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| Journal of the New Zealand Medical Association,
29-June-2012, Vol 125 No 1357
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Motor vehicle traffic crashes are the leading cause of
mortality and morbidity among adolescents and young adults (15–25 years of
age) in New Zealand 1,2 and many other
developed countries.3 In response to the high
motor vehicle crash rate among young drivers in New Zealand a Graduated Driver
Licensing System (GDLS) was introduced in
1987.4
The key elements of the GDLS are: a 6 months learner licence
stage of supervised driving; a restricted licence stage of 18 months that allows
unsupervised driving except at night-time (10pm–5am) or with young
passengers in the car; a full licence stage with no restrictions. Further
details of the New Zealand GDLS have been described
elsewhere.5
Despite a significant improvement since graduated licensing
was introduced5, traffic related injury remains
the leading cause of death and hospitalisation among young New Zealanders,
especially young Māori.1,2 Most New
Zealand research on young drivers to date has used routinely collected crash
data, such as the police traffic crash reports and the national hospital
inpatient records, but has been limited in that these databases do not, and
realistically cannot, include the level of detail required to ensure that
learner driver policy and programmes are based on sound scientific evidence
applicable to young drivers in the current New Zealand
context.6
There have been a small number of studies examining the role
of parents/caregivers in the learning to drive process. These studies have shown
that high parental monitoring of driving by teens can reduce risky driving and
crashes, especially during the high risk first few months of driving, and
interventions designed to encourage parental monitoring have shown
promise.6 None of these studies were undertaken
in New Zealand. The lack of relevant information relevant to young drivers in
New Zealand lead to the establishment of the New Zealand Driver’s Study
(NZDS).
The NZDS is a prospective cohort study designed to explore
the relationship between a comprehensive range of driving and traffic safety
related factors and subsequent traffic crashes and convictions among newly
licensed drivers. The study is designed to capitalise on the unique
characteristics of New Zealand’s multistage graduated driver licensing
system with the interview stages for collecting exposure data coinciding with
the three testing stages of graduated driver licensing (Figure 1)
The protocol for this study has been described in detail
elsewhere.6 Briefly, study participants are
recruited at the learner licence stage and complete a self-administered baseline
questionnaire. They are then followed-up at the restricted and full licence
stage, by telephone interview. Official traffic crash and offence outcome data
are obtained from the authorities that maintain these databases, and
self-reported crashes are obtained at the restricted and full licence interview.
The study was established following a comprehensive pilot
study which included: consultation with Māori; consultation with agencies
involved in driver licensing; trialling procedures for recruitment; and
developing, pre-testing and piloting
questionnaires.6,7
In this paper we describe:
Goals and
objectives—The goal of the New Zealand Drivers Study is to
provide an evidence base that would facilitate efforts aimed at reducing
traffic-related injury in New Zealand, and especially among the high-risk
adolescent/young adult population. The primary objective of the study is to
explore the relationship between a comprehensive range of driving and traffic
safety related factors (e.g. driving experience, motivation for
driving/licensing, driver training, alcohol use, risk-taking) and subsequent
traffic crashes and convictions among newly licensed drivers, and from this
identify specific areas that can be targeted to reduce traffic-related injury
among this high-risk group.
The secondary objectives are to:
An overview of the study design is provided in
Figure 1.
Figure 1. Overview of study
design
 Study population size—We aimed to
recruit 5,000 newly licensed drivers of whom 1500 (minimum 1000) would be
Māori, over a 12-month period. In reality we recruited 4282 over a 2-year
period but 290 were ineligible (e.g. overseas licence conversions, truck
licences, unsigned consent form) leaving a final cohort of 3992, of whom 825
self-identified as Māori.
Although the total cohort, and the number of Māori,
were less than our original targets, the power calculations as given in the
study protocol6 show that these numbers provide
adequate statistical power to show effect sizes of around 1.5 to 2.0 for policy
issues (e.g. full cohort) and 2.0 to 2.5 for programme issues (e.g. Māori
cohort), which we considered satisfactory.
Recruitment methods—Our aim was to
recruit participants from a diverse range of locations throughout NZ. During the
pilot study7 recruitment was undertaken by the
University research team. Given the differing needs of the local populations, we
anticipated that different methods would be required in different regions.
In the larger urban areas, (Auckland, Christchurch,
Dunedin), research assistants (RAs) (mostly postgraduate students) were employed
part-time to undertake face-to-face recruitment at the local driver licensing
centres. For example, in areas with a relatively high proportion of Māori,
such as South Auckland (Manukau), and on the East Coast of the North Island
(Hastings/Napier, Wairoa, Gisborne and Ruatoria) it was preferred that the
recruitment be done by local Māori community groups. Six such groups, with
whom we had consulted and established a collaborative partnership during the
pilot, were subcontracted to undertake the recruitment among their local people.
All of these community groups had many years of experience
delivering learner licence courses, and the source of participants for the NZDS.
Five of these community/iwi groups were also subcontracted to undertake
recruitment at their local driver licensing centre.
To boost the number of rural participants, postcards were
distributed to all NZ Automobile Association (NZAA) licensing centres, except
those in the large urban centres. On this postcard was an invitation for all
newly licensed learner drivers to take part in the study, a mention of the $20
petrol voucher all participants would receive for each completed interview, and
our contact details (email address, a free telephone number, and cell phone
number) which they needed to contact if they wished to take part. This
recruitment method was reasonably successful, and in particular helped boost the
number of rural participants.
The licensing centres chosen for the recruitment were those
that had issued the highest number of learner licences in the previous year.
However, as there was no booking system for learner licence testing, there was a
degree of uncertainty as to how many potential study participants would turn up
at any centre, at any given time. Most of the recruitment took place at NZAA
licensing centres, and although the recruitment took two years instead of one,
the NZAA staff were very accommodating and supportive of the research staff.
This highlighted the importance of having established a good rapport with the
NZAA head office personnel during the pilot study.
Recruitment commenced on 1 February 2006 and finished on 31
January 2008. Of the 3992 study participants, 2685 (67%) were recruited at a
licensing centre, 916 (23%) at learner licence courses, and 391 (10%) by
postcards. At the licensing centres, the recruitment rate across all research
assistants was approximately 75% and for the learner licence courses it was
around 90%. We were unable calculate a recruitment rate for postcards as we did
not know the denominator.
Explanatory/exposure measures—Figure
1 provides an overview of the explanatory and the exposure measures, and the
stage of the study when this information is collected.
Outcome measures—We intended that
official data be sourced for traffic offences (any recorded breach of the Road
Code), and injury crashes. We got participants consent to access crash and
offence data at learner, restricted and full licence stage for up to 10 years
after full licence. Our pilot work indicated a significant portion may not give
consent for access to official data, especially for police records, and the
period for which we sought it. In reality, this was not a problem. Of the 3992
study participants 98% gave consent to access all the listed traffic and injury
records, 1% refused access to all records, and 1% denied access to some
records.
For the 98% who gave consent, we have successfully accessed
the traffic infringement and offence and convictions notices from the Driver
Licence Registry (with assistance from the Department of Justice who hold these
records, and not the NZ police as stated in the protocol) using the driver
licence number. As at 31 December 2010, 1589 participants had at least one
traffic offence, and there were 5751 offences in total.
We have also been successful in accessing crash information
from the NZ Crash Analysis System (CAS), which is maintained by the Ministry of
Transport. Crashes in which the study participant was a driver are identified
using their driver licence number, and verified by personal data such as name
and date of birth.
To identify crash records with no licence number name, date
of birth, and address variables were used in the linkage. These variables are
recorded on the bulk of the crash records. As at 31 December 2010 the
participants had been involved in 143 police reported crashes.
Since official crash reports are known to have significant
biases associated with them 8 we also sought
information from the participants about injury crashes. The intention is to use
official and self-report information to derive a single outcome measure of crash
experience.
Follow-up of the cohort—Central to
the study design was the expectation that participants who had obtained their
learner licence would be followed up when they got their restricted and full
licences. Critical to achieving this was the establishment of a study specific
database by the New Zealand Driver Licence Registry (DLR) that would allow us to
track the participants’ progress as they moved through the licensing
system. With the cooperation of the DLR we achieved this objective. Each week
the DLR provide us with an updated electronic file of any changed records in the
database. A changed record indicates progression to the next licence stage, and
the eligibility of the participant for the next licence stage interview.
Figure 2 is a snapshot in time of participants’
progress through the licensing stages and demonstrates that our tracking has
been very successful. As at 30 April 2011, 74% had passed their restricted
licence test and 87% of these completed the restricted licence interview, and
39% had passed the full licence test and 93% of them had completed the full
licence interview.
Figure 2. Cohort progression as at 30 April
2011
 After a minimum of 2.5 years had elapsed since obtaining a
learner licence we conducted telephone interviews to ascertain reasons for
non-progression to the restricted licence. Up to six attempts were made to
contact non-progressors. As at 30 April 2011 we had attempted to contact all
non-progressors. Of these 548 (46%) have been interviewed, 101 (8%) refused, and
540 (45%) could not be contacted.
Sociodemographic and behavioural characteristics of
the cohort—One of the objectives of the recruitment process was
to recruit a cohort that reflected the geographic and cultural diversity of the
newly licensed driving population of New Zealand, and to recruit sufficient
Māori to be able to conduct a separate analysis for Māori. As noted
above, to achieve this, recruitment was undertaken in regions that represented
both rural and urban areas, North and South Islands, in ethnically diverse
communities, and included several regions with a relatively high proportion of
Māori in the population.
Table 1 shows the sociodemographic characteristics of the
study population at the learner licence stage. Gender was evenly distributed
and, the majority were young with 77% aged 17 years or younger. The results show
we successfully recruited an ethnically diverse cohort, with 825 (21%) and 516
(13%) identifying as Māori and Pacific origins, respectively. It should be
noted that participants could identify with as many ethnic groups as they
wished.
Table 1. Sociodemographic characteristics of
the NZDS participants at learner licensing stage
The residential address of each study participant was
classified according to Statistics New Zealand “Urban/rural
profile”.9 The majority (89%) lived in an
urban location.
The NZDep2006 score10 is a
measure of socioeconomic deprivation created by combining nine variables, which
reflect eight dimensions of deprivation, from the 2006 census. The scores in
Table 1 show a reasonably even distribution (8%–10%) across all levels,
except for the level of highest (16%) and lowest (12%) deprivation.
Table 2 shows a similar degree of diversity within
behavioural factors. Scores for impulsivity/sensation seeking and
aggression/hostility were derived using Zuckerman’s personality
measure.11 Of note, there are substantial
numbers at the extremes on measures of impulsivity/sensation seeking and
aggression/hostility. The cohort also includes significant numbers of hazardous
drinkers (measured by the AUDIT-C12 ) and
occasional and regular drug users. Also of significance is that half the cohort
had driven (illegally) on a public road prior to obtaining their learner licence
and approximately a quarter have either been in a crash themselves or know
someone who had.
Table 2. Behavioural characteristics of the
NZDS participants at learner licensing stage
 Parents’ study—One objective of
this study was to examine the role of parents/caregivers as supervisors of newly
licensed drivers. As planned a parent or caregiver was eligible for the
parents’ study if their child:
In the protocol we proposed to recruit
the first 1000 eligible parents. This, however, ran the risk of recruitment bias
as 15–17 year olds recruited in the latter part of the recruitment phase
would have had insufficient time to progress to the restricted licence stage,
and therefore their parents would not be eligible for the study. Accordingly
recruitment for the parent study continued until August 2008, seven months after
the NZDS cohort recruitment phase finished. This meant that every 15–17
year old held their learner licence for the minimum length of time (6 months),
and thus could progress to the restricted licence stage, should they have chosen
to.
Figure 3 shows the numbers that met the successive
recruitment criteria. In all, 85% of the parents invited to take part in the
study agreed to do so, resulting in 1200 parent participants (773 mothers, 427
fathers).
DiscussionCohort studies provides excellent means of providing insight
into the factors associated with adverse outcomes as young drivers commence
their driving careers. There are major challenges to mounting such a study,
nevertheless we have been successful in mounting such study, albeit with some
limitations.
While the study population size was less than we had planned
for, as indicated above, it has not seriously compromised the statistical power
of the study. It was particularly pleasing that we were able to achieve
sufficient numbers of Māori to undertake a separate Māori analyses.
This is important for the credibility of the study in the New Zealand context.
Internationally, given the dearth of injury epidemiology which is focused on
indigenous populations we are well placed to add significantly to knowledge in
this area.13 This process has already commenced
with an examination of unlicensed driving among Māori
14 and another on attitudes and opinions of
newly licensed drivers. 13
The greatest challenge to the study to date has been the
recruitment. By extending the recruitment period to two years instead of one, we
were able to achieve an adequate number of study participants. This was
especially important for Māori, as an analysis solely for Māori, was a
specific objective of this study. The Māori community/iwi groups that
undertook the recruitment must be given credit, for making this possible. Our
experiences confirm those of others who have shown community engagement and
local assistance is critical to successful recruitment of indigenous people.
16,17
Critical to the success of this study is our ability to
track outcomes for all participants, via official data sources. Our very high
levels of consent to access outcome data (traffic crashes, infringements and
offences) during the licensing years, and for ten years after obtaining a full
licence, means we can access outcomes for 98% of the cohort, on an ongoing
basis. Also, as we are able to link the official outcome data to our study data
using driver’s licence numbers, because the majority of crashes do have
the driver licence number recorded on the file, we are confident we can access
complete data. The risk of bias in terms of those we do not have outcomes
information for is minimal.
A major threat to validity of the findings of a cohort study
such as this is attrition bias due to failure to monitor some participants
through the licensing stages. Figure 2 shows that we have been very successful
in tracing participants and subsequently obtaining follow-up information from
them. It should also be noted that the response rates to interviews at the
restricted and full licence stages have been very high at 87% and 93%,
respectively. Failure to follow-up at the restricted licence stage should not be
interpreted as precluding follow-up at the full licence stage.
Currently, we have successfully followed up at the full
licence stage two thirds of those we failed to follow-up at the restricted
stage. In this context it is important to note that we have followed-up
approximately half of those who have failed to progress from the learner to the
restricted licence stage, to ascertain their reasons for failing to progress
through the graduated licensing system and to obtain information on driving
behaviour comparable to that obtained from those who have progressed to the
restricted licence stage. .
The characteristics of the study population showed
substantial heterogeneity in demographic characteristics at the learner
licensing stage. For example, the range of ages potentially allows us to
determine the independent contributions of age (maturity) and driving experience
to crash risk. The relatively small number of participants from rural settings
will restrict our ability to determine rurality as a factor for various
outcomes. Aside from the sociodemographic factors, the results show we have
heterogeneity in terms of impulsivity/sensation seeking, aggression, alcohol and
other drug use, and road safety experience. Of particular note is that we have a
significant number of the cohort who had been driving (illegally) on public
roads prior to obtaining their learner licence.
Our results also show high response rates, and thus low risk
of bias, for all the items in Table 2. Even the more sensitive issues, such as
drug use, had less than 4% who did not complete the item.
The recruitment for the parent study was successful with 85%
of parents agreeing to participate. This is a unique feature of this cohort
study of young drivers and will enable us to determine how central parents are
in how young drivers progress, or do not, through the graduated licensing
system.
A limitation of this study is that we have not recruited a
representative cohort. This means that the prevalences we report cannot be
considered to represent young drivers in New Zealand. This decision was
deliberate since recruitment of representative cohort would have required us to
have a far larger cohort to ensure, for example, we had sufficient Māori to
allow separate analyses. This would have been very expensive and not an
efficient means of addressing our primary objectives. Irrespective we considered
the recruitment of a representative cohort would have been extremely difficult,
if not impossible, given high refusal rates for certain sub-populations of young
drivers. As has been demonstrated by Dunedin Multidisciplinary Health and
Development Study non-representative cohorts have the potential to make a
substantial contribution to our knowledge.
Although analyses addressing the primary objectives of the
study have only recently commenced other results from the NZDS are achieving our
main aim, that is to influence young driver policy in New Zealand. Results from
the NZDS have been used by the Ministry of Transport in the development of New
Zealand’s young driver policy18, and
learner driver policy currently under development.
Results from the parents’ study have been requested by
the New Zealand Transport Agency to assist in the development of a young driver
programme for parents. We are also currently preparing a number of papers of
direct relevant to policy. For example, the government recently indicated that
as one of its supporting actions to increase the safety of young drivers it
intended to investigate the introduction of maximum licence time limits for
learner and restricted licences.19
Despite the fact that staged progression is central to
graduated licensing schemes there is limited information on the sociodemographic
and behavioural characteristics which are associated with non-progression,
whether non-progression results in negative traffic outcomes, and why
non-progressor choose not to progress. We have recently addressed this issue and
the findings have been forwarded to the Ministry of
Transport.20
ConclusionA significant investment was made in pretesting and piloting
for the NZDS. This has paid off as there have only been minor variations to the
protocol for the main study. Moreover the study population displays significant
variation in range of factors at the learner licensing stage. The NZDS study has
already made a contribution to the development of young driver policy and, as
has been demonstrated here with the example of non-progressors, it is well
placed to significantly increase that contribution.
Competing interests: None
declared.
Author information: John Langley,
Professorial Research Fellow1; Dorothy Begg
Senior Research Fellow1; Rebecca Brookland,
Research Fellow1; Shanthi Ameratunga,
Professor2; Anna McDowell, Statistical
Analyst3; John Broughton, Associate
Professor4
Acknowledgements: This
study was funded by the Health Research Council of New Zealand, Accident
Compensation Corporation, the Road Safety Trust (2005–2009) and is
currently funded by the Health Research Council of New Zealand and the Road
Safety Trust (2009–2013). The authors also gratefully acknowledge the
assistance of all the research assistants and community organisations that
recruited the study cohort, and the study participants for their ongoing
contribution to this study.
Correspondence: Professor John Langley,
Injury Prevention Research Unit, Dept of Preventive and Social Medicine, Dunedin
School of Medicine, University of Otago, PO Box 56, Dunedin 9054, New Zealand.
Fax: +64 (0)3 4798337; email: John.Langley@otago.ac.nz
References:
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