Our analysis showed that more than four-fifths of New Zealanders used a private motor vehicle to travel to work on Census day in 2006. Only one in fourteen people walked to work and one in forty cycled. Increased car use from 1991 to 2006 occurred at the expense of active means of travel as the prevalence of using public transport remained unchanged during that period. We found important differences in active travel patterns by region, age, gender and personal income.
This is one of very few papers reporting population-based active travel behaviour in New Zealand. One of the major benefits of using Census data is that it is a near-complete survey of the general population (96.3% response rate in 2006) and the people's transport activity nationally, regionally and across different population subgroups over time may be compared. When interpreting these results, however, some limitations need to be considered. First, the Census question asked only for 'main means of travel to work' and did not take into account multiple transport modes, for example, walking and taking a bus in one journey. This means the contribution of walking to the journey to work may be under-estimated. Second, the 1991-2006 Census questions were date-specific and the data may be biased seasonally, although the timing of Census day has been similar year to year. People's active transport activity may be overestimated in this case as the Census is usually in March when the weather is warm and relatively dry. Third, we were not able to adjust for potential confounders as only aggregate data were available for this analysis. For example, personal income may be related to an individual's age, gender and residential area, all of which independently, influence choice of travel to work. Finally, the findings may be affected by the "ecological fallacy" as averaged aggregate data were used to infer relationships, for example, between various regional characteristics (such as average distance to work) and the proportion of cycling and walking to work. These questions may be addressed in future studies which obtain individual level data.
Despite these limitations, our findings are consistent with and extend the evidence gained from previous research. Parallel to decreasing trends in active travel to work behaviour, overall travel mode share for cycling and walking has been declining steadily in New Zealand (from 4% and 21% respectively in 1989 to 1% and 16% respectively in 2006) . During the same period, the annual distance driven in light 4-wheeled vehicles has been increasing - particularly among the 45-64 age group . From 1990 to 2006, total greenhouse gas emissions increased by 25.7%, and emissions from road transport increased disproportionately (by 66.9%) . In 2006, transport accounted for 42% of total emissions from the energy sector . A recent report indicates that the air quality in Auckland is worsening due to emissions from increasing use of motor vehicles .
A study from the US shows that CO2 emissions from the transport sector will continue to rise unless vehicle kilometres travelled can be substantially reduced, as present trends in car use will overwhelm the gains that may result from technological advances such as changes in fuel type (e.g., biodiesel fuel) and motor vehicle efficiency (e.g., hybrid cars) . The findings are unlikely to be different in the New Zealand context given the country's dispersed population (4.3 million people spread over 268,680 km2), low density cities and automobile centred transportation system.
Other studies have found that New Zealanders rarely cycle or walk even when travelling short distances. Walking represents only 39% of all trips under two kilometres and cycling accounts for three percent of all trips under two kilometres and two percent of all trips between two and five kilometres in the 2004-2007 household travel surveys . Only one-fifth of New Zealanders surveyed in 2003 strongly endorsed plans to replace car trips with active modes such as cycling and walking on at least two days per week and less than half of the latter considered cycling for short distances [39, 40]. Although a variety of factors can influence public attitudes and behaviour , these findings are likely to reflect decades of under-investment in public transport and cycling and walking infrastructure. In Auckland, the construction of motorways has been favoured consistently over alternative modes in transport planning over the past 50 years .
We observed regional differences in patterns of cycling and walking to work. Such differences may be partly explained by aspects of the physical environment such as weather, climate and topography (hilliness) [43–45] and distance to work . The influence of environmental factors such as average temperatures and rainfall, however, should not be over-emphasized. A number of cities in North America and Europe have reported substantial increases in the prevalence of walking and cycling in the last decade, for example, daily ridership doubled in New York between 2001 and 2006 , yet have climates much less favourable than those of most parts of New Zealand.
We found low rates of cycling to work in regions with long average distances to work (≥ 10 km). Statistics New Zealand reported that on the Census day in 2006, 83% of people who walked to work travelled less than 5 km and 89% of those who cycled to work travelled less than 10 km . Although distance to work is not easily changed, increased housing density, availability of public transport and investment in active transport infrastructure such as bicycle lanes and shared paths may improve engagement in active travel modes.
Two New Zealand regions that bucked the overall trends by revealing increasing levels of walking warrant further comment. Regional strategies in Wellington and Nelson have made substantial investments in active transport. Wellington has proposed an urban development strategy , based on the idea of a "growth spine" (a strip of land along which more intensive urban development is encouraged), a bus lane programme  and school, workplace and community travel plans . In Nelson, pedestrian, cycling and urban growth strategies have been implemented with integration between transport planning and urban development teams . Future research will be required to investigate the effectiveness of these and other active transport strategies being implemented.
Studies from other automobile dependent countries such as the US, UK and Australia have also reported a comparatively low level of cycling and walking to work [52–56], with important sociodemographic variations in the patterns of active travel. In general, men are more likely to cycle than women; and women are more likely to walk than men. Younger people are more likely to walk and cycle compared with older age groups. This is important because it will be necessary to boost walking and cycling rates in the older age groups to realise the potential health benefits of active transport. The cardio-protective effects of exercise relate much more closely to current activity than to past exposures . Our study shows that walking is more common in lower income groups, whereas socioeconomic status does not appear to influence cycling. Similar patterns were observed in previous research [53, 58, 59]; however, others reported associations between cycling and income [44, 60].
In contrast, in most European countries, walking and cycling make up at least one-fourth of all urban trips (45% in the Netherlands) and active travel patterns are universal across different segments of society; walking increases with age, cycling declines only slightly, women cycle as much as men, and people from all income classes cycle [11, 58, 61–64]. The success of European countries in promoting cycling and walking is attributed to the "coordinated implementation of the multi-faceted, mutually reinforcing set of policies" in the past few decades [58, 61]. Components include: provision of better facilities for pedestrians and cyclists, extensive traffic calming of residential neighbourhoods, increased traffic regulation and enforcement, people oriented urban design, integration with public transport, comprehensive traffic education and training, restrictions on car ownership, use and parking [58, 61] and workplace travel plans .
In countries like New Zealand, significant barriers exist to implementing such comprehensive measures to promote active commuting but much could be achieved in the short term. For example, although Australia has sprawling cities and a high rate of car ownership, the prevalence of cycling to work has increased substantially in some states in the last decade together with growing investments in bicycle infrastructure (for example, there was a 43% increase from 2001 to 2006 in cycle commuters in Melbourne) [66–68]. Likewise, we found an increasing trend of walking to work in the two New Zealand regions that have invested in sustainable transport strategies.
As an important initial step at the national level, a project has begun to build a cycleway network running the length of New Zealand . While primarily intended to enhance tourism, the initiative has the potential to promote active commuting if a comprehensive cycle network plan is incorporated to strengthen connections between residential areas and key activity centres in urban and rural New Zealand. A potential way to move toward more attractive environments for active commuting without major infrastructural change is reducing the speed limit in residential streets, which currently is 50 km/hr in New Zealand compared with 30 km/hr (or less) in European countries . Given a favourable trend in cycling and walking as a recreational activity in New Zealand , another useful step would be offering interventions promoting a modal shift, i.e., from using cars to walking and cycling, tailored to recreational cyclists and walkers. The effectiveness of such targeted behaviour change programmes has been reported in a previous review . An Australian study showed that participants in mass cycling events, particularly novice riders and first-time participants, cycled more frequently in the month after the event .