Life cycle environmental impact of a high-speed rail system in the Houston-Dallas I-45 corridor

The Houston-Dallas (I-45) corridor is the busiest route among 18 traffic corridors in Texas, USA. The expected population growth and the surge in passenger mobility may result in a significant impact on the regional environment. This study uses a life cycle framework to predict and evaluate the net changes of environmental impact associated with the potential development of a high-speed rail (HSR) System along the I-45 corridor through its life cycle. The environmental impact is estimated in terms of CO2 and greenhouse gas (GHG) emissions per vehicle/passenger-kilometers traveled (V/PKT) using life cycle assessment. The analyses are performed referring to the Ecoinvent 3.4 inventory database through the phases: material extraction and processing, infrastructure construction, vehicle manufacturing, system operation, and end of life. The environmental benefit is evaluated by comparing the potential development of the HSR system with those of the existing transportation systems. The vehicle component, especially operation and maintenance of vehicles, is the primary contributor to the total global warming potential with about 93% of the life cycle GHG emissions. For the infrastructure component, 56.76% of GHG emissions result from the material extraction and processing phase (23.75 kgCO2eq/VKT). Various life cycle emissions of HSR except PM are significantly lower than for passenger cars.


Introduction
Environmental degradation is one of the world's most critical issues. Therefore, concerns toward emissions have continuously increased over the last decades. Global Climate Change and its consequences have resulted in a fundamental shift in Extended author information available on the last page of the article some economic and social decisions. One of the leading sectors with continuous growth in emissions of air pollutants and greenhouse gases (GHGs) is transportation. According to the 2014 International Panel of Climate Change (IPCC) report, the transportation sector contributed 14% of the global GHG emissions (Edenhofer et al. 2015). The U.S. Environmental Protection Agency's (EPA) report on Inventory of U.S. Greenhouse Gas Emissions and Sinks shows that transportation currently takes the first place of the total U.S. GHG emissions, with a 28% share (USEPA). Transportation accounts for 10% of the gross domestic product, 70% of all petroleum use, and 57% of the total transportation emissions stem from lightduty vehicles (Hodges 2010;USDOE 2015). The Texas Commission of Environmental Quality (TCEQ) report shows that mobile sources contributed 67% of nitrogen oxide (NO x ) emissions, and 23% of volatile organic compounds (VOC) emissions in the Greater Houston Area. This result is directly linked to an increase in population and the use of light-duty vehicles in this region (TCEQ 2019). Texas has the highest energy-related carbon dioxide emissions by state (USEIA 2019a). The increases in criteria pollutants, particularly nitrogen oxide (NO x ), carbon monoxide (CO), and particulate matter (PM), originated from the regional population growth and increased fossil (fuel) use, especially in the transportation sector. According to the Texas Urban Triangle report on Framework for Future Growth, the period of 1960-2004 registered an overall transportation emissions increase in the share of 18-23% (Neuman and Bright 2008).
Increased urbanization rates inflict stresses on transportation infrastructure, and Texas has three of the ten largest metro areas in the U.S. (Houston, San Antonio, and Dallas), with an estimated population growth of 70% by 2050 (Zhang and Chen 2009). With the increase in population, the total mobility in passenger kilometers traveled (PKT), in the Triangle region, is projected to grow four times more than during the 2000s annualy. Moreover, the per-capita PKT is expected to reach values higher than the North American regional average. Therefore, it is pertinent to consider the development of an HSR system to accommodate the travel demand and mitigate the environmental impact of the transportation sector in this region. One of the most successful strategies often used in Japan, Europe, and China is the migration of road traffic onto the railway, which is generally more environmentally beneficial (Jones et al. 2017;Zhao and Yu 2018). Studies comparing the environmental impact of both transportation modes show considerable benefits toward reducing the energy used and pollutants Horvath 2010, 2012). In the effort to develop sustainable transportation modes, legislators initiated significant steps toward the implementation of an HSR system via the Shinkansen N700 series trains. However, the construction phase requires a significant amount of energy, material, and consequently, an increase in environmental impact (Liu and Li 2012). A cumulative assessment of the overall environmental impact from the proposed HSR system requires a life cycle assessment (LCA) study that accounts for all emissions generated over its lifetime, including phases such as raw material extraction and processing, manufacturing and construction, operation and maintenance and end of life. LCA is one of the most effective methods that estimate the environmental impact and evaluate the mitigation methods and technologies (Chipindula et al. 2018;Khasreen et al. 2009). Miyauchi et al. (1999) conducted a basic LCA survey by comparing three HSR vehicles where most of the impact was attributed to vehicle operation (Miyauchi et al. 1999). With the rapid development of HSR in the world, various case studies of HSR were carried out in different counties with a focus on the evaluation of environmental impacts (Bilgili et al. 2019;Chester and Horvath 2012;Dalkic et al. 2017;Jones et al. 2017;Miyoshi and Givoni 2014;Robertson 2016;Song et al. 2014). For example, Yue et al. (2015) conducted the research of China's HSR system by including manufacturing, construction, operation, and disposal of vehicle and infrastructure materials, and they found that 70-91% of the overall environmental impact was attributed to vehicle operation (Yue et al. 2015). Similar results were reported by Chester and Horvath (2010), who found the contribution of emissions from the operation phase in the range of 70-90% (Chester and Horvath 2010). Further investigation of Chester and Horvath's work revealed that the main cause of operation emissions is due to the electricity generated from coal power plants (Grossrieder 2011;Hodges 2010;Hoehne and Chester 2017;USEIA 2019b). With respect to HSR maintenance using a carbon-efficient strategy (Kaewunruen et al. 2015), GHG emissions of HSR using non-ballast track are less than the conventional track using ballasted track over the infrastructure lifespan (Krezo et al. 2016). Studies conducted in Europe indicated that apart from the operation, infrastructure took a significant share of the total impact, which could be dependent on the ridership volume (Bueno et al. 2017). Some researchers also indicate that there is no significant difference between the environmental impacts of road and railways infrastructure because the material used is similar (concrete consumption levels) (Federici et al. 2008). However, the net environmental impact from the operation phase is expected to compensate for the infrastructure-related emissions (Bueno et al. 2017;Chester and Horvath 2010). Studies comparing the road, air and railway systems conducted in Europe, Asia, and the U.S., indicate the rail transportation as one of the most sustainable modes that have significantly lower releases of air pollutants and GHGs (Andrade and D'Agosto 2016;Chan et al. 2013;Feigenbaum 2013;Haas 2014;Hodges 2010;Schipper et al. 2011). Specificly in the U.S., Kamga and Yazici (2014) comprehensively compared nation-wide HSR with two popular transportation modes of car and airplane, and explored the environmental sustainability and social benefits of building a high-speed rail network across the U.S. Like the different case studies of HSR in China (Chang et al. 2019;Lin et al. 2019), the environmental impacts of the I-45 HSR in Texas should be different from the HSR in California Horvath 2010, 2012;Matute and Chester 2015). Therefore, it is of vital importance that a quantitative environmental analysis with a life-cycle perspective that includes all phases (raw material extraction, manufacturing, transportation, construction, operation and maintenance, and end-oflife) is conducted for the first HSR system in Texas. Our work focuses on quantifying mid-point impacts of the planned HSR in the I-45 Corridor, and life cycle emissions comparison with car, bus and airplane with regard to criteria pollutants CO 2 , NO x , SO x , and PM. These emissions of HSR are expected to be sensitive to the resources of an electricity grid mix in the region along the I-45 corridor. In our work, the LCA methods are guided by the International Organization for Standardization (ISO) 14040, which includes data acquisition for material and fuel consumption throughout the life process of railway vehicles and infrastructure. The rest of the article is structured as follows: Sect. 2 includes the LCA methodology applied to the HSR system, and data collection from the literature of the HSR project, Sect. 3 provides the LCA results and some discussion on the HSR system, and our conclusions are given at the end.

Methodology and data
This life cycle study was conducted as per the framework developed based on ISO 14040-14043. The developed LCA framework is used to perform the HSR life-cycle analysis in three components shown in Fig. 1: vehicle; infrastructure; and a combination of both. Each component accounts for various phase life cycle processes including raw material extraction and processing, vehicle manufacturing, material distribution, construction, operation and maintenance, and end-of-life. In addition, the system boundary also accounts for the phase study of facilities and auxiliary equipment used during the operation and maintenance of the HSR system. The LCA also includes the analyses of alternative transportation modes of airplanes and roads. The base case begins with the Ecoinvent 3.4 process for transportation services, adjusted to reflect the actual conditions of the Dallas-Houston HSR system. Other specific data, such as the electricity mix for the operation phase, distance, material, and energy were also included to reflect the number of maintenance services along the Dallas-Houston corridor. The project accounts for 7 Shinkansen vehicles, and the infrastructure includes rail track, bridges, culvert, stations, trainset maintenance facilities (TMF), and maintenance-of-way (MOW). The alternative modes (road and air freight transportation) include vehicle/aircraft lifetime correspondent to fuel amount in passenger-kilometers traveled. All modes account for emissions during manufacturing, operation and maintenance, and the infrastructure constructions of each system. The evaluation in transportation migration was performed, taking into consideration the yearly average percentage of people traveling between I-45 corridors by mode. In our primary work of LCA of the HSR system between Houston and Dallas, we only evaluated the environment impacts of HSR without the comparison with other transportation modes and a sensitivity study (Chipindula et al. 2019). Currently, the I-45 highway is shared between car and bus with 89% and 2% of the total passenger volume share, respectively (USDT-FRA 2017). Aircraft transportation is assumed to be the remaining 9% of the total volume. Car input reflects the manufacturing and road network for average-sized gasoline cars in Texas. For the analysis, a large-size passenger car (2000 kg) with an engine capacity of around two liters is assumed as an average-sized car, selected based on the typical distribution of vehicle sizes in the region from small passenger cars to large sports utility vehicles (SUVs) and Trucks. The rate of five passengers per car is assumed. For buses, a low sulfur diesel vehicle is assumed with manufacturing and operation conditions of a clean bus technology with advanced engine design and exhaust and a capacity of 40 passengers. The aircraft transportation accounts for the remaining 9% of the population, which was modeled for the average capacity of 247 passengers per plane. For HSR emissions, the calculation of pollutants accounts for a lifetime of 20 years of vehicle operation and 60 years for infrastructure. For car, bus and aircraft the default models were directly used without further modification from the existing SemaPro module, as it is considered to cover most of the relevant emissions, calculated in terms of passenger share for each respective mode. However, for the HSR system, the study was further extended to reflect the required materials and energy used during the infrastructure construction and vehicle operations phase. Equation (1) expresses the calculation for individual system emissions, where E desribes the emissions of a pollutant in vehicle-kilometer traveled per year (VKT/year), a key measure in transportation planning that helps identify differences in travel demand compare travel modes, project congestion levels/impact and support transportation planning. TEmis denotes the total lifetime emission of a given pollutant and D t denotes the total lifetime distance traveled (km/ years). In addition, the study also examines the three most relevant categories for both vehicle and infrastructure. Out of the 15 mid-point categories in the Impact 2002? assessment method, the three most impacted areas include global warming (GW), respiratory inorganic (RI) and energy demand (E). The selected categories assess the significance of Criteria Air Pollutants (CAP), Greenhouse Gases (GHG) emissions and the energy use per passenger kilometer traveled with focus on pollutants like carbon dioxide (CO 2 ), nitrox oxide (NO x ), sulfur dioxide (SO 2 ) and particulate matter (PM).
The quantification of mid-point results presented as PKT, was calculated by multiplying the vehicle kilometers traveled by the average number of occupants for each vehicle. Equations (2) and (3) are measured for comparing emission impacts at mid-point category. These mid-point results are normalized to reflect the lifetime for the vehicle (20 years)-presented as Y Vehicle at utilization rate (R). The infrastructure lifetime is presented as Y infrastructure for 60 years of operation. The average distance of the proposed high-speed rail route is 386.24 km (d), with an average passenger utilization rate of 70% out of 400 seats.
The total distance traveled reflects the initial operating condition of seven HSR vehicles with eight cars and a seating capacity of 400 passengers operation in a route of 386.24 km (d). The route runs on the edge of the Great Plains in Texas, and there is no need for tunnels and great bridges across rivers (shown as Fig. 5 in the Appendix). The vehicles are scheduled to operate 18 h a day, leaving the other 6 h for system maintenance and inspection. Considering that the HSR uses electricity as an energy source, the source of electricity mix scenarios on the environmental impact from the operation phase is also analyzed during the vehicle's lifetime. Generally, the train operation phase does not generate direct emissions. However, the electricity generation and transition produce pollutants that can be reduced with the change in the electricity mix. Emissions for light-duty vehicles traveling to and from the station are not part of this study. The complete life cycle evaluation accounts for all emissions generated over the vehicle and infrastructure lifetime.
Under these conditions, this study evaluated 15 mid-point categories, and emissions were calculated to reflect the total lifetime of vehicle and infrastructure, where E vehicle/infrastructure is the vehicle and infrastructural emissions per passenger kilometers traveled, Q is the vehicle/infrastructure lifetime emission of a given pollutant, p is passengers, d is the distance of HSR, R is the vehicle utilization ratio, and Y is the lifetime of vehicle (20 years) or infrastructure (60 years).

Material extraction and processing
Material extraction and processing are among the most critical phases of the product life. Therefore, most of the product emissions are determined by decisions made during the design phase of a product. Similarly, passenger vehicles are made of different materials, some of which are not always recyclable, increasing the total environmental burden of any product. This LCA inventory used for this research includes reliable data for all-natural resources, the processing, and transportation of the materials. The inventory references previous life cycle study data and the inventory in the Ecoinvent 3.4 database. The HSR vehicle used in this study is the Shinkansen N700 train manufactured in Japan. Due to a lack of information on Japan's vehicle inventory, this assessment considers a similar size train manufactured in Germany, in which inventory is available in Ecoinvent, as per the approach used by Yue et al. (2015). Most of the energy and material data reference the information on the Dallas-Houston HSR Environmental Impact Statement Report sponsored by the Department of Transportation and by the Texas Central Railroad (TCRR) engineers, which also provide the values for energy used during the extraction and processing phase (USDT-FRA 2017). The infrastructure data is a mix of previous studies and quantities of material used in the track, stations, maintenance, equipment and service facilities.
The LCA processing module includes inputs of raw materials, energy and onsite transportation of the product. A minor impact is allocated to transportation in the processing zone, because the route takes place in a very small distance, compared to the other transport processes. The HSR track choice is a non-ballast, with viaducts and bridges, above the threshold level, avoiding interference with the existing transportation system. For this reason, concrete and steel are the predominant materials in the railway infrastructure. The track selection was based on infrastructure lifetime (60 years), safety, security, and reliability. Moreover, it has been reported that fixed track construction consumes 89% less energy than the ballast track (Liu and Li 2012). This project does not include tunnels because of the flat surface along the Dallas-Houston region.

Manufacturing/construction
The manufacturing of vehicle parts and infrastructure materials require carbonbased energy, which is associated with the release of CO 2 , NO x , SO x , O 3, and PM emissions. For this study, the Shinkansen vehicles are assumed to be manufactured in Japan using the available energy in the region. The inputs used for vehicle manufacturing were primarily electricity and processed aluminum, steel, organic and non-organic materials, such as glass, plastic and resin which represents vehicle main materials and the manufacturing module from SimaPro Ò . Infrastructure accounts for the total materials and energy used during the years of construction. The railway infrastructure includes track, bridges, culvert, stations, trainset maintenance facilities (TMF) and maintenance-of-way (MOW) facilities. Signal housings that monitor train traffic, signaling cables and power supply for equipment are also part of the infrastructure system.

Transportation
Materials for track, stations and other support facilities are transported to the construction sites by diesel heavy pickup trucks and the Houston railroad connection system. Given the required materials, the extension of the track and the kilometers per passenger, high consumption of energy (diesel and gasoline) and consequently a high percentage of fossil fuel emissions are expected. Therefore, this study assumes that construction materials are obtained within the proximity of track. On the other hand, the Shinkansen vehicles were considered to be transported by ship from the Kinkisharyo manufacturing in Osaka, Japan to Galveston port via the Panama Canal. Rail, reinforcement steel, structural steel, and aggregate are transported to the sites via rail. Transportation emissions are calculated by multiplying the distance traveled by the weight of the materials. The average miles, passengers, and material used in the infrastructure construction and vehicle transport are as shown in Tables 1 and 2.

Operation and maintenance (O&M)
This phase was modeled considering all the energy and material required to operate and maintain the railway system during the initial phase. The scope of this phase included activities required to support the operation and maintenance of vehicles and facilities throughout their lifetime. The input data accounts for daily power consumption from vehicles, train stations, maintenance facilities and signaling using consumption rates from project engineers. In addition, the O&M vehicle modeling also accounts for diesel used in the multiple unit trainsets, during the monthly and quarterly inspections. The end-to-end route distance was estimated to be approximately 384.63 km operated at the speed of 329.91 km-hour. Initially, the seven trains are expected to depart every 30 min during peak hours and hourly during the off-peak for a total of 18 h of operations and 6 h of system maintenance and inspection. The electricity consumption demand for the train is assumed to be a single phase running through a wiring installation above the track and distributed to each train using a catenaries distribution system. At the initial phase, the total daily train electricity consumption is estimated to be 448.87 MWh. Electricity for the entire facility operation, including maintenance (163.7 MWh) is estimated to be  (Massetti et al. 2017). Though this phase uses mostly electricity, during the 20 years, other products such as lubricants, diesel, paint, water, and metals are also used, on a smaller scale.

End of life
The end-of-life assessment model was established considering the disposal and recycling mode of material and energy used throughout the life cycle phase. Out of the total of materials used in vehicle construction, only a small amount was recycled. Most of the materials are scrapped and disposed of in the end-of-life phase. Materials that are part of stations and catenaries (steel and aluminum) are among the ones with the highest percentage of recycling rate. Energy consumption in vehicle scrapping and the recycling process was retrieved from the Ecoinvent 3.4 database. Railway track and road infrastructure are considered to be unutilized which results in zero end-of-life effects. Since there is no data inventory for track dismantling, the process suggests that the infrastructure is left on-site at the end of life (Chan et al. 2013).

Quantitative mid-point impacts
This section outlines the life cycle impacts of the Dallas-Houston HSR system in terms of Impact 2002?, mid-point categories, as shown in Table 3. The resulting impacts across the 15 categories are based on defined boundaries and required inventory information established at the goal and scoping phase. Regarding the CO 2 equivalent emissions, the vehicle component accounts for 92.77% in the overall life cycle GHG emissions of the HSR system. Our emission evaluation agrees well with some previous studies which reveal that the dominant energy consumption occurs  Horvath 2010, 2012;Lin et al. 2019). As per infrastructure, 56.76% of GHG emissions are contributed by the material extraction and processing phase (23.75 kgCO 2 eq/VKT), and the rest is shared by the infrastructure construction and maintenance phases. Evaluating the total HSR system, the characterization assessment indicates that vehicle operation is the largest contributor to the overall impact, accounting for more than 50% of almost all mid-point categories. The significance of vehicle emissions results echoes the large share of electricity and fossil-fuel usage during the vehicle operation and maintenance phase. In the infrastructure, GHG emissions reflect the amount of material and energy used during the material extraction and processing phase. At this phase, infrastructure accounts for 81% of the total material used in the HSR system reflecting the amount of copper, concrete, steel, rebar, and energy (electricity, fuel, and lubricants) used during the four years of non-ballast track and facility construction. The increase in particulate matter, mostly from anthropogenic sources or pollutants emitted from the power plant, resulted in a high impact on human health and the environmental damage potential. Global warming was impacted by hard coal/lignite mining operation, electricity and fuel consumption from vehicle operation, the use of heavy equipment and the transportation phase. The phase-wise distribution of impacts for the infrastructure is shown in Fig. 2. To characterize the scale of impact levels and the mitigation required to reduce the intensity of targeted pollutants, each mid-point category, represented in Table 1, (a) to (o), quantified emissions by their impact mechanism. Except for the non-carcinogens (b) mineral extraction (o), and land occupation (j) categories, the vehicle phase contributes to the environmental impact by a large margin. The overall vehicle phase results in a share of 67.83% in the total life cycle across the 15 mid-point impact categories. Respiratory organics also have a major impact on the vehicle component. The increase in respiratory organic (f) emissions is linked to the O&M phase and the high quantities of oil and gas products consumed. The life cycle distribution of infrastructure (Fig. 2) confirms the percentage contribution of the material phase. Apart from the pollutants originated from mining, material processing, and construction, the power source also impacts the total system emission. HSR trains are frequently powered by electricity and depending on the source, low or high emission electrical generation system, the long-term impact may be significant. A previous environmental assessment conducted in a non-renewable source, such as coal power plants, has revealed higher emission values than those from renewable Fig. 2 Percentage distribution of phase-wise mid-point impact category for infrastructure sources, like wind or hydroelectric (Schipper et al. 2011). Therefore, even though HSR has proven to be more efficient than other transportation modes (cars and planes), its long-term operation may be compromised by the available energy source in the region. Like on the other energy-related studies, the assessment of the I-45 high-speed rail system shows that the increase in global warming (carbon-related emissions) is strictly related to the increase in fossil fuel use which suggests that the emissions by vehicle operation can be reduced by introducing a more renewable energy source. Less carbon-intense electricity and a higher average ridership are the main factors to minimize GHG emissions per PKT (Andrade and D'Agosto 2016). Figure 3 shows the cumulative energy demand for vehicles and infrastructure per PKT in the I-45 corridor. The negative energy represented as others in the infrastructure accounts for a credit to the overall energy used, which is beneficial to the overall system.

Life cycle emissions of HSR
The effects on occupancy and passenger migration to the HSR system control the environmental efficacy of developing the HSR system. For the Dallas-Houston corridor, the projected increase in population and the station location are relevant contributing factors to the increase in HSR ridership rate. Moreover, the American study in HSR ridership shows that station location tends to increase the migration of air travelers to the HSR system (Haas 2014;Todorovich and Hagler 2011). However, one study reveals that in Texas, air travelers are less likely to switch into the HSR system, Fig. 3 Analysis of cumulative energy demand for vehicle and infrastructure at 70% ridership considering that the majority of passengers who choose to travel by airplane are business travelers (Liu and Li 2012). To elucidate the environmental benefits of travelers' switch from other transportation to HSR, the compassion of life cycle emissions from four major transportation modes (car, bus, aircraft, and HSR) was conducted. The energy use was also evaluated, and they were calculated per PKT at the full vehicle occupancy. The full capacities of car, bus, aircraft, and HSR are listed in Table 4, and the obtained energy use and emissions are depicted in Fig. 4.
In the previous study, Chester and Horvath (2012) explored the environmental benefits of the HSR in California by comparing the GHG emissions and acidification potential of HSR with car and aircraft. Our work extended that to include the major criteria air pollutants of GHGs, CO 2 , NO x , SO 2 , and PM and consumed energy. In this study, the selected module for car, bus, and aircraft reflects the most recent technologies that combine aspects of vehicle efficiency and the implementation of renewable energy. Therefore, their environmental concerns are not sorely impacted by the increase in the occupancy rate, but also by the technology and change in the energy used. The total energy use of HSR is comparable to bus and aircraft and is lower than those of cars by 27%. Global warming potential (GWP) emissions of HSR take the second high; however, it would be the lowest CO emissions with a 75% reduction from those of cars. NO x emissions of HSR are much lower than those of cars and are slightly higher than those of buses and aircrafts. Although the SO 2 emissions of HSR are slightly higher than those of buses and aircraft, it is much lower than those of cars. However, the HSR system performs very poorly in terms of PM emissions in comparison to the ones of cars, buses, and air travel, as observed in Fig. 4f. These non-greenhouse gas air pollutions (NO x , SO 2 , and particulate matter) are key environmental quality issues which are directly associated with fossil fuel electricity generation in the United States (Massetti et al. 2017). Moreover, since fossil fuel use is the primary source of CO 2 , the burning of coal, oil, and natural gas for electricity, the generation the global greenhouse emissions also increases. This anomaly is due to the heavy reliance on electricity produced from fossil fuels. As reported by Chester and Harvath (2012), PMs are often generated by mining activities of raw material and the emissions by the operation phase, including the supply chain combustion process and electricity generation. PM which is frequently formed from the complex chemical reaction of sulfur dioxide and nitrogen oxides can also be emitted from construction sites and fossil fuel burning stacks. Under these study conditions, the electricity mix by using more solar and/or wind power is the most promising consideration to optimize the environmental impact of the HSR system. The highest quantitative input of HSR is from electricity production, so unless renewables are used in producing the electricity used to power the trains, PM emissions will not decrease.
The statistical data shows that there were about 16 million one-way journeys made between Houston and North Texas in 2017 (Transforming Travel In Texas). After the Dallas-Houston HSR is built, in the middle or later 2020s, the HSR system will attract about 6.5 million passengers in one way, which is 25% of the total travelers between Houston and North Texas at that time. By 2050, it is estimated that it will take about 13 million passengers in one way, accounting for 35% of the total journeys. The currently planned 68 one-way trips of the project will accommodate about 9.9 million passengers annually. The trip numbers will be increased when the train occupancy rate reaches 100% in the 2030s. The increase in passengers will result in lower emissions per PKT than the project initiation phase. Over the years, public transportation has proven to reduce fuel use and total emissions generated by the transportation sector. However, the environmental benefit of bus, aircraft or HSR implementation, is dependent on fuel usage and the passenger kilometer traveled by mode. For this study, the benefit of HSR implementation is strictly related to the migration of passengers from cars to the HSR system. Car riders represent the highest percentage of current passengers traveling between Dallas-Houston, which affects the energy consumptions and the emissions generated, per passenger kilometer traveled. Therefore, the estimated increase of 35% on HSR ridership, by 2050, will increase the number of trips and consequently reduce the HSR emissions, per passenger kilometers traveled. While comparing the HSR system with other transportation modes, the efficiency and the type of fuel used, was not taken into consideration.

Sensitivity analysis
The sensitivity analysis was conducted to evaluate the environmental benefits resulting from the change in the source electricity mix. Operation and maintenance contribute the most to the overall vehicle emissions, and the electricity mix is the main driver that increases pollutant emissions. The electricity mix varies by country and region. The current share of U.S. and Texas electricity mix distribution does not reflect the actual SimaPro Ò inventory. The U.S. electricity share has the highest share for electricity from coal and lignite, which significantly increases the impact of vehicle and HSR system, in general. The U.S. Electric Reliability Council of Texas (ERCOT) mix is mostly originated from gas sources that have a much lower impact than the electricity generated by coal or lignite. To evaluate the actual contribution of main pollutants, the main vehicle emissions were assessed using the actual share of each fossil fuel source in Texas and the U.S. Results show that the actual emissions for the HSR system will be much lower than the one calculated with the Ecoinvent 3.4 database. Table 5 shows that vehicle operations will potentially reduce the CO 2 contribution by 64%, SO 2 by 78%, NO x by 60% and the N 2 O emissions by 57%. Considering that the electricity mix is the main driver to the increase in vehicle emissions by switching the Ecoinvent 3.4 data mix with the Electric Reliability Council of Texas (ERCOT), an improvement in the overall vehicle emissions was expected. Reduction in vehicle emissions by changing the electricity mix to the less impacted source has previously been proven by other HSR/train environmental impact assessments conducted in Europe and North America. The change in the electricity mix was proved to be one of the most efficient ways of reducing the long-term impact of the electricity mix (Chan et al. 2013;Jones et al. 2017). At the endpoint, the major reduction is observed in the potentials of climate change (62%) and human health (44%). This result reflects the reduction in respiratory inorganic emissions (NO x , PM, and SO 2 ) which normally stem from coal electricity sources and fossil fuel. In a case study of short-haul air travel replaced with HSR in Australia, it was confirmed that the adoption of renewable energy technologies could further alleviate the carbon footprint of HSR (Robertson 2018). There are abound wind power resources in the Texas region. Thus various emissions associated with the operation of the studied HSR can be further reduced when a large amount of electricity is generated from offshore wind and applied to the I-45 HSR in the future (Chipindula et al. 2018).

Conclusions
We investigated the life cycle environmental impact of the potential HSR system in the Houston-Dallas (I-45) corridor, which is ranked as the busiest route among 18 traffic corridors in Texas. The study determined the number of total emissions for potential HSR. The study also analyzes the break-down of the emissions per each phase in the life cycles and per the source of emissions. The benefit analysis is performed to compare the total emission of the potential HSR system and the existing highway transportation system. The study is assisted by the SimaPro LCA software with Ecoinvent 3.4 database and Impact 2002? impact assessment method. The estimated net changes with an assumption of Shinkansen N700 show that the vehicle phase is the largest contributory phase across 12 of the 15-mid-point impact categories, except non-carcinogens, land occupation, and mineral extraction. Vehicle operations and the electricity consumed in this stage are the control factor to determine the environmental efficacy of the system in terms of respiratory inorganics (PM 2.5 ) emissions. Vehicle component contributes with 14.50 kgCO 2 eq/ VKT, of which fossil-fuel usage during operation is the primary contributor with 98% of the greenhouse gas (GHG) emissions. For the infrastructure component, 56.76% of GHG emissions are contributed by the construction phase (23.75 kgCO 2eq/VKT). The energy use of HSR is comparable to the bus and aircraft and is 27% lower than those of cars. Various life cycle emissions of HSR except PM are significantly lower than those of a passenger car, which is the dominant transportation mode currently adopted by travelers between Dallas and Houston.