**1. Introduction**

In recent years, European governments including the UK, have committed to addressing the adverse air pollution in their cities, by setting targets to decrease the carbon dioxide (CO2) levels. Since then, the use of diesel vehicles has been incentivised and these are favoured over petrol vehicles in several European cities through the 1998 agreemen<sup>t</sup> between the European Automobile Manufacturers Association and the European Commission [1,2]. As a result, diesel vehicles now form a significantly increased share of the UK's licensed vehicle fleet. Since 1994, the proportion of diesel vehicles increased from 7% to about 40% in 2016, reaching a total of more than 12 million vehicles [3]. Although previous research has asserted that diesel vehicles are more CO2 efficient than petrol vehicles [4,5], a very recent study revealed that diesel vehicles produce similar "real-world" CO2 emissions to petrol vehicles [6]. In the same study, the authors indicated that diesel vehicles have not demonstrated significant advantages over petrol vehicles in terms of CO2 emissions since 1995 in Europe. The policy of favouring diesel vehicles over petrol vehicles came at the cost of a considerable increase in air pollution in many cities, particularly in London, since diesel vehicles emit toxic pollutants such as

nitrogen oxides (NOx) and particulate matter (PM) [4], which are recognised as the most dangerous to the human health [4,7]. Thus, there is an urgen<sup>t</sup> need to take action against the continuation of diesel vehicles.

Studies have shown that the transport sector emits the greatest amounts of greenhouse gas (GHG) emissions [8], whereby 95% of transport sector emissions are caused by the road transport [9]. In 2014, the road transport was responsible for 20% of all global CO2 emissions [10]. Although the scope of this paper focuses primarily on road transport emissions due to the scale of their contribution to London's air pollution [2], it is imperative to point out that there are other sources of air pollution that originate from within the transport sector such as aviation, rail, and maritime activities as well as other sectors including industrial and energy productions [2,11,12]. Therefore, it is argued that addressing the emissions produced by all of the aforementioned sources should be considered when developing policies for a sustainable zero-carbon transport system. Otherwise, failure to reverse the current trends in the transport emission concentrations will only exacerbate the situation and lead to further adverse consequences in the future.

London's air quality is considered the worst among European cities [13], and the UK has been threatened with legal action and fines if it fails to reduce its air pollution emissions in some cities including the capital [14]. As a result, successive governments and policymakers have tried to implement new policies to tackle air pollution in London but, regrettably, the crisis still prevails [15]. In addition, recent studies have revealed that the vast majority of Londoners (82%) agreed that tackling air pollution must be a priority [16]. We therefore emphasise that e ffective new transport policies, tailored to the challenges that the capital faces, must be developed and introduced as a matter of urgency. This situation underpins the salience of this research paper.

In light of the current air pollution situation, the Mayor of London set a goal of achieving a zero-emission road transport by 2050 in the Mayor's Transport Strategy (MTS) [17], and the UK governmen<sup>t</sup> is committed to lowering GHG emissions by 80% by 2050 through the 2008 Climate Change Act [11], as articulated in the Agenda 2030 for sustainable development [18]. It is argued that achieving this ambitious target can be attained by the decarbonisation of the transport sector [11]. Consequently, the UK governmen<sup>t</sup> has announced its intention to ban the diesel and petrol vehicles by 2040 [19] in order to counteract air quality issues. The governmen<sup>t</sup> justified the forthcoming ban by publishing reports [20] predicting that, by 2040, (1) high NOx levels in the UK will be one of the biggest environmental risks to people's health in terms of diabetes, asthma, bronchitis, lung cancer, and heart disease; and (2) air pollution in the UK is expected to destroy 50% of the plant life as well as 40% of the wildlife habitats. However, there is a continuing debate about whether banning diesel vehicles can provide the answer to tackling air pollution. On the one hand, some German politicians as well as car companies, are not in favour of the ban and feel that it is "excessive" [21,22]. On the other hand, transport and environmental experts [7,15,23] realise the importance of the ban and have therefore made a strong case for policymakers to implement it as soon as possible. Recent research shows that banning diesel vehicles would have a significant impact on reducing the NOx and PM2.5 levels as well as improving the public health [7,15].

Studies examining the public opinion concerning the ban are scarce. Previous literature paid much attention to interventions such as the creation of low emission zones (LEZs) [24–26], but the authors concurred that LEZs do not bring about the desired improvements in the local air pollution. One recent study investigated the potential economic and health benefits of banning diesel vehicles in Dublin [7], while others have attempted to predict the attitudes of UK drivers towards switching to electric vehicles (EVs) after the ban is introduced [20]. However, to the best of our knowledge, there have not been any empirical investigations, specifically using London as a case study, to gauge public opinion regarding the forthcoming ban on diesel vehicles in 2040. The public opinion with respect to any putative ban has ye<sup>t</sup> to be fully explored. Furthermore, it is argued that the reluctance to implement the ban in the UK is a contributing factor to thwarting the early uptake of EVs as well as weakening the EV policy [15,23]. For this reason, our research is significant, as it investigates whether the ban can be implemented earlier than 2040. The outcome of this investigation will provide clearer signals regarding the future of diesel vehicles, which in turn will strengthen the EV policy and uptake.

We argue that addressing London's air pollution is fundamentally linked to three factors, namely: (1) A strong political will, which includes strategies and action plans on the city, national, and international levels; (2) the cultural paradigm, e.g., city branding. This comprises the practices and marketing of the city, which has a direct influence on sustainability and people's behaviours [27,28]; and (3) public participation. This paper takes into consideration the third factor, i.e., public participation, paying particular attention to the case study of London using a bottom-up approach.

This paper aims to fill the gap in the literature by establishing the public opinion and attitudes towards the ban. It also aims to investigate the rate at which the Mayor should implement the ban, and to explore what factors may influence the public acceptance of this policy. Although the forthcoming ban is intended to apply to both diesel and petrol vehicles, the core focus of this paper is on the diesel vehicles since they produce more toxic emissions that require further attention.

### **2. Case Study, Data and Methods**
