Air quality: key behaviours report

4 Reducing car use

4.1 Evidence on air quality impacts of reducing car use

The transport sector is responsible for a large share of air pollution in Scotland, as it is in the UK as a whole and internationally (Li et al., 2017; Marinello et al., 2021; NAEI, 2023). In 2019, transport was responsible for approximately 59% of Scotland's total NOx emissions (Hector et al., 2022). Of these transport emissions, passenger cars (mainly diesel cars) were responsible for approximately 28%, with the rest coming from rail, aviation, shipping, and other road transport (Hector et al., 2021). Transport was also responsible for approximately 25% of the total PM2.5 and 20% of PM10 emissions (Hector et al., 2022). It is evident that transport is a key contributor to air pollution, but from these data, drawn from the National Atmospheric Emissions Inventory, it is not clear what share of Scotland's emissions are associated with private car use (as opposed to car use for business, taxis etc.).

Reducing car use is the most common focus in literature linking behaviours of the public and air quality (Keyvanfar et al., 2018; Quarmby et al., 2019; Riley et al., 2021), and road transport emissions form the focus of most air quality strategies and technologies (Quarmby et al., 2019). There has been much interest in the recent literature on the impact of reduced travel due to COVID-19 restrictions on air quality. The COVID-19 lockdowns can be viewed as a natural experiment on the impacts of reducing car use (see Box 1). Interventions such as Low Emission Zones/Ultra Low Emission Zones and congestion charging, designed to reduce traffic in priority areas, can make significant impacts on air quality (Chamberlain et al., 2023; Holman et al., 2015; Mudway et al., 2019; Quarmby et al., 2019) although mixed results are also reported (Holman et al., 2015). As well as impacts on air quality outcomes, positive impacts on behaviours (reductions in car use and shifts to active travel) are also reported (Tarriño-Ortiz et al., 2022).

Box 1: Impacts of COVID-19 restrictions on air quality

Numerous studies around the world report the impacts of COVID-19 lockdowns on air quality. Many report large reductions of observed ambient concentrations of air pollutants, especially NOx, during lockdown periods (Han et al., 2023), but such reductions vary widely across studies. An early review by Marinello et al. (2021) found that reductions, where they were recorded, ranged from 6-90% for NO₂, from 9-60% for PM10, and from 7-86% for PM2.5. Very few case studies recorded unchanged pollutant levels. The largest reductions in pollutants were recorded for large Asian cities where pre-COVID concentrations were high (Marinello et al., 2021). It is difficult to establish the extent to which improvements in air quality during COVID-19 were a result of reductions in car use, as opposed to reductions in other types of transport and industrial activity. Wang et al. (2020) attempted to attribute air quality improvements during COVID lockdown in China to changes in different types of activity. Generally, reductions in industrial activity were more strongly associated with air quality improvements, although both transport and industry were seen to contribute significantly. For NOx emissions, there was evidence that reductions in transport activity explained more of the improvement in air quality than did reductions in industrial activity.

Kramer (2021) analysed impacts of COVID-19 on air quality in Scotland. Comparing the observed data to a business-as-usual scenario for the period they estimate that the first lockdown reduced NOx and NO₂ by 58% and 52% respectively, with the largest reductions occurring during the daytime (particularity at times that would normally have been rush hour periods). This aligns with the decrease in cars on the road observed (Transport Scotland data reported car traffic levels fell to 25% of 2019 levels but had almost recovered to pre-pandemic levels after 6 months). The estimated decrease in urban PM2.5 because of lockdown was very small, and estimates are more uncertain due to the complexity of modelling PM, given regional transport of PM and the variety of natural and anthropogenic sources. These findings for Scotland reflect findings for the UK more broadly, for which several studies reported significant reductions in NOx (Mohajeri et al., 2021; Ropkins & Tate, 2021; Wyche et al., 2021), but less clear impacts on PM2.5 (Wyche et al., 2021). A study comparing four UK cities – Edinburgh, London, Cardiff and Belfast – found that the greatest reductions in air pollution were found in Edinburgh, where the greatest reductions in driving and public transport use were observed (Mohajeri et al., 2021).

Quarmby et al., (2019), in a review of air quality strategies and technologies, highlight improvements in active travel infrastructure as one of the most promising interventions for improving air quality, but noted a lack of evidence quantifying the impacts of switching to cycling. Modelling carried out for Sustrans, aiming to estimate air quality-related health impacts of improvements in active travel infrastructure across the UK, found mixed results (Ballinger et al., 2017). The impacts of schemes depended on levels of active travel uptake associated with the scheme, exposure to air pollution when walking/cycling, population density and other factors. The greatest estimated health impacts related to the Glasgow scheme, which involved the completion of the 'Bridge to Nowhere' across the M8 motorway. The air quality health impact of the reduction in car journeys associated with the scheme was estimated at equivalent to £104,820 a year, with the majority of the benefits experienced by those switching to active travel for their work commute. The authors estimate that, extrapolating from the benefits of the Glasgow case study, achieving a target of 10% of all journeys in Scotland by bike could result in air quality benefits equating to £364 million per year (Ballinger et al., 2017).

International studies modelling large-scale shifts to active travel (mainly cycling) indicate that shifting from car journeys to active travel has the potential to reduce air pollution (especially NOx) and result in overall benefits to the population in terms of reductions in mortality, respiratory disease, cardiovascular disease, cancer, adverse birth outcomes, loss of productivity associated with illness, and road traffic fatalities (C. Johansson et al., 2017; Mueller et al., 2015; Rabl & de Nazelle, 2012; Rojas-Rueda et al., 2012). In these studies, increased physical activity is often found to be responsible for the majority of the health benefits (Mueller et al., 2015). Additionally, the modelling studies highlight that where cycle routes are not separated from motorised traffic there can be negative impacts for those switching to active travel, as a result of increased exposure to pollutants, which can offset the health benefits experienced by individuals (Ballinger et al., 2017; Mueller et al., 2015; Rabl & de Nazelle, 2012).

Quarmby et al. (2019) highlight the importance of public transport networks, including frequent services and integrated transport hubs, for air quality improvement. In their profiling of three case study cities ranking highly for air quality (which included Edinburgh), public transport networks were identified as a critical success factor along with a good cycle network, and financial incentives to buy electric vehicles (see section 5). Per passenger kilometre, overall NOx and CO emissions are substantially lower for urban public transport than car travel (Potter, 2003). Modelling studies estimate air quality improvements from increasing public transport by bus. An evaluation of a national financial incentive scheme for public transport use in Germany finding that the scheme reduced air pollution index scores by 6% (Gohl & Schrauth, 2022). A study in China estimated that for every 1% increase in buses on the road, air quality index score drops by 0.08% (Sun et al., 2019). Other studies indicate that public transport provision is, on its own, not necessarily effective in improving air quality (Ma et al., 2021). Evaluation of the air quality impacts of the Jubilee Line Extension of the London Underground found only small impacts on air pollution and only at some locations (Ma et al., 2021). The air quality impacts of behaviour changes in use of public transport (as opposed to specific infrastructure improvements) will also vary depending on public transport mode and fuel types (e.g. diesel-powered or electric buses or trains, hydrogen-powered buses) (Potter, 2003).

Remote, hybrid and flexible working arrangements have the potential to reduce car use significantly. A review by Moglia et al. (2021) highlights air quality benefits of remote working, particularly with respect to the wide-scale adoption of home working practices during the COVID-19 pandemic, however Spear et al. (2022) noted that no studies have yet measured the specific contribution of working from home to the air quality improvements observed during COVID-19 lockdowns. Kylili et al. (2020) modelled the life cycle impacts of three different scenarios: office working, working from co-working spaces, and remote (home office) working. In both the co-working and remote working scenarios, it was assumed that employees would still have to work in the office at least 2 days per week. Their analysis found that in the home working scenario, NOx emissions were reduced by more than 50% compared to the office working scenario, with the co-working scenario associated with a reduction of more than 40%, with results for PM10 and PM2.5 emissions of a similar magnitude for the different scenarios. This analysis aligns with other life cycle assessments of greenhouse gas emission reductions associated with home working practices (Spear et al., 2022), however it should be noted that building energy consumption associated with the different working practices was assumed to be equal in the Kylili et al. assessments.

Ge et al. (2018) modelled impacts of workplace sharing initiatives/flexible working hubs, reporting CO₂ emissions reductions associated with the reduced commuting travel. We can anticipate that the environmental impacts of such schemes would also extend to reduced emissions of other pollutants. As rush hour congestion is largely a result of commuting, flexible working arrangements (including flexible working hours as well as locations) has considerable potential to reduce air pollution through reducing congestion (Yu et al., 2019), as well as reducing emissions through reduced car use overall.

Particularly relevant to commuting, car-sharing /car-pooling has received interest as a route to reducing car use. Car-sharing has the potential to lower both the emissions per passenger by promoting shared travel as well as reducing the overall number of vehicles on the road (Correia & Viegas, 2011). There is, however, a lack of evidence on the effectiveness of car-sharing initiatives for reducing emissions (Keyvanfar et al., 2018).

Car use reduction can also be achieved through minimising travel for non-commuting journeys. Accessing shops and services locally can mean reduced journey lengths and greater opportunity for switching from car to active travel for these journeys. Improvements in air quality are one of the potential impacts of '20 Minute Neighbourhoods' (20MN) i.e. places that enable people to live more locally through easy access to services and amenities particularly by active travel (Al Waer & Cooper, 2023). The 20MN concept highlights that behaviour change in reducing car travel to everyday services and amenities relies on supportive built environments and planning processes. While it is important to highlight the value of staying local as part of the behavioural changes involved in reducing car use, we return to the factors affecting people's ability to do so, as highlighted by the literature on 20MN, in the COM-B analysis of the following section. Minimising unnecessary travel has large potential impacts in terms of reducing car use, however we did not find specific literature on the associated air quality impacts beyond that focusing on commuting to work.

Overall, it is clear that reducing car use is a key behavioural category to prioritise for air quality improvement. In addition, reducing car use has multiple social and environmental benefits, including for public health, communities and mitigating climate change. From the review we identify three key behaviours to prioritise, plus one additional behaviour for consideration:

Reducing car use

Key behaviours

Walking, cycling or wheeling for short journeys

Using public transport instead of driving

Working flexibly or from home

Additional behaviour for consideration

Using local shops and services

4.2 Factors influencing key behaviours around car use reduction

A range of factors (both motivating and constraining factors) influencing the key car use reduction behaviours were identified from the literature. These are outlined in relation to Capabilities, Opportunities and Motivations in Table 3-6 below. It should be noted that, throughout this report, the factors outlined in the tables represent those that were highlighted from the literature reviewed and do not necessarily represent an exhaustive list of the multiple factors influencing each of the key behaviours.

Table 3: Capability, Opportunity and Motivation factors influencing switching to active travel

Walking, cycling or wheeling for short journeys

Capability

Physical abilities and mobility constraints

Choosing active travel for short journeys can depend on individuals' mobility and physical ability to undertake journeys on foot, by bike etc. Physical capabilities can be a major barrier to walking and other forms of active travel in certain groups, particularly older people and disabled people (Currie et al., 2021; Centre for Ageing Better, 2021). Enabling environments can reduce barriers associated with physical capability constraints.

Cycling skills

Cycling requires certain skills and competencies. Developing cycling skills in childhood and adolescence, before young people are old enough to drive may be particularly important for developing a culture of cycling (Colley et al., 2022).

Confidence in abilities

Individuals' feeling of self-efficacy - beliefs about their own capability to walk/cycle/wheel - influence active travel behaviour. This includes confidence and self-belief in one's abilities (Voorheis et al., 2023). Lack of confidence can be a particular barrier to cycling (Centre for Ageing Better, 2021).

Opportunity

Cycling infrastructure

Low-stress facilities (e.g. protected bike lanes, slow or car-free streets) makes cycling a more attractive option (Buehler & Pucher, 2023). Safe cycle networks are important to support cycling both in urban and more rural environments (O'Gorman & Dillon-Robinson, 2021).

Walkable environments

While destination proximity is associated with more active travel behaviour (Aldred, 2019), the walkability of neighbourhoods is also influenced by factors such as land-use mix, street connectivity, pedestrian safety, and residential density (Dovey & Pafka, 2020).

Access to equipment

Cycling requires access to suitable equipment. Schemes providing bikes, including bike share schemes can lead to increased active travel. Improving access to e-bikes, cargo bikes, and bikes with seats for children, as well as traditional bikes, can promote cycling to more diverse groups, including families (Colley et al., 2022).

Social networks

Voorheis et al (2023) stress the importance of social environment on active travel behaviours (especially for the older population), e.g. companions to walk with, support of neighbours, walking as a generalised social norm.

Motivation

Safety concerns

Concerns about safety reduce motivations for active travel. This is a major constraint, particularly for cycling (Colley et al., 2022). Concerns about personal safety can be a barrier to walking for women especially, including use of green active travel routes (Irvine et al., 2022).

Weather conditions

Warm and dry weather has a positive impact on active travel behaviours, while rain, snow, windy, cold and too hot weather encourage a switch to sheltered transport modes (Bocker et al., 2013).

Perceived (in)convenience

Switching to active travel can be perceived as less convenient than alternatives. A recent survey of Scottish residents found this to be the most commonly reported barrier to walking, cycling or wheeling instead of travelling by car or van, stated by 49% of respondents (BMG Research, 2023).

Habit

For frequently-made journeys, the choice of mode is often one which is habitual and automatic rather than a decision which is considered each time. Feeling that habits are hard to break was the second most commonly cited barrier to active travel (reported by 39% of respondents) in a recent Scottish survey (BMG Research, 2023).

Table 4: Capability, Opportunity and Motivation factors influencing switching to public transport

Using public transport instead of driving

Capability

Knowledge about public transport

Information available to people influences their choice of using public transport (transport lines, stops, time of service, estimated travel duration) (Chowdhury & Ceder, 2016). Personalised travel plans can help increase knowledge about public transport options for regular journeys.

Confidence in using public transport

Confidence can be a barrier to using public transport for some people. Getting supported first-hand experience of public transport can help children to develop the competencies to travel independently by public transport (Colley et al., 2022).

Opportunity

Public transport networks

The provision of a low-carbon infrastructure of public transport can encourage individuals' transport mode change (Javaid et al., 2020). Well-developed public transport networks, which include frequent services and integrated transport hubs are necessary to support high uptake of public transport (Quarmby et al., 2019).

Social norms

Seeing low-carbon travel behaviours as the norm and as socially desirable will encourage people to use more public transport (Chowdhury & Ceder, 2016; Javaid et al., 2020).

Cost

The cost of public transport, compared with the use of private cars, influence travel mode choice (Chowdhury & Ceder, 2016). Fare subsidies and concessions can support an increase in share of trips made by public transport, although there is also risk that free public transport can result in shifts from active travel for short journeys (Colley et al., 2022).

Motivation

Perception & experience of public transport

People's perception and lived experience of public transport can (de)motivate them to use public transport, such as the reliability of transport, safety, waiting time for connections, comfort (Chowdhury & Ceder, 2016). Perceived inconvenience is the barrier to switching to public transport cited most commonly by Scottish residents (BMG Research, 2023). Such perceptions also interact with opportunity and capability factors as lower levels of both will likely increase perceptions inconvenience.

Habit

As with active travel, habits can form a barrier to switching from car to public transport for frequent trips, with 30% of Scottish residents reporting habits as a barrier (BMG Research, 2023).

Table 5: Capability, Opportunity and Motivation factors influencing working flexibly or from home

Work flexibly or from home

Opportunity

Job type

Many are unable to work flexibly or from home due to their occupation and the nature of their work. Working from home is primarily an option for those doing office-based work (Beck & Hensher, 2021).

Systemic support for flexible working or work from home

Achieving a successful transition to more flexible working method requires systemic support, such as redesigning physical and digital workplaces to accommodate the diverse needs of employees (Babapour Chafi et al., 2022; Bentley et al., 2016; Bérastégui, 2021). In a recent survey of Scottish residents, the fact that this behaviour relies on the agreement of others (in this case primarily employers) was the most commonly cited barrier to working from home (BMG Research, 2023).

Digital infrastructure

Working flexibly or from home often depends on access to the internet, particularly high-speed internet, as well as appropriate IT equipment (Beck & Hensher, 2021).

Post-pandemic remote working norms

The COVID-19 pandemic led to a significant rise in remote work, and this trend is continuing in the post-pandemic period (Alfanza, 2021; Hu et al., 2021). The development of remote working norms during COVID provides a conducive social environment for the continuation of such working practices.

Home environment constraints

Working from home requires suitable space for working, which is a constraint experienced by many working from home during the COVID pandemic (Beck & Hensher, 2021).

Motivation

Individual benefits from flexible/remote working

There is an increasing preference on working from home, due to the improvement of work-life balance (Erro-Garcés et al., 2022; Nguyen & Armoogum, 2021), commute time saving (Beck & Hensher, 2021) and productivity (Aczel et al., 2021).

Social connectedness

Maintaining social connectedness to colleagues and face-to-face collaboration is a motivator for people to spend at least some of their working time at a physical workplace. Remote working hubs may provide opportunities for in-person social contact closer to home (Beck & Hensher, 2021).

Table 6: Capability, Opportunity and Motivation factors influencing use of local shops and services

Using local shops and services*

Opportunity

Suitable local infrastructure and amenities

Proximity of local amenities is key to reducing private car use (Chau et al., 2022), shorter distances makes it more viable to access amenities by active travel or public transport (Handy, 2017). As well as proximity, the accessibility of amenities is important – a destination may be close by but not accessible due to e.g. main roads, railway lines (O'Gorman & Dillon-Robinson, 2021). The number of local amenities, their variety and the specific types of amenities available also impact on whether people can meet their needs without having to travel by car (Elldér et al., 2022). The level of quality and experience, not just their presence or absence, will play a part in whether they are used (O'Gorman & Dillon-Robinson, 2021).

In 20-minute neighbourhoods, people are more likely to walk for transport than those not in 20-minute neighbourhoods. Across Scotland there are communities that have suitable infrastructure and services allowing them to be 20-minute neighbourhoods, although less is known about the quality of services available and whether these communities are functioning as 20 minute neighbourhoods (O'Gorman & Dillon-Robinson, 2021).

Cost

In a survey of Scottish residents, 40% reported cost to be a barrier to shopping more locally/minimising travel (BMG Research, 2023).

Motivation

Habit and lack of desire for change

The choice to use local shops and services may depend on existing habits. In a survey of Scottish residents, when asked about barriers to shopping more locally/minimising travel, 32% of respondents stated liking how things are now and 31% stated that it is a hard habit to break (BMG Research, 2023).

*Factors influencing active travel and public transport use are also interconnected with choice of where to go for shopping and services (see Tables 3 and 4).