4.1. The Population in the Urban Area of Barcelona: Differences between Core and Peripheral Rings
Table 1 shows that population has grown more in the AMB’s outer ring than in the inner one, and more in that inner ring than in the city of Barcelona. However, as already explained, the AMB only comprises the most central part of the Barcelona functional urban area (FUA) (see
Figure 1 again). As we intended to analyse the influence of metropolitan population growth and its spatial distribution on daily mobility, it was necessary to adopt this second and broader definition of metropolitan limits, since FUA is precisely delimited by daily movements due to work or study—i.e., commuting.
Table 2 shows the population of the Barcelona FUA—around 5 million inhabitants—of the core and of the 10-km rings, on 1 January of each year. In the first two decades of the 21st century, the population of the metropolitan area of Barcelona continued to grow. However, it has not grown in all of its areas [
32]. The population of the city of Barcelona and the contiguous municipalities (located at less than 10 km away from the capital city) fluctuated during the economic expansion, crisis and post-crisis periods. It grew between 2000 and 2009, and between 2015 and 2020, and decreased between 2009 and 2015. Annual growth between 2000 and 2020 has been very low, both in the core (0.53%) and in the first metropolitan ring (0.24%). In contrast, the suburban municipalities located 10 km or more from Barcelona grew throughout the entire period analysed. In absolute figures, municipalities located between 10 and 20 km from Barcelona are the ones that have grown the most. However, in relative terms, the highest growth corresponds to the municipalities situated further away (30 km or more).
Figure 4 shows that, during the economic expansion period (prior to 2009), suburban municipalities located 10 to 20 km from Barcelona were the ones that increased their population the most in absolute numbers. These were followed by those located 20 to 30 km from Barcelona and, thirdly, came the city of Barcelona itself. At the other end of the spectrum were inner ring municipalities—less than 10 km from Barcelona—that gained the least population.
During the economic recession phase (from 1 January 2009, to 1 January 2015), Barcelona lost the most population, followed by the closest towns and those located more than 50 km away from the central city. The rest of the rings continued to gain population, especially those located between 10 and 20 km, followed by those situated between 20 and 30 km away from Barcelona. The more remote municipalities gained less population, showing that suburbanisation lost momentum during the economic crisis, probably because the estate bubble burst and the construction industry collapsed. Finally, the new economic recovery phase, beginning on 1 January 2015, has shown different characteristics from the expansion one. Barcelona and the inner ring have been gaining the most population in absolute numbers, followed by the municipalities situated between 10 and 20 km from the central city (
Figure 4). However, this is not the case in relative terms (
Figure 5). This graph shows that suburbanisation has slightly recovered in recent years, but not as intensely as before 2009, and that it particularly benefits those municipalities located more than 30 km away from Barcelona.
The demographic changes described in the previous paragraphs explain that, in the first two decades of the 21st century, the proportion of the metropolitan population residing in the core has progressively decreased. It has gone from representing 35.4% of the population in 2000 to 32.7% in 2020 (
Figure 6). The same happens with the contiguous inner ring, as residents within 10 km of Barcelona go from 19.7% to 17.2% in the same years. However, the percentages have stabilized—and even increased slightly—in recent years. The opposite occurs in outermost rings, gaining demographic importance since the beginning of the century. For instance, the 20 to 30 km ring goes from representing 15.5% in 2000 to 16.5% in 2020, and the 30 to 40 km ring from 4.1% to 6.3%, respectively.
In these two decades, not only has the population living in the metropolitan periphery grown more than that of the core city, suburbanisation has also produced a change in age structure. While suburban towns are mainly composed of families—with working-age parents and children—the population of Barcelona is older. However, there are also differences within the city. In the more central neighbourhoods, where many national and foreign immigrants have settled, there is a process of rejuvenation—and, in some neighbourhoods, also gentrification. However, in the working-class ones, built in the 1960s and 1970s, the population is ageing [
33,
34]. This ageing process will also reach the peripheral municipalities in the future. In the meantime, the existing differences in age structure are relevant in terms of the means of transport used. Indeed, as it will be observed in the following section, there are not only differences according to the geographical place of residence, but also to the age of the population that uses them.
4.2. Daily Mobility in Barcelona: Towards Metropolitan Transport Sustainability?
The public authorities of the city of Barcelona and its metropolitan area have been implementing policies to achieve the transition to a more sustainable mobility, for several years. The most recent are the low emission zone (ZEB-Rondes) enforced in 2020 in Barcelona and some contiguous municipalities, and interventions on physical urban space. These latter ones intend to reduce the circulation of private vehicles by, among others, limiting the number of traffic lanes in several streets and vehicle speed. Both measures were rapidly enforced during the COVID-19 lockdown. However, other measures, aiming to increase the use of public transport and of micro-mobility, had been previously implemented: the expansion of the metro (subway) system, the creation of a new orthogonal bus network, and the addition of new kilometres of bicycle lanes. But what were the results of these policies until 2019? Has private vehicle traffic decreased?
Starting with the total number of daily trips, these have increased since the beginning of the 21st century. Despite that they slowed down with the economic crisis (when many people lost their employment), they grew again with the economic recovery and fell again in 2020 due to COVID-19 (
Figure 7).
These changes have also been observed in the number of trips per person, reaching an average of 3.97 in 2019 (
Figure 8). Outer ring inhabitants are those that are currently making more daily movements (4.03 movements per person) and those who live in the inner ring, the ones that do it the least—3.91 times. However, differences among the three areas are small.
This increase in mobility during the post-crisis has been observed in all means of transport. Starting by public transport, both the annual number of passengers travelling by bus and by metro has grown, since 2012 or 2013, after a few years of falling because of the economic crisis (
Figure 9). The thresholds of 400 million and 200 million annual metro and bus passengers, respectively, were surpassed in 2018.
Active mobility users—those who move on foot, by bicycle or on a scooter—have also increased, especially in the case of short distance moves. For instance, daily bicycle journeys have grown from 163,492 in 2013 to 211,993 in 2019 (
Table 3). These forms of micro-mobility have benefited from the constant growth of the bicycle lane network (reaching 236 km in 2020,
Figure 10) and of other types of pedal or cyclable paths, such as pacified streets and green paths (992 km and 174 km, respectively, in the AMB in 2019). Similarly, the network of secure bicycle parking areas (Bicibox) has also spread throughout the AMB. The growing number of its registered users indirectly shows that the amount of people using bicycles as their main means of transport in the urban area has increased (
Figure 11).
Another key element of metropolitan policies to achieve the transition towards sustainable mobility is promoting electric vehicles to progressively replace combustion engine ones. To that end, a growing network of electric charging stations—similar to petrol ones, but where electric vehicles can recharge their electric batteries—has been established, reaching a total of 10, in the whole AMB.
The number of its users has been growing gradually in just five years, reaching 581 in 2019. Consequently, the number of recharges and kWh distributed has also grown to 37,835 and 383,889, respectively, in 2019 (
Figure 12). These electric recharges would cover 2.7 million km and avoid emitting 356.47 tons of CO
2 and 1508.14 kg of NO
2 to the atmosphere [
36].
However, electric vehicles are still a tiny part of those vehicles moving in Barcelona and its metropolitan area. Furthermore, despite public policies promoting the use of an increasingly sustainable mobility, daily trips by private vehicle have grown more than those made by sustainable means of transport, during the post-crisis. This can be observed in
Table 3, showing that the total number of daily movements has grown in virtually all means of transport in the AMB between 2013 (one of the deepest crisis moments) and 2019. However, as the table continues to point out, the rate of growth of the number of daily trips by private vehicle more than doubles the growth in the use of public transport or active mobility, in 2019. In fact, these journeys by private vehicle represent more than a quarter of the total moves (
Table 3). While central city dwellers use active mobility more frequently than inner ring ones, private vehicles are predominant against any other means of transport in the outer ring (
Table 4).
Why has the use of private vehicles increased despite the deployment of policies to reduce them in Barcelona and its metropolitan area? Possible causes are developed in the following paragraphs.
One of the plausible reasons is population ageing. EMAF 2019 mobility survey data indicates that young people between the age of 16 and 29 are those that use public transport the most. Indeed, in 2019, they made 39.3% of all their trips on this kind of vehicle, compared to 21.2% of the adults between the age of 30 and 64, and to 19.3% of those over 64 [
36]. Therefore, the older the population of a municipality, the less public transport is used. However, active mobility increases to percentages ranging from 42.2% of all trips in young people, to 47.2% in adults and 65.7% in the elderly people. By contrast, adults between the age of 30 and 64 are the biggest private vehicle users (31.6% of all daily moves compared to 18.5% of young people and 15% of elderly people, in 2019).
This growing use of private vehicles by adults could be related to their greater participation in the labour market. Therefore, a second factor, daily mobility by cause, and its possible changes, must be analysed. Two out of every three trips are for reasons other than to work or study, and only a third is to commute. This latter percentage decreased during the economic crisis—when the number of unemployed increased considerably—but has recovered since 2013; the percentage is higher in 2019 (
Figure 13 and
Figure 14).
Though differences are not huge, the highest percentage of commuting trips is observed in outer rings, and the lowest in the city of Barcelona (
Figure 13). The opposite occurs in daily trips for reasons other than commuting (
Figure 14). Once more, we can assume that these differences between the core city and peripheries may partly respond to age structure, since people living in Barcelona are relatively older, while suburban municipality residents, especially those in the outer ring, are mainly of working age or young people who study [
33].
What would this have to do with the number of private vehicles used, the vast majority (still) being powered by a combustion engine?
Table 5 shows that daily movements for reasons other than commuting are mainly active ones—on foot or by bicycle—while commuters mainly use public transport or their private vehicle. The increasing proportion of commuting trips among daily movements between 2013 and 2019, combined with growing suburbanisation, would explain, to a large extent, that percentages of those using active mobility or public transport to move in the AMB, have decreased. In fact, those who move in private vehicles have increased by almost a point and a half.
Does this mean that policies promoting public transport have failed? Not necessarily, though the use of private vehicles for reasons other than commuting has risen slightly, this has not been so in the case of commuting. In fact, active mobility is the form of mobility that has grown the most between 2013 and 2019, while the use of public transport has decreased and that of private vehicles has remained stable. In other words, it is the greater proportion of commuting in relation to overall mobility and the greater number of trips for work or study reasons that explains the increased use of private vehicles in recent years, not the fact that they are increasingly used for commuting.
Undoubtedly, the relation between age structure and reasons for daily mobility must affect the way the different means of transport are used in each area into which the AMB is divided. To this end, we analysed the diverse forms of moving within the AMB, distinguishing the three areas into which it is divided, and how they have changed over time (
Figure 15).
The results showed that it is in outer ring suburban towns where active mobility and public transport are used the least and that the highest proportion of daily trips are made by private vehicle. Furthermore, it is also in these municipalities where private vehicles are most used, even in daily mobility for reasons other than commuting [
31]. This would explain why this type of daily mobility, made by private vehicles, has increased (
Table 5). By contrast, public transport and mobility on foot and by bicycle are the most used in Barcelona. In this sense, the rest of the inner ring municipalities are in an intermediate position.
An indicator summarising data discussed in the previous paragraph is the public transport/private transport ratio. A result bigger than 1 means that public transport is used more, and a result smaller than 1 indicates that private vehicles predominate.
Figure 16 shows that it is in the city of Barcelona that public transport is predominantly used, while, in peripheral municipalities—especially outer ring ones—private vehicles are used more. In the whole AMB, this indicator is around 1, showing a certain equilibrium in the use of both transport modes, though the indicator tends to decrease, therefore, pointing to an increasingly predominant use of private transport. Given that the three lines corresponding to the three geographical areas under analysis in
Figure 16 show great stability, the downward trend of the ratio for the whole AMB could be explained—as it has already been stated—by a progressively greater demographic importance of the peripheries with respect to the core of the urban area, the city of Barcelona.
Another factor that may explain this tendency towards a greater use of private vehicles is a change in the spatial patterns of traffic, showing a gradual increase in longer daily trips. This can be analysed using several indicators. For instance, that of municipal self-containment, giving the percentage of daily trips originating in and with destination to the municipality of residence (
Figure 17). Obviously, the highest municipal self-containment values (close to 90%) are observed in Barcelona, the largest, most populated, and densest municipality, that has the largest job market and more services. At the other end of the spectrum are periphery municipalities (with values below 60%), in many of which it is necessary to leave to work, consume, visit a doctor, etc. More significantly, municipal self-containment has reduced in all three geographical areas since 2013, because of economic and labour market recovery. Indeed, this is clearly shown by the line in the graph indicating self-containment for the whole AMB. In other words, increasingly more people leave their municipality to work, study or other reasons, implying that the distance travelled has grown.
Another interesting indicator is destination-origin ratio, found by dividing the number of daily trips with destination in of each area by those originating in each of them (
Figure 18). The result for the city of Barcelona is more than 2. This indicates that, at least, twice as many daily trips have this destination as those originating in it. The opposite occurs in contiguous municipalities to Barcelona, the rest of the inner ring, where the number of trips in which the destination is the inner ring is almost half of those originating in it. These are mainly residential municipalities, where many of the people living in them commute to Barcelona. By contrast, in outer ring towns, the indicator shows a more balanced result. Indeed, many of their inhabitants move out of their municipality of residence to work or for other reasons, but many people also arrive to them daily, for instance, to work. In fact, numerous industrial and logistic parks are situated in this outer suburban area. The line in the graph shows that values for the whole AMB are more than 1 (there are more moves that have this destination than those originating in it) and this trend grows. This could suggest that the AMB is increasingly open to the world, implying longer daily trips.
The last indicator, average duration of daily trips by public transport (
Figure 19), shows an increase in distances travelled. Obviously, the lowest values of this indicator correspond to the core city (less than 35 min) and the highest ones to outer ring municipalities (more than 45 min). In addition, it should be also noted that figures tended to rise. In other words, a longer time travelling can be synonymous with a longer distance covered and could also imply a potential advantage for using private transport in metropolitan areas. In fact, the average duration of journeys in private transport in the AMB (23 min) is much lower than that in public transport, lasting, on average, more than half an hour, or 34.8 min, to be exact. In fact, private vehicle journeys are only longer than daily trips on foot, bicycle, or scooter (15.8 min). As each mean of transport is used differently in the core city, in the rest of the inner ring and the outer ring, the average duration of daily trips is very similar in the three areas under examination—22.4, 22.0 and 21.8, respectively, for an average duration of 22.2 min in the whole AMB. In other words, despite that population has grown more in the periphery with respect to the centre, and despite the increased travel distances that this entails, there would not have been an increase in the average duration of trips due to the greater use of private transport in the outer ring.
In short, the public policies implemented have, at the same time, increased the use of active mobility and public transport, between 2013 and 2019. However, that of private transport has augmented even more, probably due to a combination of several factors: population ageing, an increasing percentage of the population living in peripheries—mostly working (adults) or studying (young people)—an increment of commuting with respect to other reasons for daily movements, and of the distance travelled. In the following section we will observe its impact on air quality.
4.3. Air Quality in the AMB: Towards (Insufficient) Pollution Reduction
Air pollution is an important cause of mortality and morbidity; it is the environmental factor that most damages health. Pollution increases the risk of contracting respiratory, cardiovascular, immunological, neurological, or oncological diseases and increases the risk of dying from any other cause at any age. According to the Barcelona Public Health Agency (ASPB), excess air pollution causes about 1000 deaths in the city per year, a figure representing 7% of all deaths [
37].
One of the air pollution components are substances in the air, which are the ones that can cause the greatest health damage. They are mainly caused by anthropogenic action, namely, by industrial and transport emissions.
The pollutants found in the air are: suspended particles (PM
10 or PM
2.
5); sulphur compounds (S0
2 sulphur dioxide; H
2S hydrogen sulphide); nitrogen compounds (NO nitric oxide, NO
2 nitrogen dioxide, NOx nitrogen oxide, NH3 ammonia), carbon compounds (CO carbon monoxide, CO
2 Carbon dioxide, CH4 Methane, THC total hydrocarbons), halogens and chlorine compounds (Cl
2 chlorine, HCl hydrochloric acid, HF hydrofluoric acid, CFCs chlorofluorocarbons) and photochemical oxidants (O
3 ozone, peroxides, aldehydes). The pollutants affecting this metropolitan area the most are: nitrogen dioxide NO
2—an irritating gas generated by road traffic, especially diesel-powered vehicles—and suspended particles (PM
10 or PM
2.
5), emitted by combustion vehicles [
38].
Among the many different indicators measuring pollution, the ICQA index—elaborated with data on the main air pollutants (SO
2, NO
2, O
3, CO and PM
10) —was chosen to analyse air pollution, in this study. Data come from the Directorate of Environmental Quality belonging to the Department of Territory and Sustainability of the Generalitat de Catalunya. Its values vary between −100 (worst air quality) and 100 (best air quality). Shown in
Table 6 are the mean values of the ICQA index for those AMB municipalities where there are measurement stations connected to the Atmospheric Pollution Prevention Surveillance Network of Catalonia.
In most municipalities, the values of the index exceeded 50% during the entire period under analysis, reaching the highest values—best air quality—between 2010 and 2014, the economic crisis years. In general terms, air quality is better than 15 years before. As for geographical location, the highest rates were observed in the inner ring municipalities such as Sant Coloma de Gramanet, Sant Andreu de la Barca, L’Hospitalet de Llobregat and also Sant Cugat del Vallès—the latter, part of the AMB, but situated in the outer ring. However, in the city of Barcelona, air quality was lower. The main components of atmospheric pollution in all these municipalities are nitrogen dioxide (NO2) and PM10 particles. In the city of Barcelona, air quality is measured in two types of stations, urban traffic stations and urban background stations. The first measures pollution from street traffic emissions. As for urban background ones, they measure contamination from different origins. They are situated away—300 m—from main roads with more than 10,000 vehicles per day, or from any other nearby emission source.
According to data from the Barcelona Public Health Agency and the Department of Climate Action, Food and Rural Agenda of the Generalitat de Catalunya [
37], pollution levels in the city of Barcelona have been decreasing since 2004. Indeed, data reflect the positive results of the implementation of measures to reduce pollution both in the city of Barcelona and in the AMB. However, NO
2 emissions by urban traffic (
Figure 20) are above the limits for the protection of human health. In fact, they are above the limits set by both the European Environment Agency—40 µg/m³ (in force since 2005)—and the WHO, which is much more demanding, i.e., 20 µg/m³. Only urban background stations show NO
2 levels below 40 µg/m³ (
Figure 21). The case of PM
10 particles is different, as they have decreased, reaching half the limit considered harmful to health (
Figure 22 and
Figure 23).
Therefore, the reduction in pollution and the improvement of air quality in the city of Barcelona and the municipalities of the AMB was confirmed; indexes were more positive than fifteen years ago. Vehicle fleet composition changes (in terms of environmental standards) due to car renewal may be partly responsible for this improvement: according to the Barcelona City Council, in 2019, only 25.4% of the vehicles registered in Barcelona did not have the environmental sticker identifying the most environmentally friendly vehicles. This reduction in pollution was faster in the crisis years and slowed in the economic recovery ones. Moreover, reductions were lower in traffic stations and with regard to pollutant NO2, emitted mainly by diesel vehicles. Finally, the latest available data show that the restrictions caused by lockdowns and the sharp reduction in mobility during the pandemic led to an unprecedented improvement in air quality. Nevertheless, once activity has resumed, it will gradually recover pre-pandemic levels.