels and droplet gels, showing that some specific properties of the primary colloidal particles play a role in the development of the rheological properties of the gels.
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http://dx.doi.org/10.1016/j.jcis.2020.05.004DOI Listing
November 2020

DNA-Coated Functional Oil Droplets.

Langmuir 2018 08 17;34(34):10073-10080. Epub 2018 Aug 17.

Optoelectronics Group, Cavendish Laboratory , University of Cambridge , J. J. Thomson Avenue , Cambridge CB3 0HE , U.K.

Many industrial soft materials include oil-in-water (O/W) emulsions at the core of their formulations. By using tuneable interface stabilizing agents, such emulsions can self-assemble into complex structures. DNA has been used for decades as a thermoresponsive, highly specific binding agent between hard and, recently, soft colloids. Up until now, emulsion droplets functionalized with DNA had relatively low coating densities and were expensive to scale up. Here, a general O/W DNA-coating method using functional nonionic amphiphilic block copolymers, both diblock and triblock, is presented. The hydrophilic poly(ethylene glycol) ends of the surfactants are functionalized with azides, allowing for efficient, dense, and controlled coupling of dibenzocyclooctane-functionalized DNA to the polymers through a strain-promoted alkyne-azide click reaction. The protocol is readily scalable due to the triblock's commercial availability. Different production methods (ultrasonication, microfluidics, and membrane emulsification) are used with different oils (hexadecane and silicone oil) to produce functional droplets in various size ranges (submicron, ∼20 and >50 μm), showcasing the generality of the protocol. Thermoreversible submicron emulsion gels, hierarchical "raspberry" droplets, and controlled droplet release from a flat DNA-coated surface are demonstrated. The emulsion stability and polydispersity is evaluated using dynamic light scattering and optical microscopy. The generality and simplicity of the method opens up new applications in soft matter, biotechnological research, and industrial advances.
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http://dx.doi.org/10.1021/acs.langmuir.8b01828DOI Listing
August 2018

Thermal Hysteresis and Seeding of Twisted Fibers Formed by Achiral Discotic Particles.

J Phys Chem B 2017 10 13;121(42):9920-9928. Epub 2017 Oct 13.

Materials and Engineering Research Institute, Sheffield Hallam University , Howard Street, Sheffield S1 1WB, United Kingdom.

In this paper, molecular dynamics simulations of simple disc-shaped particles are used to investigate the free self-assembly of defect-free fibers. Depending on the choice of particle shape and interaction strength, the formed fibers are reproducibly either straight or, for reasons of packing efficiency, spontaneously chiral. As they grow radially, increasing stresses cause chiral fibers to untwist either continuously or via morphological rearrangement. It is also found that, due to the kinetics of fiber initiation, the isotropic solution has to be significantly supercooled before aggregation takes place. As a result, the thermal hysteresis of one formed fiber extends to 13.9% of the formation temperature. In the presence of a three-thread seed cluster of 15 particles, however, monotonic fiber growth is observed 9.3% above the normal formation temperature. Thus, as in many experimental systems, it is the kinetic pathway, rather than the thermodynamic stability of the final assembly, that dominates the observed behavior.
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http://dx.doi.org/10.1021/acs.jpcb.7b05316DOI Listing
October 2017

Magnetically aligned supramolecular hydrogels.

Chemistry 2014 Dec 24;20(50):16484-7. Epub 2014 Oct 24.

Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD (UK).

The magnetic-field-induced alignment of the fibrillar structures present in an aqueous solution of a dipeptide gelator, and the subsequent retention of this alignment upon transformation to a hydrogel upon the addition of CaCl2 or upon a reduction in solution pH is reported. Utilising the switchable nature of the magnetic field coupled with the slow diffusion of CaCl2 , it is possible to precisely control the extent of anisotropy across a hydrogel, something that is generally very difficult to do using alternative methods. The approach is readily extended to other compounds that form viscous solutions at high pH. It is expected that this work will greatly expand the utility of such low-molecular-weight gelators (LMWG) in areas where alignment is key.
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http://dx.doi.org/10.1002/chem.201405500DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4497324PMC
December 2014

A microrheological study of hydrogel kinetics and micro-heterogeneity.

Eur Phys J E Soft Matter 2014 May 27;37(5):44. Epub 2014 May 27.

Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, UK,

The real-time dynamic heterogeneity of the gelation process of the amino acid derivative Fmoc-tyrosine (Fmoc-Y) is studied using particle tracking microrheology. To trigger gelation, glucono-δ-lactone (GdL) is added, which gradually lowers the p H over several hours. The onset of self-assembly in the system is signified by a sharp drop in the mean-squared displacement of embedded particles, a phenomenon that is found to correlate with the p H of the system reaching the pK(a) of Fmoc-Y. The gel point is identified and found to be dependent on the GdL concentration. Analysis of embedded probe particle dynamics allows the heterogeneity of the sample to be quantified, using three metrics: the heterogeneity ratio (HR), the non-Gaussian parameter of the van Hove correlation function (N and the bin distribution of the mean-squared displacement (MSD) of single particles (f(z)). Results from the three techniques are found to be approximately comparable, with increases in heterogeneity observed in all samples for incubation times t(w) = 0-3 hours. The final heterogeneity in all samples is found to be remarkably low compared to other systems previously reported in the literature.
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http://dx.doi.org/10.1140/epje/i2014-14044-yDOI Listing
May 2014

Microrheology and microstructure of Fmoc-derivative hydrogels.

Langmuir 2014 Apr 11;30(15):4483-92. Epub 2014 Apr 11.

Cavendish Laboratory, Department of Physics, University of Cambridge , JJ Thomson Avenue, Cambridge, CB3 0HE U.K.

The viscoelasticity of hydrogel networks formed from the low-molecular-weight hydrogelator Fmoc-tyrosine (Fmoc-Y) is probed using particle-tracking microrheology. Gelation is initiated by adding glucono-δ-lactone (GdL), which gradually lowers the pH with time, allowing the dynamic properties of gelation to be examined. Consecutive plots of probe particle mean square displacement (MSD) versus lag time τ are shown to be superimposable, demonstrating the formation of a self-similar hydrogel network through a percolation transition. The analysis of this superposition yields a gel time t(gel) = 43.4 ± 0.05 min and a critical relaxation exponent n(c) = 0.782 ± 0.007, which is close to the predicted value of 3/4 for semiflexible polymer networks. The generalized Stokes-Einstein relation is applied to the master curves to find the viscoelastic moduli of the critical gel over a wide frequency range, showing that the critical gel is structurally and rheologically fragile. The scaling of G'/G″ as ω(0.795±0.099) ≈ ω(3/4) at high frequencies provides further evidence for semiflexible behavior. Cryogenic scanning electron micrographs depict a loosely connected network close to the gel point with a fibrillar persistence length that is longer than the network mesh size, further indications of semiflexible behavior. The system reported here is one of a number of synthetic systems shown to exhibit semiflexible behavior and indicates the opportunity for further rheological study of other Fmoc derivatives.
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http://dx.doi.org/10.1021/la5005819DOI Listing
April 2014

The influence of the kinetics of self-assembly on the properties of dipeptide hydrogels.

Faraday Discuss 2013 ;166:101-16

We discuss the effect of the kinetics of pH change on the mechanical properties of dipeptide hydrogels. Data from other peptide-based low molecular weight gelator (LMWG) systems suggest that the rheological properties are often highly dependent on the assembly rate. To examine kinetics here, we have used the hydrolysis of glucono-8-lactone (GdL). The hydrolysis of GdL to gluconic acid results in a decrease in pH, the rate of which is temperature sensitive. Hence, we can adjust the rate of pH decrease, whilst achieving the same absolute final pH. Our data shows that at all temperatures the rheological profile is very similar, with an increase to a plateau, followed by a second increase in moduli, despite very different kinetics of assembly. Surprisingly, the final mechanical properties are very similar in all cases. We also show that the structures formed at the plateau can be accessed by adjusting the pH using CO2. By carefully balancing the pKa. of the gelator with the pH achievable using CO2, flexible hydrogel membranes can be formed as opposed to a bulk gel. The rheological characteristics of the membranes are typical of a highly entangled polymer network. These membranes can be rigidified by post-addition of GdL to further lower the pH.
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http://dx.doi.org/10.1039/c3fd00104kDOI Listing
March 2014

Mixed biopolymer aqueous solutions--phase behaviour and rheology.

Authors:
William J Frith

Adv Colloid Interface Sci 2010 Dec 18;161(1-2):48-60. Epub 2009 Aug 18.

Unilever Discover Colworth, Colworth House, Sharnbrook, Bedfordshire, MK44 1LQ, UK.

Mixed biopolymer solutions are found in many food systems and household products, and are also employed in industrial processes such as bio-separation and purification. They display a rich phase behaviour, ranging from association and precipitation to the more common segregative phase separation into two liquid phases. Understanding the underlying physics of their phase behaviour and of the rheology-morphology relationships of the resulting phases is a topic of interest and importance in terms of being able to reliably design and produce products containing mixed biopolymer solutions and predicting their behaviour. The science of mixed biopolymer solutions is complicated by the fact that they are ternary systems, typically comprising mostly water, and that the biopolymers themselves are liable to structural transitions such as gelation. Both of these factors can play an important role in the phase behaviour of the mixtures, and the morphology of the resulting phases. In the following, an introduction is given to the physics of mixed biopolymer solutions and the behaviour of their phases, with a view to highlighting the unique aspects of such materials in comparison to other liquid-liquid mixtures, such as emulsions and polymer blends, and also the more interesting topics for future research in these fascinating materials.
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http://dx.doi.org/10.1016/j.cis.2009.08.001DOI Listing
December 2010

Emulsification mechanism and storage instabilities of hydrocarbon-in-water sub-micron emulsions stabilised with Tweens (20 and 80), Brij 96v and sucrose monoesters.

J Colloid Interface Sci 2009 Oct 6;338(1):201-6. Epub 2009 Jun 6.

Department of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.

The influence of both the nature of the surfactant and surfactant concentration on the processes of droplet break-up and coalescence in the formation of decane-in-water nano-emulsions in a high-pressure homogenizer was investigated. Emulsions were produced using a Christison Scientific M110-S microfluidiser with an impinging jet high-shear chamber. For all six surfactants studied (Tween 20, Tween 80, Brij 96v, sucrose monolaurate, sucrose monomyristate and sucrose monopalmate), the droplet size decreased with increasing surfactant concentration reaching a limiting droplet size at a surfactant concentration of 15 mM. The limiting droplet size for the different surfactants used were; Tween 20 (approximately 250+/-30 nm), Tween 80 (approximately 320+/-40 nm), Brij 96v (approximately 200+/-20 nm) and the three sucrose monoesters had very similar sizes of approximately 250+/-20 nm. A hydrophobic fluorescent dye (1-undecylpyrene) was used to establish the extent of competition between droplet break-up and coalescence in the emulsification process. For all the emulsifiers studied, droplet coalescence in the process reduced as the amount of emulsifier increased, becoming zero at concentrations of about 15 mM, i.e. the same concentration as that required to produce the limiting minimum droplet size. This shows that in the emulsification process droplet size is determined by both break-up and re-coalescence events, and at lower surfactant concentrations (<15 mM) that the final droplet size is probably a consequence of multiple break-up events. Emulsion stability over 200 h was investigated by measuring changes in the droplet size using dynamic light scattering. The increase in droplet volume was shown to be linear with respect to time, indicating an Ostwald ripening process. The observed ripening rate for the three sucrose monoesters (monopalmitate, monomyristate and monolaurate) was approximately 20 nm(3) s(-1), which is the ripening rate calculated using the Lifshitz-Slesov-Wagner (LSW) theory. This ripening rate is the change in radius that results from movement of the oil through the continuous phase, taking into account the oil solubility in water and the diffusion coefficient of the decane-in-water. The ripening rate for Brij 96v was about three times larger than the calculated rate and there is an indication that the ripening rate increases slightly with increasing surfactant concentration, indicating that some enhancement due to the presence of micelles has occurred. With Tween 80 and 20 the ripening rates were 20 and 40 times, respectively, larger than those calculated using the solubility and diffusion coefficients. The increased rate has been shown to be first order with respect to the surfactant concentration indicating micelle mediated ripening. It is hypothesized that an optimum formulation for the sub-micron emulsion with these types of surfactant, will balance surfactant concentration to minimize droplet size during processing while aiming to minimize or prevent Ostwald ripening.
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http://dx.doi.org/10.1016/j.jcis.2009.05.077DOI Listing
October 2009

Controlled release from modified amino acid hydrogels governed by molecular size or network dynamics.

Langmuir 2009 Sep;25(17):10285-91

Department of Chemistry and Centre for Materials Discovery, University of Liverpool, Crown Street, Liverpool L69 7ZD, U.K.

Hydrogels can be prepared using the commercially available Fmoc-phenylalanine or Fmoc-tyrosine as the gelator. Gelation is triggered by careful adjustment of the pH of the solution using glucono-delta-lactone (GdL). Model dyes have been entrapped in the hydrogels, and the release of the dyes from the hydrogels has been monitored. The release ratios indicate that the systems are under Fickian diffusion control. A range of dyes with different radii of gyration diffuse from the Fmoc-phenylalanine hydrogels with similar diffusion coefficients, implying that the network is not specifically retaining even relatively large (5 nm) dyes. On the other hand, the larger dyes are restricted in their diffusion from Fmoc-tyrosine hydrogels. These results correlate with the rheological measurements for the hydrogels, where those formed from Fmoc-tyrosine were shown to have significantly higher storage moduli than those formed from Fmoc-phenylalanine. In addition, the frequency-dependent behavior of the hydrogels demonstrates that Fmoc-tyrosine shows the classic response of a strong gel with a storage modulus that is nearly independent of frequency. However, for Fmoc-phenylalanine, the frequency dependence of moduli is very strong and very similar to that displayed by a transient network, where the interconnections between junction zones in the network are highly flexible and able to withstand large deformations.
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http://dx.doi.org/10.1021/la9011058DOI Listing
September 2009

Rheology of gelling and yielding soft matter systems.

Soft Matter 2008 May;4(6):1133-1140

Unilever Corporate Research, Colworth Park, Sharnbrook, Bedfordshire, United Kingdom.

Yielding and gelling soft matter materials are ubiquitous throughout biological and geological systems, the most commonly encountered examples being in food and other household products. In this Highlight, we attempt to summarise the rheological properties that are characteristic of structured soft matter systems, including their universal flow behaviour and viscoelastic response as well as appropriate methods of characterisation. We also discuss how the mechanical response depends on the materials' microstructure. Soft matter of this nature is typically in a non-equilibrium state, which means that it can be modified during processing and storage (aging) to obtain new structural states and rheologies. Several universal features have recently been observed experimentally for the linear and non-linear response of structured soft matter, and such developments are assisting in the development of suitable models to characterise their behaviour.
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http://dx.doi.org/10.1039/b719677fDOI Listing
May 2008

Direct measurement of the effective charge in nonpolar suspensions by optical tracking of single particles.

J Chem Phys 2007 May;126(19):194503

School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom.

The authors develop an ultrasensitive method for the measurement of the charge carried by a colloidal particle in a nonpolar suspension. The technique uses the phenomenon of the resonance of a particle held in an optical tweezer trap and driven by a sinusoidal electric field. The trapped particle forms a strongly damped harmonic oscillator whose fluctuations are a function of gamma, the ratio of the root-mean-square average of the electric and thermal forces on the particle. At low applied fields (gamma<1) the particle is confined to the optical axis, while at high fields (gamma>1) the probability distribution of the particle is double peaked. The periodically modulated thermal fluctuations are measured with nanometer sensitivity using an interferometric position detector. Charges, as low as a few elementary charges, can be measured with an uncertainty of about 0.25 e. This is significantly better than previous techniques and opens up new possibilities for the study of nonpolar suspensions.
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http://dx.doi.org/10.1063/1.2734968DOI Listing
May 2007

Synergistic interaction in emulsions stabilized by a mixture of silica nanoparticles and cationic surfactant.

Langmuir 2007 Mar 23;23(7):3626-36. Epub 2007 Feb 23.

Surfactant and Colloid Group, Department of Chemistry, University of Hull, Hull, HU6 7RX, United Kingdom. [email protected] hull.ac.uk

Using a range of complementary experiments, a detailed investigation into the behavior of dodecane-water emulsions stabilized by a mixture of silica nanoparticles and pure cationic surfactant has been made. Both emulsifiers prefer to stabilize o/w emulsions. At high pH, particles are ineffective emulsifiers, whereas surfactant-stabilized emulsions become increasingly stable to coalescence with concentration. In mixtures, no emulsion phase inversion occurs although synergism between the emulsifiers leads to enhanced stability at either fixed surfactant concentration or fixed particle concentration. Emulsions are most stable under conditions where particles have negligible charge and are most flocculated. Freeze fracture scanning electron microscopy confirms the presence of particle flocs at drop interfaces. At low pH, particles and surfactant are good emulsifiers alone. Synergism is also displayed in these mixtures, with the extent of creaming being minimum when particles are most flocculated. Experiments have been undertaken in order to offer an explanation for the latter synergy. By determining the adsorption isotherm of surfactant on particles in water, we show that surfactant addition initially leads to particle flocculation followed by re-dispersion. Using suitable contact angle measurements at oil-water-solid interfaces, we show that silica surfaces initially become increasingly hydrophobic upon surfactant addition, as well as surfactant adsorption lowering the oil-water interfacial tension. A competition exists between the influence of surfactant on the contact angle and the tension in the attachment energy of a particle to the interface.
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http://dx.doi.org/10.1021/la0634600DOI Listing
March 2007

Characteristics and risks of drivers with low annual distance driven.

Traffic Inj Prev 2006 Sep;7(3):248-55

Department of Public Health, Wellington School of Medicine and Health Sciences, University of Otago, Wellington, New Zealand.

Objectives: It has been noted by several authors that risk (defined only in terms of total expected numbers of crash involvements per total distance driven) paints a misleading picture of crash liability, particularly for the young and the old, as their high risk is associated with risky driving patterns typical of people who drive low annual kms. This article sets out to analyze these driving patterns of low-km drivers and to evaluate the risk of these patterns. As licensing programs tend to focus on young and old drivers, who tend to drive lower annual distances, income and employment data are also analyzed for low-km drivers. This is to provide a better picture for policy makers of the sort of people and the sorts of transportation requirements that their policies may affect.

Methods: Crash data and travel data were disaggregated by driver characteristics and by driving conditions (road type, day and night, weekend and weekday) and combined to form estimates of risk for typical driving patterns of driver groups. Characteristics of driving patterns and of the drivers themselves were derived for groups defined by age and by the amount of annual driving undertaken.

Results: Older drivers who drive less tend to have higher risk per km mainly due to their predominantly urban trips. Nevertheless, because older drivers on average manage to reduce their risk per distance driven by choosing driving patterns that are safer than the driving patterns of other age groups, the risk of older drivers as a group is not overestimated.

Conclusion: Despite being quite different from one another, the low- and high-km driving patterns of younger drivers were found to impose identical risks.
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http://dx.doi.org/10.1080/15389580600672836DOI Listing
September 2006

The contribution of alcohol to night time crash risk and other risks of night driving.

Accid Anal Prev 2005 Sep;37(5):816-24

New Zealand Ministry of Transport, PO Box 3175, Wellington, New Zealand.

Many studies show that driving at night is more risky in terms of crash involvements per distance travelled than driving during the day. The reasons for this include the more prevalent use of alcohol by drivers at night, the effects of fatigue on the driving task and the risk associated with reduced visibility. Although the consumption of alcohol prior to driving occurs most commonly at night, drink-driving is not inherently a night time risk factor. This study decomposes the New Zealand risk of driving at night into risk associated with alcohol and risk associated with inherently night time factors. The overall risk associated with alcohol use by drivers was shown to decrease with increasing age for the most risky situation analysed (male drivers on weekend nights). Given the levels of drinking and driving on weekend nights, the overall effect of alcohol was shown to contribute almost half of weekend night time risk for drivers aged under 40 on lower volume roads, but to contribute little to overall risk on higher-volume roads, consistent with other research showing that higher-volume roads are not favoured by drinking drivers. Risk at night relative to risk during the day (excluding risk associated with drinking and driving) was shown to decrease with age. Roads with illumination at night are less risky at night relative to during the day than roads without illumination. The risks estimated in this paper reflect the behaviour of the road users studied and their prevalence on the roads under the conditions analysed.
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http://dx.doi.org/10.1016/j.aap.2005.03.021DOI Listing
September 2005

Association between older driver characteristics, on-road driving test performance, and crash liability.

Traffic Inj Prev 2004 Jun;5(2):112-6

Land Transport Safety Authority, Wellington, New Zealand.

From May 1999, a new system for licensing older drivers was introduced in New Zealand. It included a practical on-road driving test with expanded scope, to be completed every two years from the time the driver turns 80. The relationship between crashes and test performance needed to be studied to inform the debate regarding the testing system. The population studied was all drivers who entered this licensing system during its first three years of operation. They were defined as crash involved if they were involved in an injury crash during the two years following their first licensure under the new system. Logistic regression was used to describe the risk of crash involvement in terms of driving test performance and other driver characteristics. Each driving test failure was associated with a 33% increase in the odds of crash involvement (95% CI 14% to 55%), controlling for age, gender, minor traffic violations, and whether the older driver lived with another licensed driver or not. Minor traffic violations in the two years following the driving test were associated with twice the odds of crash involvement. These results suggest that the new on-road driving test does identify older driver behaviors or limitations that are related to crash liability. It is anticipated that the results presented here will provide essential information for discussing older driver licensing systems, whose impact will grow in importance as the population of drivers ages.
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http://dx.doi.org/10.1080/15389580490435006DOI Listing
June 2004

Older driver crash rates in relation to type and quantity of travel.

Traffic Inj Prev 2004 Mar;5(1):26-36

Land Transport Safety Authority, Wellington, New Zealand.

It is a well-established phenomenon that, notwithstanding their overall good crash record, older drivers have a higher than average rate of involvement in injury crashes when the rate is calculated by dividing crash numbers by distance driven. It has been hypothesised that at least some of this higher crash rate is an artefact of the different nature of driving undertaken by many older drivers. For example, driving in congested urban environments provides more opportunities for collisions than driving the same distance on a motorway. However, there have been few opportunities to investigate this theory, as relevant data are difficult to acquire. High-quality data from the New Zealand Travel Survey (1997/1998) were combined with crash data to enable a statistical model to estimate the risk of driver groups under various driving conditions characterised by the type of road used, time of day, day of week, and season of year. Despite elevated crash risks per distance driven compared with middle-aged drivers for most road types, older drivers were as safe as any other age group when driving on motorways. Accounting for the fragility of older drivers and their passengers in the risk estimates for other road types, older drivers appeared to have daytime risks comparable to 25-year-olds and night-time risks as low as any other age group. The driving patterns of older drivers (in terms of when and where they drive) were estimated to minimize their risks in comparison with the driving patterns of other age groups. These results are of interest to both policy makers and transportation planners working against the background of inevitable increases in the number of older drivers as the population ages.
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http://dx.doi.org/10.1080/15389580490269146DOI Listing
March 2004

The influence of alcohol, age and number of passengers on the night-time risk of driver fatal injury in New Zealand.

Accid Anal Prev 2004 Jan;36(1):49-61

Land Transport Safety Authority, Research and Statistics, P.O. Box 2840, Wellington, New Zealand.

Breath alcohol measurements and other data collected at randomly selected roadside sites were combined with data on fatally injured drivers in crashes occurring on the same weekdays and times (Friday and Saturday nights) at locations matched by the size of the nearest town. A logistic model was fitted to these data for the years 1995-2000 to estimate the effects of alcohol, driver's age and the influence of passengers carried on the risk of driver fatal injury in New Zealand. The estimated risks increased steeply with increasing blood alcohol concentration (BAC), closely following an exponential curve at levels below about 200mg/dl (i.e. 0.2%) and increasing less than exponentially thereon. The model fitted to data for drivers under 200mg/dl showed that risks at all BAC levels were statistically significantly higher for drivers aged under 20 (over five times) and for drivers aged 20-29 (three times) than for drivers aged 30 and over. Further, controlling for age and BAC level, driving with a single passenger was associated with approximately half the night-time risk of driver fatal injury relative to driving either solo or with two or more passengers. According to a recent travel survey, the types of passengers carried at the times of night and days of week studied appear to differ significantly from the types of passengers carried generally, which may lead to different passenger effects on driver behaviour. The high relative risk of teenage drivers means that they reach high risk levels commonly regarded as unacceptable in the field of road safety even at their current legal limit of 30mg/dl, particularly when more than one passenger is carried in the car.
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http://dx.doi.org/10.1016/s0001-4575(02)00114-8DOI Listing
January 2004

Further results from a trial comparing a hidden speed camera programme with visible camera operation.

Accid Anal Prev 2002 Nov;34(6):773-7

Land Transport Safety Authority, Research and Statistics, Wellington, New Zealand.

As described in a previous paper [Accident Anal. Prev., 33 (2001) 277], the hidden camera programme was found to be associated with significant net falls in speeds, crashes and casualties both in 'speed camera areas' (specific signed sites to which camera operation is restricted) and on 100 km/h speed limit roads generally. These changes in speeds, crashes and casualties were identified in the trial area in comparison with a control area where generally highly visible speed camera enforcement continued to be used (and was used in the trial area prior to the commencement of the trial). There were initial changes in public attitudes associated with the trial that later largely reverted to pre-trial levels. Analysis of 2 years' data of the trial showed that falls in crash and casualty rates and speeds associated with the hidden camera programme were being sustained. It is not possible to separate out the effects of the concealment of the cameras from other aspects of the hidden speed camera programme, such as the four-fold increase in ticketing. This increase in speed camera tickets issued was an expected consequence of hiding the cameras and as such, an integral part of the hidden camera programme being evaluated.
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http://dx.doi.org/10.1016/s0001-4575(01)00077-xDOI Listing
November 2002