The Configuration of the Transport Network

To analyze how the different historical transport networks were configured, in this article we will identify which areas and cities were most well connected and, on the contrary, which territories were isolated. In this article, “connectivity” refers to the capacity for cities and areas to be connected, through different means of transport allowing the mobility of people and commodities. The public expenses devoted to the construction and maintenance of transport infrastructures represented a very huge economic effort for the governments in each historical period. For these reasons, all the well-connected cities should offer solid political, economic, or social justifications to those investments. The evolution of the network structures and its connectivity shows different patterns of political design, favoring some regions and hindering others. The differences between each period will serve us to explain some of each period’s government rulers.

To obtain these calculations, we have analyzed the centrality degree of each city and urban settlement of the Iberian Peninsula. The theoretical basis for the calculation of centrality in this project has been based on the graph theory and the use of network analysis (Dicks Reference Dicks1972; Freeman Reference Freeman1979; Hagget and Chorley Reference Hagget and Richard1969). This theory has already been widely applied in many areas and also in archaeology (Knappett Reference Knappett2013; Mills Reference Mills2017). Taking into account this theory, every element of our reality can be converted into a relationship of nodes (points) and edges (lines) that are connected to each other. Thus, each element is represented by a node and the relationship between them by an edge. If there is a relationship between two nodes, then there is an edge. Otherwise, obviously not. From this set of relationships, a matrix can be created. From this premise, this methodology can be perfectly applied to the case of roads and cities in different historical periods. Each city and crossing represent a node and each route (whether terrestrial, fluvial, or maritime) is represented by an edge.

Initially, as in many other investigations on network analysis, the network created was understood as a binary system by nature, that is, evaluating connections between nodes with a value of 1 if a connection existed and 0 if no connection existed (Isaksen Reference Isaksen2008; Keay and Earl Reference Keay, Graeme, Patrick Le and Pierre2006). Unfortunately, this application of binary networks doesn’t reflect the complexity of a transport network and the different impact (or benefits) of each mean of transport could offer to each city. As not all the means of transport were equally important for the political, economic, and social development of a territory (or a city), not all the transport systems should have the same importance in the calculations of connectivity. For this reason, a weighted network analysis system (Barrat et al. Reference Barrat, Barthelemy, Pastor-Satorras and Vespignani2004) was created in this project. According to this system, the edges of a network are evaluated not only if they are absent or not (binary calculation) but also if they have different values and, therefore, the weight of a node is calculated from the sum of the different values of the edges that are communicated. For nonsocial networks, the weight often represents the capacities of the ties, for example, the airports’ connections (e.g., the number of seats among airports; Colizza et al. Reference Colizza, Romualdo and Alessandro2007; Opsahl et al. Reference Opsahl, Vitoria, Pietro and José2008).

Applying a value to each of the edges depending on the means of transport they represent has been a very discussed and tested element in the development of this project and in previous projects (Carreras and de Soto Reference de Soto2010, de Soto Reference de Soto2010; de Soto and Carreras Reference Carreras2015). The established weighted edges represent the different impacts (in terms of economy and urban development) of each mean of transport. From the historical data obtained from the study of the different transport systems (Carreras and de Soto Reference de Soto2010) it has been established in each historical period that the maritime system was the most economic and fastest mean of transport and offers the greatest transport capacity, until the arrival of the railway. Second, river transport, especially downstream, offered fast and very economical navigation, although not upstream navigation, which was very expensive and slow. Finally, land transport has been divided into the main roads and secondary roads (in this project we also have distinguished tertiary roads for Early Modern and nineteenth-century networks). To determine the weight of each one of the edges, several tests were carried out with different values to observe which values offered more credible and understandable results. This analysis is not intended to evaluate transportation costs as it will be in the following analysis, instead it pretends to analyze the connectivity of each city. Although, at first, it seemed interesting to use weighted values directly depending on transport costs, it could be observed that the results were too exaggerated because the difference in costs (sea 1, river 5/10, land 50Footnote ⁸ ) was too high (in the same way than other intermediate values). For this reason, it was decided to test various qualitative values until it was determined that the optimal ratio resulted from the choice of minimum values between 4 and 1 (sea 4, river 3, main road 2, secondary road 1) to develop the calculations. Previously to this project, we’ve tested the application of this methodology, in less detailed networks in the Iberian Peninsula and in other Roman territories with interesting results (Carreras and de Soto 2013).

In this project, we have sought the calculation that best helps to visualize and understand the efforts of the different governments to increase the communicability of their diverse territories and the reduction of connectivity of other territories due to the lack of investment in their infrastructures. To do this, we have chosen the calculation of centrality based on obtaining the value k of each node of the network that represents the sum of all communication channels available to a city, settlement, or crossing. As we understand that not all edges had the same value depending on the transport system they represent, we created a weighted network, a study that was assimilated and deepened by several authors (Newman Reference Newman2004). The study of the degree of centrality understood as the value of the sum of connections available to a central node is simple and has the advantage of not needing to know the entire structure of the network (Opsahl et al. Reference Opsahl, Filip and John2010). In our project, this calculation is even more valuable because we know the entire network and the precedent and subsequent centrality situations that allow us to create historical comparisons.

In a weighted network, connectivity data can be represented mathematically by an adjacency matrix where the values are not only zero or one, but are equal instead to the weights on the edges:

$${{\rm{A}}_{ij}} = {\rm{weight}}\,{\rm{of}}\,{\rm{connection}}\,{\rm{from}}\ {{\rm{}}_i}\,{\rm{t}}{{\rm{o}}\ _j}.{\rm{}}$$

To calculate the degree k of a node in a weighted graph, the procedure is similar to a normal unweighted graph. The degree k of a vertex _i in a weighted network is the sum of the weights of the edges attached to it:

$${k_i} = \sum\limits_j^{} {{A^{ij}}} $$

For example, the Roman Provincial capital of Tarraco (Tarragona) was connected through a seaport, 3 main roads, and 1 secondary road. With a binary network, we could say that this city had a k connectivity equal to 5 (1 seaport + 3 main roads + 1 secondary road = 5 connections). Using weighted networks where each mean of transport has different values, its k connectivity is equal to 11 (1 seaport = 1*4 + 3 main roads = 3*2 (6) + 1 secondary road = 4 + 6 + 1 = 11). On the other side, an inland secondary city like Osca (Huesca) was connected to 2 main roads and 4 secondary roads. If only the number of connections was considered, Osca (2 main roads + 4 secondary roads = 6) should have more connectivity than a bigger and more political city as Tarraco (5). But, if we use weighted networks, the Provincial capital of Tarraco had more connectivity (11) than Osca (2*2 + 4*1 = 4 + 4 = 8).

Once we have a detailed transportation networks of the Iberian Peninsula for each period, it has been easy to transform them into a weighted network to work with (figure 3). As it has been explained in the following text, each edge representing different means of transport has a different weight.

Figure 3. Maps with the weighted networks analyzed in this article.

Notes: Roman times (upper-left), medieval era (upper-right), Early Modern (bottom-left), and mid-nineteenth century (bottom-right). Dark blue: sea connections; light blue: river connections; thick black: main roads; thin black: secondary roads.

Thanks to the calculations made on the connectivity of the different settlements of the Iberian Peninsula in each of the historical periods it is possible to create a map to visualize the results (figures 4-7). From the interpolation of the data obtained in the calculation of the degree k for each one of the different urban settlements and crossings of the transport network, a series of heat maps can be created on the connectivity of the different territories. Thus, the most well-connected (blue color) and the most isolated areas (red color) can be observed.

As can be seen from the observation of the obtained map, during the Roman period the peripheral areas of the peninsula and the river valleys were well connected (figure 4). From the territorial reorganization that Rome undertook in the Iberian Peninsula, the road policies were designed to allow the communication of all the territories of the peninsula. In any case, the best communication of the coastal areas and the river valleys was because to an interest of Rome in prioritizing communications across the coast and the great waterways, especially the Guadalquivir, Ebro, Tajo and Duero Rivers because they made easy the communication with the sea and therefore with the capital of the empire, Rome (Molina Vidal Reference Molina Vidal1995). To complete the access to all the Iberian territories, Rome also created a dense road network in a long construction process various routes that crossed the peninsula (Solana and Sagredo 2006). This fact favored the appearance of diverse crossings of roads with relatively high values of connectivity.

In fact, it seems clear how the Roman structure did not break with some previous indigenous routes that have survived thanks to Greek sources (Timaeus De Mirabilibus Auscultationibus, Avienus Ora Maritima) and that describe routes preserved in the Roman transport network (Alfaro Reference Alfaro2001; Blázquez Reference Blázquez2006). A particular case can be found in the design of communications in the north of the peninsula. These territories were the last to be conquered by the Roman armies thanks to the victory in the Cantabrian Wars (29 BC–26 BC). Firstly by military reasons and secondly due to economic motivations allowed the consolidation of an important horizontal road axis that communicated the rich gold mining areas of the northwestern tip of the peninsula with land routes to Rome through the Pyrenees (Argüelles Reference Argüelles2016).

Figure 4. The Roman Iberian Peninsula connectivity.

Figure 5. The Iberian Peninsula connectivity in Middle Ages.

Figure 6. The Early Modern Iberian Peninsula connectivity.

Figure 7. The mid-nineteenth-century Iberian Peninsula connectivity.

Figure 8. Connectivity differences of the territories of the Iberian Peninsula. In red, loss of connectivity; in blue, increased connectivity.

Notes: Left: from Roman to medieval times, center: from medieval to modern times, right: from modern times to the nineteenth century.

Figure 9. Main Roman urban foundations in the Central Pyrenees.

Source: Digital Atlas of the Roman Empire: https://dh.gu.se/dare/.

Figure 10. Main Pyrenees roads in Catalonia in early medieval times.

Source: Bolós Reference Bolós1991: 124.

Figure 11. Connectivity maps of the northeastern Iberian Peninsula.

Notes: Roman times (upper-left), medieval era (upper-right), Modern Era (bottom-left), and nineteenth century (bottom-right).

In this way, it can be observed how the great river corridors of the Iberian Peninsula were used intelligently, avoiding the mountainous obstacles of the interior areas. This design adapted to the Iberian geography conditioned the road network structure, creating a great triangle between Tarraco, Asturica Augusta (Astorga), Emerita Augusta, and its extension to Hispalis, from which all the other roads were ordered. In this triangle were included the main administrative centers—provincial governments (legatus) and procurers (procuradores), besides the most outstanding economic centers (Houten Reference Houten2018). From this transport network, the main major cities of the Iberian Peninsula were developed (Carreras Reference Carreras1996).

In medieval times (figure 5), the political situation had varied significantly. Between the eleventh and the thirteenth centuries, there was a political and cultural expansion of the Christian kingdoms across Europe against enemies of other religions. It happened in the Crusades in Holy Land; in the Northern Crusades against pagan Baltic, Finnic, and West Slavic peoples; and in the “Reconquista” of the Iberian Peninsula (Porrinas Reference Porrinas2011). During the twelfth century in the Iberian Peninsula, different kingdoms coexisted and maintained changing relationships between them (Álvarez Borge Reference Álvarez Borge2003; Bartlett Reference Bartlett2003; García de Cortazar Reference García and José1983; Viguera Reference Viguera and Julio1999). On the one hand, in the northern part of the peninsula were several kingdoms that intended to expand to the south as the kingdom of León, Castilla, Navarre, Aragon, and Portugal. On the other hand, the peninsular south was in a convulsive moment because al-Andalus was divided into small Taifa kingdoms that in the first years of the twelfth century were under the Almoravid Empire and later under the Almohads (Viguera Reference Viguera and Julio2007). All this complex political situation shows how many centuries ago there was no political entity in the Iberian Peninsula that managed the entire territory. This situation caused the lack of global design policies that could imply major changes in the transport networks. Instead existed a continuity in the use of the same roads and communication routes created by the Romans among the creation of short new roads to complement the connectivity of the new political hub settlements (Diago and Ladero Reference Diago and Miguel Ángel2009). The roads did not respond to a general plan of terrestrial communications, they obeyed to military, repopulation, pilgrimage, agrarian, commercial, and urban expansion imperatives (Madrazo Reference Madrazo1984). There were, however, some modifications due to changes in the use of communication channels. Routes that in Roman times represented great axes of communication, during the eleventh through thirteenth centuries lost that function, and in the opposite case, Roman routes considered of a secondary character became great axes of communication during the medieval period. A clear example of this situation can be seen, for example, in the appearance and consolidation in medieval times of an axis of capital communication for the Christian society, the Camino de Santiago (Laliena and Duque Reference Laliena and Ángel1994). This axis, which already existed in Roman times, increased the connections of the territories where it was built while the main Roman route that was a few kilometers further south partially lost its importance.

In this period, coastal areas continued to have good connectivity. Each of the peninsular coastal territories was opened internationally through the sea. Thus, the Catalan-Aragonese crown began its expansion toward the Mediterranean through the ports of Barcelona and Tarragona (Ferrer Reference Ferrer, Andrea and Sílvia2009; Santamaria Reference Santamaría1991). The peninsular south coast served as a connection between the territories of al-Andalus and the territories of North Africa (Constable Reference Constable1994). If we look at the map of differences between periods (figure 8), we can observe all these changes more clearly. First, the improvement of connectivity experienced by areas such as the eastern area of al-Andalus in the south where later the kingdom of Granada will be established, the northern Meseta and the area of the medieval kingdom of Portugal, the Cantabrian coast, and, mainly, the consolidated axis of the Camino de Santiago.

The situation in the seventeenth century changed again in a very important way (figure 6). In this period, in the Iberian Peninsula are already settled and differentiated between the two great kingdoms of this territory, Spain and Portugal (Elliot Reference Elliott and Mark1991). Thanks to his rights of succession and a military action, the Spanish monarch Felipe II unified the two kingdoms during some decades, although there was still a clear political, legal, and economic division between them. This was a century of deep crisis in the Iberian Peninsula, mostly in the Spanish kingdom, due mainly to an important population decline, a crisis in the productive system and political decisions harmful to economic development (Ringrose Reference Ringrose1972). This economic situation induced an urban regression from the sixteenth century situation and deep demographic changes (Gelabert Reference Gelabert, Thompson and Bartolomé Yun1994). The setting of the kingdom capital in Madrid and the growing population of this city created the basis to settle the first radial network in the Iberian Peninsula (Alvar Ezquerra Reference Alvar Ezquerra1989; Ringrose Reference Ringrose1973).

As regards the connectivity of the different territories, it can be observed how an incipient radial system began to be glimpsed, which from the peninsular center allows communication with the rest of the territories of the periphery. In the communications, the transport structure fossilized the importance of the Castilian cities during the previous century, especially in the north (e.g., Burgos, Valladolid, Segovia). Those routes heading the north facilitated the transport of European imported commodities to reach Madrid (Diago and Ladero Reference Diago and Miguel Ángel2009). At the same time, the growing importance of coastal territories can be visualized, where the recession was lower, and the beginning of economic improvements began earlier. The infrastructures of the seventeenth century combined the preservation of the previous communicativeness structures in the interior of the peninsula and the connectivty improvements of the peripheric territories along the coast, from the Mediterranean territories to the Portuguese and the Cantabrian coast. In comparison with the previous period studied (figure 8), the areas most favored by the infrastructures created since the sixteenth century are the northern area of Castile and, mainly, the southern territories such as Seville. In this period, we can observe in Spain the substitution of a transport network based on economical conditions for another model focused on political factors like the decision to fix the capital in Madrid (Ringrose Reference Ringrose1973).

Finally, communications in the mid-nineteenth century represent the end of the eighteenth century economic crisis and the beginning of the creation of the contemporary transport system (figure 7) (Madrazo Reference Madrazo1984, 2001; Ringrose Reference Ringrose1972). In Portugal, the connectivity of Lisbon with productive territories of the country increased, like Porto in the north or the Algarve in the south. In the case of Spain, the railway established the radial transport model, where the mainland routes have their origin (or destination) in the capital, Madrid (Mojica and Martí-Henneberg Reference Mojica and Jordi2011). From this neuralgic nucleus, the rest of the territories had good connections depending on their situation regarding the radial axes. In this scheme, the Madrid-Zaragoza-Barcelona, Madrid-Córdoba-Sevilla, and Madrid-Valladolid-León axes stood out for their continuous connectivity. It is interesting to note the continued isolation of the border between the two Iberian countries with long fringes with limited connections. In the map of differences regarding the situation of the seventeenth century (figure 8), the role of the railway in improving the communicativeness of the different peninsular territories stands out significantly (Gómez Mendoza Reference Gómez Mendoza1982, 1989). The appearance of this new transport system affected and redistributed the communications scheme in the new states. In the center of the peninsula, the territories that were far from the new railway infrastructures, declined in connectivity. The economic crisis that they suffered and the lack of investment made their situation within the transport network worse. It is also interesting to observe how the different historical routes that linked territories of the peninsular periphery without going through the center were definitively cut. The horizontal roads such as the Camino de Santiago or the southern stretch of Via Augusta that linked the Mediterranean coast with the west of Andalusia disappeared. The Roman route known as Vía de la Plata, which linked Hispalis (Seville), Emerita Augusta (Mérida), and the north, also disappeared. In the nineteenth-century Spanish road system, all the communications should pass through the capital, Madrid.

Medieval Continuity: Evolution of the Former Infrastructures

The medieval period was continuity in terms of the development of transport infrastructures. Most Roman roads were still in use and were distinguished by a width of 4.5 meters and good drainage, which was enough for traveling wagons; that is why they were called “carreres” (wagon roads). The “carreres” were public roads under royal protection (camins rals) common in the plains and wide valleys such as the one in Cerdanya traced by the Segre route. These medieval roads were made of pressed earth, so they became muddy in winter and difficult to transit (Riera Reference Riera2002: 445–46).

In the higher mountains or places with steeply gradient, there were animal tracks (“de bast”), which were normally 1.10 meters wide with some wider places for crossing animals in both directions. These were the typical Pyrenees roads, which were normally public but there were also private tracks with toll payment.

Regarding the towns, some old Roman centers such as Iacca (Jaca) or Iluro (Oloron) were occupied again and became the main stops in the route crossing Somport’s pass, perhaps the most successful route in medieval times together with the two coastal corridors. From Oloron, traders could go to Tarbes, Toulouse, and Pau. In the eighteenth to nineteenth century, traders carried wool, species, olive oil, salt, and pack animals toward the North and ovicaprids, textiles, Occitan cake, and English wool toward the South.

On the contrary, the Catalan central routes were not so popular. The Segre route connecting Perpignan to Lleida was still in use, but with an important change. The city of Iulia Lybica (Llívia) lost its importance as a prominent settlement in favor of the new medieval towns of the nearby Puigcerdà (thirteenth-century foundation) and Seu d’Urgell (the capital of Urgell’s bishopry). Puigcerdà was also the end of the Llobregat routes that connected Barcelona with Manresa and Berga or the Besós route following the Osona plain and the river Ter up to Ripoll, and from here to Puigcerdà crossing the Collada de Tosses (figure 10).

Medieval sources reveal that the critical points of those routes were the river crossing. There were only a few free fords to cross the Pyrenees rivers, whereas the rest had bridges and barges for crossing. In both cases, merchants had to pay a toll to the authority, either local noblemen, towns, or monasteries. For instance, the Segre route included pay toll bridges at Vilafranca de Conflent to cross the river Tet, at Puigcerdà to cross the Arabó, and at Pont de Bar, Seu d’Urgell, Tresponts, Organyà, and Balaguer to cross the Segre. Some of the newborn towns were at those crossing river points (Vilafranca de Conflent, Puigcerdà, Seu d’Urgell, Organyà, and Balaguer).

Thanks to the Roman road policy, the medieval era was undoubtedly the time of best communications in the Pyrenees. The political process of the peninsula from the eighth century, and especially from the ninth and tenth centuries with the consolidation of the Marca Hispanica, led to a boom in mountain communications in the area of the Pyrenees. Just from the consolidation of the Carolingian Empire with the coronation of Charlemagne, it was necessary to establish a march, a border, between the Frankish Carolingian Empire and the territories of al-Andalus ruled by the Umayyad Caliphate (Viguera Reference Viguera and Julio2007). This march was formed by the creation of some small lordships and counties beyond the rule of the Frankish imperial rule located in the Iberian Pyrenees. The small political fragmentation of this Pyrenean territories, but under the rule of the Frankish Carolingian Empire (in the early medieval times) until the tenth century when those counties were practically independent from the kingdom of France (Abadal Reference Abadal2007), favored the consolidation of the main roads built by the Romans connecting with septentrional territories and at the same time the construction of some small mountainous routes that allowed the military and commercial connection between those counties. This is reflected in the connectivity of this entire territory (figure 11). Although, obviously the main commercial routes continued to be located in routes closer to the coast, in this historic period important cities in the mountain developed.

From modern times, the Pyrenees begin to lose prominence mainly due to the lack of economic productivity in the mountainous areas and their limited road access. The road system gradually degraded over the centuries in favor of the main coastal routes and the route from Barcelona to the peninsular interior where the capital was located, passing through Lleida and Zaragoza. However, although the southern part of the Pyrenees lost connectivity in relation to the Spanish transport network, it still maintained continuous contact with the French territories (Pojada Reference Pojada2012). Thus, various commercial documents and notarial acts show that there was a trade in modern times that crossed the Pyrenees in both directions. Wool, salt, and oil were transported from the south of the Pyrenees (Codina Reference Codina2005) and the famous Spanish horses were illegally traded. In these routes, some populations condensed the products to transport them in caravans. These populations were Puigcerdà, Andorra, and Vall d’Aran (Blaquière Reference Blaquière1963; Vidal Reference Vidal1984). In the opposite direction, from France an important trade of some animals like the mules (Poujade Reference Poujade2007), products derived from the fish like Cod, sardines, or conger eel or some products manufactured like cloaks was developed (Larguier Reference Larguier2005; Pojada Reference Pojada2012). But there was also a wide variety of products that came to Catalonia mainly from Toulouse such as hats, leather goods, copper, lead, cheese, sugar, tobacco, spices, religious objects, combs, cowbells, stockings, threads and ribbons, various needles, knives, nails, shovels, hooks, and containers that the merchants, often emigrants, went to search for in Toulouse and returned to Catalonia (Pojada Reference Pojada2012).

This tendency of loss of connectivity of the Pyrenees within the Spanish transport network was even more stressed with the reforms carried out by Spain between the eighteenth and nineteenth centuries (Madrazo Reference Madrazo2001; Ringrose Reference Ringrose1972) prior the creation of the railway (figure 9). The radial road system defined from Madrid connects the capital with the main peripheral cities but leaves in the background the rest of the territories, as in this case the Pyrenees. The system is connected to the coastal and inland manufacturing cities, such as Manresa or Terrassa. However, it is difficult to reach populations located in the closest zones to the Pyrenees.