Longer Truck to Reduce CO₂ Emissions: Study and Proposal Accepted for Analysis in Spain

Abstract

The transport industry in the European Union plays a key role in the economy. However, due to persistent political, social, and technological changes, examining optimization strategies in transportation has become a crucial task to minimize expenditure, promote sustainable solutions, and address environmental degradation concerns. This study analyzes the effectiveness of a new truck trailer design, adapted from existing European models, which improves load capacity through an extended trailer length. The increased length (and, by extension, volume) is expected to reduce the number of vehicles for freight transportation, thereby improving road congestion and reducing environmental impacts, which include GHG emissions and overall carbon footprint. To achieve this objective, a comprehensive analysis of current European regulations on articulated vehicles and road trains was carried out, alongside a review of related case studies implemented or under development across the European Union member states. Additionally, a pilot study was conducted using the proposed 18 m semi-trailer across 14 real-life freight routes involving loads from several suppliers and manufacturers. This study therefore demonstrates the economic benefits and reduction in pollutant emissions related to the extended design and evaluates its impact on road infrastructure conditions, given the total length of 20.55 m.

1. Introduction

The transport industry in the European Union plays a critical role in the economic sector; however, ongoing political, social, and technological changes have made the study of transportation optimization strategies a priority. There are efforts to reduce energy costs, promote sustainable solutions, and address growing societal concerns about environmental impacts [1,2]. One of the main objectives of the European Union’s transport policies, as set out in the White Paper, is to promote clean, safe, and efficient transport [3]. The European Commission also adopted the “European Green Deal”, a strategic framework with a time horizon of up to 2050 to achieve sustainable solutions in transport [4].

In 2015 the United Nations General Assembly made a commitment to the 2030 Agenda, with 17 Sustainable Development Goals (SDGs), as a way of improving the quality of life for countries and their population. Goal number 13 is based on concerns about carbon dioxide (CO₂) and other greenhouse gas (GHG) emissions in all countries and the negative consequences both socially and economically [5,6]. Despite increasingly stringent regulations, heavy-duty freight vehicles account for approximately 25% of road transport emissions in the EU and 6% of total greenhouse gas emissions [7,8,9,10,11], which aligns with what is reported by the United States Environmental Protection Agency (EPA) [12].

Consequently, research should be aligned with current social needs, which compel companies to enhance their competitiveness by employing strategies that reduce transportation-specific costs. Simultaneously, these strategies must account for future resource constraints and prioritize environmental sustainability, specifically the reduction of GHG emissions.

1.1. Transport in Europe

There are five main modes of freight transport in the European Union: sea, road, rail, inland waterways, and air. Of these, road transport is the most widely used mode for conveying goods from their origin to their inland destination. According to statistics [13], road transport increased from 2013 to 2023 by 2.8%, making it the transport mode with the most favorable evolution over the last 10 years (Figure 1).

An analysis of road transport share over the past decade across European Union member states, measured in tonne-kilometers, shows a slight decrease in countries like Ireland, Poland, and Portugal. On the contrary, significant increases have been observed in countries like Lithuania, Romania, Latvia, Slovakia, Finland, and Sweden (Figure 2). This translates into growing air pollution emissions, including CO₂. Transport costs usually justify the reasons for this state of affairs; road transport of cargo is generally cheaper or more easily available than rail transport. Therefore, the search for new technologies for transporting cargo by road is fully justified.

Considering the distribution of modes of transport by country for the year 2023 presented in Figure 3, Spain’s primary modes of freight transport are sea, road, and rail, with air and inland river routes being uncommon. Amidst these, road transport is the most popular method for inland freight movement, with cargo volumes exceeding 266.5 billion tonne-kilometers [14].

1.2. Load Carried vs. Load Capacity

Efficiency in freight logistics plays a crucial role in the performance of road transport, the primary mode of inland freight movement, yielding both economic and environmental benefits [15,16]. The volume and weight of transported material have a significant influence on transport performance. According to [17] a high percentage of trucks operate at full capacity (volume) without attaining their maximum weight limits, especially in cases involving the transport of large, bulky items with a relatively low mass. The study of truck trailer dimensions (in particular, length) can have an impact on increasing the efficiency of road transport. If the load capacity increases in volume, even if it is not increased in weight, this can result in reducing the number of trips, which translates into a reduction in costs as well as the overall CO₂ and GHG emissions [18].

Carrying capacity has been analyzed from different points of view (public sector, private sector, and customers), leading to a wide variety of results, but no standard has been found to calculate the most efficient carrying capacity depending on different uses [18,19,20].

Figure 4 shows the average load, measured in tonnes, calculated by “dividing the annual freight throughput (metric ton-kilometers) by the corresponding loaded distance travelled (vehicle-kilometers, equivalent to kilometers)” [17]. This indicator shows information on the average weight of tonnes transported per kilometer over the total distance traveled with load (excluding empty trips). As shown in the graph, the average load in the European Union in 2023 was 15.7 tonnes, while, at the national level, it was 13.4 tonnes. These data indicate that based on the average load percentage, freight vehicles do not reach the allowed maximum weight, suggesting that load limitations are essentially due to volume constraints [17]. In Spain, a study found that only half of the vehicles with a maximum authorized weight of 40 tonnes exceeded 32 tonnes [14].

1.3. Objective

The objective of this study is to analyze the effectiveness of a new truck trailer design that increases the load capacity in volume (specifically by increasing its length) and is adapted from existing European models or modified for cost-effective implementation. The aim is to demonstrate the potential for reducing transport costs for logistics companies. By increasing volume capacity, the number of vehicles required and the distance traveled can be reduced, which, in turn, can contribute to lowering environmental impacts and decreasing greenhouse gas (GHG) emissions.

To achieve this objective, we first present a brief summary of the latest and most relevant European regulations in terms of weight and volume for articulated vehicles and road trains. Next, we analyze studies of longer trailers already carried out in other European Union countries, mainly Italy and the United Kingdom, before presenting our new proposal: an 18 m trailer (against the current 13.6 m). We then compare the different vehicles currently authorized in the European Union with our proposal for an 18 m trailer, determining their respective advantages and disadvantages in road transport. This article concludes with the most significant findings, limitations of this study, and future research directions.

2. Current Lorry Size and Weight Regulations in Europe

Road safety, environmental protection, and the reduction of GHG emissions have become key aspects when designing weight and length standards for heavy goods vehicles (HGVs) in Europe [1,15,16]. In addition, there are other aspects that are considered when determining the length and weight of trucks, such as the type of goods transported (weight or length), infrastructure characteristics (road width or turning angle), manoeuvrability, and the difficulty of adapting or building new trailers [21,22,23,24].

Although there are some minor differences between member countries, these guidelines and criteria align with the European Union directive [25,26]. Also, the European directive foresees exceptions for those countries that do not belong to the European Union, being able to accommodate vehicles that exceed the stipulated weight, length, height, or width of the truck cabin.

Unlike weight and length, which can be subject to more variations, which will be explained in later sections, the maximum authorized width and height of the truck cab are very harmonized. The maximum authorized height is generally 4 m, with some exceptions, where heights of 4.2 to 4.5 m are allowed. The maximum authorized width is, in most countries, 2.55 m [25].

2.1. Weight Regulations in Europe

European regulations on the maximum permissible weight of goods vehicles vary more than the length in different European countries [25,27]. There are not only notable differences when considering vehicle type (road train and articulated vehicle) but also regarding the number of axles, the type of goods, or the condition of the roads.

As for the articulated vehicle, the maximum permitted weight varies from 40 to 44 tons in most countries, with the exception of Ireland, with a maximum of 46 metric tons; the Czech Republic, which can reach up to 48 metric tons; and some countries, such as the Netherlands and Norway, which reach a maximum of 50 metric tons. In contrast, Armenia has a limit of 36 metric tons, the lowest limit of all European countries.

The road train, depending on whether its configuration is of 4, 5, or more axles, has, in general, minimum and maximum permitted limits of 36 to 40 metric tons, respectively; it can reach 46 metric tons in the case of Ireland, 48 metric tons in the Czech Republic, and up to 50 metric tons in the case of The Netherlands and Norway.

2.2. Length Size Regulations in Europe

Despite the fact that the length of heavy goods vehicles in Europe is fairly uniform, depending on the typology concerned, for articulated vehicles (16.5 m, including tractor units and semi-trailers) or road trains (18.75 m in total, with a payload dimension of 15.65 m), there are specific differences [25,28]. Distinctive variations in the length of articulated vehicles are observed in Nordic Countries, such as Norway (17.50 m), Finland (23 m), and Sweden (24 m), as well as in European countries near the East Asian border such as Azerbaijan and Russia (20 m), Ukraine (22 m), and Belarus (24 m).

The European countries that authorize greater cargo lengths for road trains are the Nordic nations—Norway, Sweden, and Finland—with maximum lengths ranging from 19.50 m to 34.50 m [29,30,31,32,33]. This is primarily due to the use of low-traffic-density roads and the considerable distances between cities. Additionally, Azerbaijan, Belarus, and Russia permit a cargo length of 20 m, while Ukraine allows a greater length, reaching up to 22 m [34].

2.3. Other Studies of Longer Vehicles in Europe

One option to respond to the problem of transport costs and emissions reduction is the development and introduction of longer and heavier vehicles (HLVs) than those set by according to the regulations of the European directive. Among the benefits are reductions in costs per unit load, labor, and GHG emissions, as well as the possibility of improving workers’ skills and increasing their salaries. On the other hand, the disadvantages include high investments in infrastructure [35].

Although the use of longer and heavier vehicles is already in place in European countries, most territories have been implementing this option through pilot tests [36,37,38,39,40,41]. These tests served to ensure a positive impact and reduce potential accidents at the time of implementation.

2.3.1. European Modular System (EMS)

The “European Modular System” (EMS) consists of modules assembled to improve load capacity. This combination of trailers is present in European legislation [25,26,42].

This alternative vehicle configuration, approved in only a limited number of countries, allows for up to five distinct combinations of 25.25 m vehicles (Figure 5). The most common combinations are a 12 m truck with a 13.6 m semi-trailer or a tractor with a 13.6 m semi-trailer and an 8 m trailer. Nevertheless, despite the fact that northern European countries (Finland, Denmark, Sweden, and Norway) and the Netherlands have been utilizing them for over two decades, their usage has only recently been expanded in Germany (in select federal states) and with special permits limiting their use on specific roads in Spain, Portugal, Belgium, and the Czech Republic [43]. This is done with the intention of reducing emissions of CO₂ and other particulates [44]. Moreover, while the movement of goods within these countries is permitted for 25.25 m trucks, the tonnage transported varies considerably, from 40–44 to 60 (in Norway and the Netherlands) and even reaching 64 in Sweden. Nevertheless, the utilization of these vehicles has not yet been fully implemented, and trials are ongoing in other countries, such as Poland and Slovakia [45,46].

2.3.2. Progetto DICIOTTO (Italy)

In 2009, a project called “Progetto DICIOTTO” was initiated in Italy and developed by the “Associazione Italiana Filiera Industria Automobilistica” [48] and other companies in the transport industry. The objective was to increase the transport load capacity by increasing only the length of articulated trucks from 16.50 m to 18 m, without extending the weight limit, 44 metric tons (Figure 6).

The conclusions of the experimental phase [48], involved over 300 vehicles and routes from 200 to 600 km, indicated that achieving load saturation in terms of both volume and weight resulted in a 12% reduction in the number of trips; this led to reduced fuel consumption per unit of goods transported and, consequently, achieved lower GHG emissions. As regards driving safety, overtaking, and manoeuvrability, no significant differences were observed between the new trailer design and the traditional vehicles.

The project was concluded with the publication of the “DL Infrastrutture of 10/09/2021”, which permitted the operation of this vehicle type on Italian territory, having demonstrated a reduction in traffic volume due to its capacity to transport 37 Europallets, four more than the previous configuration, without significant impact on daily traffic patterns.

2.3.3. High Volume Semi-Trailers (United Kingdom)

The “Longer Semi-Trailer trial” was initiated in the United Kingdom in 2012 following a proposal from the Ministry of Transport to increase the length of articulated vehicles to a maximum length of 18.75 m while maintaining the same weight limit of 44 metric tons (Figure 7). This approach provided a greater transport of pallets (1 × 1.2 m), from 26 to 30, providing a reduction in the kilometers traveled and, therefore, an economic benefit and reduced environmental impact. The project also aimed to reduce infrastructure costs, traffic accidents, and noise pollution [49].

Once the test routes were completed, the findings were very positive, with one in twelve journeys reduced, CO₂ emissions down by 48,000 metric tons, and NO_x down by 241 tons. In addition, the trials showed that there were 53% fewer road accidents than the UK HGV average [50,51]. Currently, these vehicles are allowed to circulate until 2027.

2.3.4. Double Trailer Trucks and Other Trials

At present, a number of countries have introduced regulations allowing the operation of double trailer trucks with a maximum length of 32–34.5 m and a gross vehicle weight of up to 74–76 metric tons on certain roads and under specific conditions (depending on the regulations) [30]. Examples include Sweden in 2018 [52] and Finland in 2019 [29,43]. This combined vehicle is composed of a tractor unit followed by two semi-trailers, each measuring 13.6 m in length and equipped with 9 or 10 axles (see Figure 8) and even 11 axles in some cases.

These configurations are currently being tested in countries such as Spain and the Netherlands, initially within automotive companies like Seat and Volkswagen [53]. However, several logistics operators have already extended their experimental use for the transport of other goods. Similarly, other countries, such as Denmark, are contemplating the implementation of these vehicle combinations with the aim of reducing CO₂ emissions and transportation costs [54].

3. Description of the Proposed Solution

The proposal for the semi-trailer is to extend the length of the semi-trailer, as has been studied in Italy and the United Kingdom. However, this proposal goes one step further and proposes a maximum total length that road trains carrying cars are allowed. The proposed length of car carrier trucks is 18.75 m plus 1.8 m, which sums up to 20.55 m, as the load is permitted to overhang [26]. Therefore, our proposed total length is identical to that of a road train transporting automobiles, which commonly operates on European roads. Notably, this proposal suggests a semi-trailer longer than those currently in circulation; however, it does not exceed the maximum permitted length for road trains, which are legally authorized to travel on all EU roads without the need for special permits. In accordance with the aforementioned directive, most infrastructure is already adapted to accommodate them.

In Section 3.1. the technical characteristics are explained and, in the following points, the load carrying capacities (Section 3.2) and environmental impacts (Section 3.3) according to the methodology proposed by [31,55] are discussed.

3.1. Technical Specifications of the Proposed Semi-Trailer

Figure 9 illustrates the initial semi-trailer (13.6 m) utilized for the manufacture of the proposed semi-trailer (18 m).

As illustrated in Figure 9, the rear overhang is moderately augmented (1.1 m) as well as the distance from the fifth wheel to the first axle (3.3 m), thereby enabling the length of the semi-trailer to be extended from 13.6 m to 18 m. This conversion was performed by an accredited trailer manufacturer in compliance with all safety and quality standards in vehicle manufacturing.

Initially, the investment required for converting these semi-trailers was undertaken by the transport logistics company itself, as the modifications were based on existing semi-trailers that were slightly extended. The resulting operational savings enabled the investment to pay for itself in under three months.

Table 1. Most relevant technical characteristics before and after the proposed transformation.

	Before	After
Tare weight	7200 kg	9000 kg
M 1/MAM 2	42,000/36,000 kg	42,000/35,000 kg
m 3/MAM axle 1	9000/8000 kg	9000/8000 kg
m/MAM axle 2	9000/8000 kg	9000/8000 kg
m/MAM axle 3	9000/8000 kg	9000/8000 kg
mcp 4/MAM kingpin	15,000/12,000 kg	15,000/12,000 kg
Number and size of tires	6 × 445/45 R19.5	6 × 445/45 R19.5
Total height	4000 mm	4000 mm
Total width	2550 mm	2550 mm
Maximum length	13,920 mm	18,340 mm
Rear overhang	2820 mm	3940 mm
Axle spacing 1–2	1360 mm	1360 mm
Axle spacing 2–3	1360 mm	1360 mm
Fifth wheel lead	9180 mm	12,480 mm

¹ M: technically permissible maximum laden mass. ² MAM: maximum authorized mass or gross vehicle weight (GVW). ³ m: technically permissible maximum mass on the axle. ⁴ mcp: technically permissible maximum mass at the coupling point.

presents the most notable technical specifications with regard to allowable masses (in kilograms) and dimensions (in millimeters) before and after the modification.

To analyze the dynamic behavior of the proposed vehicle, a simulation was previously conducted in accordance with the methodology proposed by [56]. The results were validated by comparing them with the calculated values for vehicles already in operation in the European Union, including articulated trucks, road trains, and EMS vehicles. These data were sourced from the Organization for Economic Co-operation and Development (OECD) report [57].

Table 2. Most relevant results in manoeuvrability and stability.

Type of Truck	Total Swept Width (m)		Lateral Acceleration Amplification Ratio	Lateral Off-Tracking During Lane Change Manoeuvre (m)
	For 5 km/h Turning Manoeuvre	Traveling on a Rough Road at 90 km/h
Articulated truck (16.5 m)	6	2.87	1.2	0.38
Road train (18.75 m)	5.78	2.95	1.39	0.89
EMS (25.25 m)	6.06	2.97	0.91	0.67
Proposed truck (20.55 m)	7.27	2.97	1.21	0.51

presents the most significant results:

• Total swept width for the low-speed turning manoeuvre (m). The ability of vehicles to navigate low-speed swept paths is of utmost importance in areas with high traffic density, where traffic circles and narrow intersections are prevalent. Failure to provide sufficient space for manoeuvring can result in dangerous situations. European regulations stipulate that vehicles must be able to complete a 360-degree turning manoeuvre at a speed of 5 km/h while remaining within an area of the roadway delineated by an “inner” circle with a diameter of 7.2 m and an “outer” circle with a diameter of 12.5 m.
• Total swept width for traveling on a rough road at 90 km/h. The operation of vehicles on uneven road surfaces can also present challenges because of the tendency of the rear trailer to deviate from the track, a phenomenon influenced by the roughness of the road and the resulting impact on the tires and suspension of the vehicle. This lateral movement can potentially pose a hazard to other road users or even result in the rear trailer leaving the paved surface.
• Lateral acceleration amplification ratio of the hauling unit vs. the trailer. The phenomenon of rearward amplification affects the tendency of a semitrailer to experience greater lateral acceleration than the tractor. It is of great consequence to road safety because lateral acceleration and the resulting “tiling” of the rear trailer can cause the entire vehicle to overturn [58]. A reduction in rearward amplification values will consequently result in a reduction in the risk of rollover.
• Lateral off-tracking during lane change manoeuvre (m). The lateral deviation of the rear trailer during a lane change manoeuvre is a significant safety concern as it has the potential to endanger other road users. As with the previous measures, reducing the lateral deviation of the rear trailer will enhance the safety of these vehicles during driving manoeuvres.

Considering the data in Table 2, it can be seen that the proposed truck has similar behavior to other vehicles already legally circulating in the European Union. The proposed vehicle has a lateral displacement during the lane change manoeuvre lower than the road train and the EMS, a lateral acceleration ratio of the trailer similar to the articulated vehicle from which it is derived and lower than the road train, and a similar width intermediate between the articulated vehicle and the road train when traveling on rough roads at high speed. It only performs worse at low speeds, requiring between 1.2 and 1.5 m more than existing vehicles, which makes it difficult to manoeuvre in built-up areas with roundabouts and narrow intersections [21,23,59]. In any case, the Spanish Directorate General of Traffic deemed these values appropriate and safe for road use when granting authorization for our proposal.

3.2. Load Capacity

From an economic standpoint, following the evaluation of the proposed truck’s stability and manoeuvrability, it is important to assess the implications of the increased load capacity. Notably, the proposed design does not imply an increase in the total authorized weight of the vehicle. However, the tare weight of the modified semi-trailer is 1800 kg more than the 13.6 m version, resulting in a corresponding reduction of 1800 kg in payload capacity. This may pose a limitation for vehicles that typically operate at maximum weight capacity. However, in practical terms, this is not a significant concern, as only 57% of vehicles on the road are saturated by weight, whereas 92% are limited by volume [55]. Therefore, although its weight capacity does not increase, extending its length (and volume) allows for the transportation of a greater quantity of goods.

Table 3. Comparison of lengths of current EU vehicles vs. proposed truck.

	Articulated Truck (16.5 m)	Road Train (18.75 m)	EMS (25.25 m)	Proposed Truck (20.55 m)
Semi-trailer length (m)	13.62	15.65	21.44	18
Increase (m)/%		2.03/14.90%	7.82/57.42%	4.38/32.16%
Semi-trailer length (m)	33.78	38.81	53.17	44.64
Increase (m)/%		5.03/14.90%	19.39/57.42%	10.86/32.16%
Semi-trailer length (m)	99.31	114.11	156.32	131.24
Increase (m)/%		14.80/14.90%	57.02/57.42%	31.94/32.16%

shows the lengths, areas, and volumes of the most common vehicles in the European Union for national and international transport (articulated vehicle, road train, and EMS) compared to the 18 m extended semi-trailer proposal. It can be seen that our proposal, which is structurally like the 16.5 m articulated vehicle, represents an increase of over 30% in loading capacity in terms of volume. Therefore, all the vehicles that circulate saturated in volume (over 92%) and not saturated in weight (over 57%) could take full or partial advantage of this increase in loading capacity.

Nowadays a huge amount of goods is transported on pallets. In Europe, the standard size of a pallet is 1.2 × 0.8 m. On the basis of these dimensions and considering the necessary clearances, it is possible to transport

• on a 16.5 m articulated truck (13.6 m semi-trailer), a maximum of 33 pallets (if not stacked high) arranged in 11 rows of 3 pallets or 15 rows of 2 pallets plus one row of 3 pallets;
• on an 18.75 m road train (truck plus trailer with a maximum loading length of 15.65 m), a maximum of 38 pallets (19 + 19);
• on a 25.25 m EMS (13.62 semi-trailer plus 7.82 trailer), a maximum of 52 pallets (33 + 19);
• on the proposed 18 m semi-trailer, a maximum of 44 pallets arranged in 14 rows of 3 pallets and 1 row of 2 pallets (Figure 10) or 22 rows of 2 pallets (Figure 11).

3.3. Environmental Impact

In Europe, today’s trucks are equipped with engines that comply with the Euro VI emissions standards, which represent the most recent and strictest standard implemented in the region. The standard has been in force since 2014 and has set very strict limits to reduce emissions of polluting gases like nitrogen oxide (NO_x) and fine particulate matter (PM). The remaining CO₂ emissions are directly related to fuel consumption [60,61]. The VECTO simulation software (version 4.3.3) [62], proposed by the European Commission, is used to calculate CO₂ emissions. This software implements Regulation (EU) 2017/2400 [63,64] concerning the determination of CO₂ emissions and fuel consumption of heavy-duty vehicles. VECTO considers several key factors, such as road characteristics, average speeds across defined road segments, and vehicle load. As a result, it is considered a reliable simulation tool. Its accuracy has been validated by various authors through comparisons with actual emissions measured using onboard equipment [60,65,66]. Due to its reliability, VECTO is widely employed in the assessment of CO₂ emissions and is also an essential tool for the certification of new vehicles and technologies [67,68,69,70,71].

While an increase in vehicle dimensions and transported weight leads to higher overall fuel consumption, it simultaneously results in a reduction in fuel consumption per tonne per kilometer [31].

In order to measure the efficiency of different types of trucks, ref. [55] defined a “fuel efficiency index” based on liters consumed per 100 km and pallets transported and a “pallet index” representing “road meters used per pallet”, including a safety distance (considering three alternatives: 70 m, 40 m, and 2½ × truck length). Then, ref. [55] calculated these indices for the different types of trucks most commonly used in the EU: single truck (12 m/26 tonnes), articulated truck/semi-trailer (16.5 m), road train (18.75 m), and EMS (25 m).

Table 4. “Road efficiency index” and “Fuel efficiency index”.

Type of Truck		“Pallet Index”		“Fuel Index”
	At 70 m Safety Distance	At 40 m Safety Distance	At Safety Distance = 2.5 × Truck Length
Articulated truck (16.5 m) (33 pallets/truck)	(16.5 + 70)/33	(16.5 + 40)/33	(16.5 + 2.5 × 16.5)/33	32/33
	2.62	1.71	1.75	0.97
Road train (18.75 m) (38 pallets/truck)	(18.75 + 70)/38	(18.75 + 40)/38	(18.75 + 2.5 × 18.75)/38	35/38
	2.34	1.55	1.73	0.92
EMS (25.25 m) (52 pallets/truck)	(25.25 + 70)/52	(25.25 + 40)/52	(25.25 + 2.5 × 25.25)/52	42/52
	1.83	1.25	1.70	0.81
Proposed truck (20.55 m) (44 pallets/truck)	(20.55 + 70)/44	(20.55 + 40)/44	(20.55 + 2.5 × 20.55)/44	34/44
	2.06	1.38	1.63	0.77

shows these indices and their calculations for our proposed truck.

Therefore, the proposed truck exhibits greater spatial efficiency for pallet transport compared to articulated trucks and road trains, and even surpasses European Modular Systems (EMSs) when considering the required safety distances between moving vehicles. This results in a reduction in both the number of vehicles needed on the road and the total space they occupy, thereby improving traffic flow and alleviating congestion. Furthermore, the proposed design enhances fuel efficiency per pallet, enabling the transport of 44 pallets without a substantial increase in fuel consumption, as the gross vehicle weight remains unchanged.

4. Analyzed Routes and Identified Savings

After conducting simulation-based analyses that confirmed the proposed vehicle’s manoeuvrability and stability and demonstrating increases in load capacity and a reduction in environmental impact—including reduced fuel consumption per pallet and fewer trucks in circulation—the Spanish Directorate General for Traffic (DGT) approved a pilot study of the 18-m semi-trailer across multiple routes, transporting real-world loads from various suppliers and manufacturers.

The initial phase of this study identified fourteen routes between suppliers and manufacturers that were being used with 13.6 m semi-trailers. This data was collected in order to have a basis for comparison with the results that would be obtained with the proposed 18 m semi-trailer. These routes encompassed both conventional two-way roads and motorways, as well as selected roundabouts and intersections. However, the routes were relatively straightforward, as the suppliers and manufacturers were situated in industrial areas and did not utilize roads that interconnected with urban centres.

Table 5. Planned routes to compare transport by articulated truck (16.5 m) and proposed truck (20.55 m).

Routes	Km/Trip	Articulated Truck (16.5 m)				Proposed Truck (20.55 m)
		Trips/Day	Trips/Year	km/Year	gCO2 Emissions	Trips/Day	Trips/Year	km/Year	gCO2 Emissions
Route 1	1.91	166.3	36,591	69,876	68,665,778	123.8	27,245	52,029	49,248,306
Route 2	80.17	22.6	4964	397,959	385,874,681	16.8	3696	296,316	268,055,683
Route 3	334.93	10.6	2324	778,378	526,848,199	7.9	1730	579,572	409,978,926
Route 4	357.82	9.4	2060	737,100	631,637,390	7.0	1534	548,837	451,427,066
Route 5	211.89	8.6	1901	402,797	362,273,787	6.4	1416	299,918	256,397,648
Route 6	549.82	7.2	1584	870,912	584,875,248	5.4	1180	648,472	423,064,637
Route 7	644.15	7.1	1569	1,010,678	995,419,330	5.3	1168	752,540	783,802,472
Route 8	129.75	7.0	1532	198,778	167,887,853	5.2	1141	148,008	118,849,779
Route 9	294.00	6.0	1320	388,080	364,207,421	4.5	983	288,960	252,526,220
Route 10	133.60	5.9	1294	172,872	156,467,658	4.4	964	128,719	116,727,773
Route 11	582.86	5.8	1268	739,066	570,240,978	4.3	944	550,300	444,369,360
Route 12	126.86	5.1	1114	141,326	97,240,492	3.8	829	105,230	74,462,555
Route 13	216.68	3.6	792	171,612	165,784,876	2.7	590	127,781	117,098,804
Route 14	240.05	1.3	291	69,855	69,480,259	1.0	217	52,013	48,218,713
	104.93	266.4	58,604	6,149,289	5,146,903,950	198.4	43,637	4,578,695	3,814,227,941

shows the data for the 14 routes proposed for this study. The data for these routes for 16.5 m articulated lorries are average calculations based on the last two years of data between the origin and destination points of each of the routes. The table is arranged according to the total number of trips per year on each of the routes. It was observed that there was one route with an average of over 160 trips per day, but with a very short distance (less than 2 km, as the supplier was practically integrated with the customer), followed by another route with more than 20 trips per day, with a distance of 80 km between the supplier and the manufacturer. Then there was a group of three routes with an average of about ten trips per day, each with a distance of 200–350 km, and another group of routes with six to seven trips per day, with distances of between 125 and 130 km (three routes), 300 km, and up to 550–650 km (three routes). The last route analyzed was the one with an average that was more than a single daily trip of 240 km. This resulted in more than 250 routes being analyzed per day, of which one was very short (2 km), but the rest ranged between 100 and 650 km, with an average distance of 300 km.

Therefore, a wide variety of routes with significantly different lengths and frequencies were evaluated. However, all selected routes shared the characteristic that the trucks were volume-constrained but not weight-constrained due to the nature of the transported goods. This made them particularly suitable for assessing the impact of increasing the semi-trailer’s length—and thus its volumetric capacity—without exceeding the maximum permitted weight limits established in Spain and most other European Union countries.

In Table 5, in addition to the distance data for each route, CO₂ emission estimates (in grams) were incorporated using VECTO software. These calculations took into account the specific characteristics of each route, such as gradients, lanes, and vehicle loads. As a result, routes with similar distances could exhibit different CO₂ emissions. Nevertheless, a reduction in average emissions of more than 25% could be observed, with decreases ranging between 21% and 30%, depending on the route.

Knowing the number of kilometers traveled and the number of trips per year, the annual costs incurred with these routes were calculated. Naturally, a decrease in the total number of kilometers traveled led to lower fuel consumption, while fewer trips per year resulted in reduced personnel expenses for each route. Finally, when comparing historical data for the 16.5 m articulated trucks with data for the proposed 20.55 m trucks in this study, significant savings were achieved over one year. These reductions, summarized in

Table 6. Summary of reductions.

	Truck 16.5 m	Truck 20.55 m	Estimated Savings
Truck trip	58,604	43,637	14,967
Kilometers traveled (km)	6,149,289	4,578,695	157,0594
Diesel consumption (L)	1,844,779	1,366,860	477,919
CO2 emissions (tCO2)	5147	3814	1333
Transport cost (EUR)	13,943,930	11,940,042	2,003,888

, included nearly 15,000 fewer trips, over 1.5 million kilometers saved, almost half a million liters of fuel conserved, more than 1300 tonnes of CO₂ avoided, and a financial impact exceeding EUR 2 million.

These arguments were sufficient to persuade the Spanish Directorate General for Traffic to authorize a pilot study involving the proposed 20.55-m vehicle on 14 selected routes. In addition to validating the simulation-predicted outcomes, this study aimed to assess various real-world parameters that were difficult to replicate through modeling. These included the incidence of traffic accidents involving this type of vehicle (both active and passive), occupational risks such as driver stress and physical overexertion, and the potential impact on road infrastructure, including deformations and wear caused by the operation of extended-length trucks.

5. Conclusions

This work allowed us to draw several conclusions. Firstly, the statistics collected for the European Union demonstrated the significance of road transport as a means of land transport to reach the final customer (Figure 1 and Figure 2). Furthermore, these same statistics indicated that the loading percentage of trucks is relatively low, despite the fact that the majority of them are operating at full capacity (Figure 3).

This issue has raised concerns among several European member states, prompting investigations into potential alternatives for the use of longer trucks. Some of these initiatives have led to modifications in European regulations, allowing the operation of 25.25-m trucks [25,26], albeit with specific restrictions, such as special permits or designated roads. However, the proposed configurations involve an increase in the authorized gross weight, necessitating enhanced infrastructure maintenance, as well as a significant rise in the cost of the combined vehicle. This increase, however, is offset by the greater load capacity, ultimately leading to a reduction in the cost per transported tonne.

Secondly, the issue of exceeding existing loading limitations is particularly significant due to the potential incompatibility of infrastructure with increased vehicle weight. Consequently, several countries have conducted studies on longer vehicles while maintaining the maximum permitted gross weight. Notable examples include the Italian study (the DICIOTTO project, ongoing since 2009) [48] and the British study (conducted between 2012 and 2021) [50]. Findings from both studies indicate an increase in semi-trailer length from 13.6 m to 15 and 15.65 m, respectively. These studies reached highly similar conclusions, noting that the vehicles maintained their safety and manoeuvrability while enabling a 15–20 per cent increase in transported volume. As a result, reductions were observed in tCO₂ emissions, fuel consumption, and transportation costs per metric tonne.

Similarly, the proposal presented and approved for study by the Spanish Directorate General for Traffic followed a comparable approach: increasing the semi-trailer length from 13.6 m to 18 m (and, consequently, its volume) while maintaining the same gross vehicle weight. This adjustment facilitated the transport of greater freight volumes on routes previously constrained by vehicle size, while preventing further deterioration of infrastructure.

One notable advantage of extending semi-trailers to 18 m, as outlined in this study, is the straightforward conversion of existing 13.6-m models. This modification involves increasing the distance between the fifth wheel and the first axle by 3.3 m and extending the rear overhang by 1.1 m. This approach enables the adaptation of existing semi-trailers to the proposed design without requiring substantial capital investment.

Thirdly, the extension of the semi-trailer to 18 m results in a 30% increase in load capacity (in length and volume) without an increase in the maximum gross vehicle weight. However, as demonstrated in Table 1, there is a slight reduction in load capacity because increasing the size of the semi-trailer also results in an increase in its tare weight. Consequently, the benefits of this proposal are only realized by vehicles that currently carry two thirds of the permitted load and are saturated in volume, as they are able to increase the amount carried in volume without reaching the weight limit.

Finally, increasing the length of semi-trailers and, consequently, the volume that can be transported will reduce the number of vehicles needed to transport the same number of pallets. This implies a reduction in the number of vehicles on the road and a reduction in fuel consumption, CO₂ emissions, and cost per pallet transported, as also stated by [31] in their analysis for Sweden. In this proposal, the planned study of the 14 routes revealed reductions of more than 25% in CO₂ emissions and fuel consumption and approximately 15% cost savings for the same quantities compared to the previous situation when transporting using 13.6 m semi-trailers.

Additionally, it is important to note that the proposed solution is particularly well-suited for transporting light cargo. This aligns with current trends, as approximately 70% of trucks operating in Europe [57] reach their available volume capacity before reaching the permitted weight limits. The proposed solution introduces a new semi-trailer concept built upon existing models already in use within the European Union for road freight transport. Extending semi-trailers to 18 m enables the development of a more sustainable and scalable transport model, contributing to a reduced environmental footprint of freight transport by lowering CO₂ emissions and minimizing resource consumption, without requiring modifications to existing infrastructure.

Despite the promising outlook for implementing the proposed semi-trailer, it is important to acknowledge certain limitations within this study. These may be addressed in future research, provided that operational permits are extended by the Spanish Directorate General for Traffic (DGT). Firstly, the conclusions that could be drawn over 12 months (on 14 routes) may be affected in the future by different demand conditions. Also, this study was carried out with car manufacturers and suppliers, although the results should be stable as long as we have trucks that are saturated in volume and do not reach two thirds of the maximum permitted load. Additionally, the present study lies in its scope, which does not extend to the consideration of regulatory costs in certain European Union countries that impose restrictions on the circulation of specific vehicle types based on road classification, although such costs are not currently in place in Spain.

In line with the identified limitations of this study, future research should focus on extending the study period, drawing upon similar initiatives such as the three-year trial in the United Kingdom and the ten-year study in Italy. Expanding the scope to encompass a greater number of routes with diverse geographical characteristics and a wider range of transported products would also enable a more robust validation of the benefits observed thus far.