Air quality: key behaviours report

6 Driving differently

6.1 Evidence on air quality impacts of changing driving behaviour

A third broad behavioural area highlighted in the literature relates to changing behaviour when driving. Changes to the way we drive, often discussed under the broad banner of eco-driving, have the potential to offer fuel efficiency gains which may translate into reduced pollutant emissions and improved air quality. A review on driving behaviour by Keyvanfar et al. (2018), highlights that driving style plays a significant role in fuel consumption, as more aggressive driving, and more stop-start driving leads to higher fuel use. Eco-driving techniques, which include slow acceleration and deceleration, early gear changes, adherence to speed limits and avoiding vehicle idling are argued to significantly reduce fuel consumption and emissions of NOx, hydrocarbons and greenhouse gases (Keyvanfar et al., 2018). Reviews on eco-driving report that fuel consumption can be significantly reduced - empirical research conducted in various contexts demonstrates an average fuel saving range of 5 - 25% (Coloma et al., 2018; Fafoutellis et al., 2020; Miotti et al., 2021; Wang & Boggio-Marzet, 2018), Alam & McNabola (2014) and Huang et al.,(2018) even report reductions up to 40%. However, The impacts on air quality are less clear. The focus in the literature has tended to be on fuel consumption and CO₂ emissions (Huang et al., 2018), whereas emissions of pollutants such as NOx, CO and hydrocarbons are known to correlate less highly with fuel consumption than CO2 emissions (Kurani et al., 2015).

Rodríguez et al. (2016) estimated reductions of 24% for NOx as a result of improving vehicle flow and reducing frequent abrupt accelerations. Fonseca et al. (2010) found no advantage of eco-driving over a 'normal' driving style in terms of NOx, however both compared favourably to aggressive driving which increased NOx emissions by an estimated 40%. The evidence of the impacts of eco-driving on air pollutants such as NOx appears mixed. Estimated impacts will likely depend on the specific driving behaviours modelled and the assumptions embedded in emissions models. In relation to driving speeds there is some evidence that reduced speed limits can help reduce emissions due to an improved flow of traffic (Connected Places Catapult, 2019; Owen, 2005; Quarmby et al., 2019). However, a review of evidence on the health impacts of 20 mph urban speed limits, found that whilst 20 mph zones are associated with greater road safety, there is less robust evidence on air pollution impacts (Cleland et al., 2020).

Whilst there are potential benefits from eco-driving, and interventions aiming to increase uptake of eco-driving are seen to be cost-effective ways to reduce emissions (Keyvanfar et al., 2018), evaluations of eco-driving schemes have shown mixed results in terms of behaviour change, especially in relation to long-term maintenance of behaviours (Alam & McNabola, 2014; Barkenbus, 2010; Quarmby et al., 2019).

Avoiding idling (running the engine when car is stationary) is often considered as part of eco-driving but can also be targeted as a behaviour in and of itself. It is reported that when a car is idling for 10 seconds or longer, the fuel consumption and emissions exceed those associated with stopping and restarting the engine (Rumchev et al., 2021). Emissions associated with idling do, however, depend on the type of car. Although both petrol and diesel cars emit CO₂ while idling, the NOx emissions associated with an idling petrol car are minimal in comparison to those emitted by idling diesel cars with PM emissions also lower (Barlow & Cairns, 2021; Shancita et al., 2014). Targeted campaigns focusing specifically on anti-idling can reduce idling behaviour, for example around schools (Rumchev et al., 2021) and at level crossings (Abrams et al., 2021; The Behavioural Insights Team, 2022), with associated improvements to air quality (Abrams et al., 2021; Mendoza et al., 2022; Rumchev et al., 2021), but more evidence is needed on their long-term effectiveness. Idling reduction technology in vehicles can also address idling emissions without the need for behaviour change (Shancita et al., 2014).

Overall, the evidence suggests that changes in driving behaviour have the potential to impact significantly on the emissions released when driving combustion-fuelled vehicles. However, it is important to note that such measures to increase efficiency are of lower impact than more fundamental changes that shift travel behaviour away from combustion-fuelled vehicles. Given the potential gains, we propose eco-driving as a key behaviour. In doing so we recognise that eco-driving is a suite of different behaviours, which could make communications to public audiences about what they comprise more complicated. Whilst the impacts of changing driving behaviour differ between petrol and diesel vehicles, encouraging these behaviours (particularly those that are most visible) across all car drivers has the potential to reinforce positive social norms.

Driving differently

Using eco-driving techniques (including stopping engine when stationary)

6.2 Factors influencing eco-driving

Motivations and barriers to the uptake of eco-driving were identified in the literature. These are summarised below in Table 8 in relation to capabilities, opportunities and motivations.

Table 8: Capability, Opportunity and Motivation factors influencing eco-driving behaviours

Eco-driving

Capability

Eco-driving skills

Generally, eco-driving skills are straightforward to comprehend and implement. It is expected that any regular car user should be able to incorporate eco-driving practices (Keyvanfar et al., 2018).

Training programmes can equip people with the skills and knowledge to use eco-driving techniques (Huang et al., 2018).

Opportunity

Institutional support

The implementation of generalised eco-driving requires systemic support (Ando et al., 2010; Ando & Nishihori, 2011), and public training is crucial to raise awareness about eco-driving (Johansson et al., 2003).

Motivation

Fuel-saving

Eco-driving is often highlighted for its dual advantages in reducing pollution and conserving fuel. The potential financial gains from fuel savings serve as motivation for individual drivers to adopt this driving approach.

Safety

Passenger safety has been strongly correlated with driving style, and eco-driving is proven to increase road safety (Alam & McNabola, 2014; Cristea et al., 2012), while aggressive driving styles provokes more accident risk (Bachoo et al., 2013; Rundmo et al., 2011, 2011).

Travel time

Research also indicates that eco-driving may lead to varying degrees of travel time increase (Coloma et al., 2018; Miotti et al., 2021), potentially reducing its public acceptability.

Driving habits

Driving habits appear to be a significant deterrent for individuals considering a shift towards eco-driving (Chung, 2015; Tseng et al., 2013). Habits are the most commonly cited barrier both to avoiding idling a vehicle (44% of respondents) and wider eco-driving behaviours (35% of respondents) amongst Scottish residents (BMG Research, 2023). However, while entrenched driving habits are hard to shift, new positive habits once established can support long-term behaviour change.