Personal automobile travel consumes a lot of energy and is a major source of polluting emissions in cities. As new transit systems are planned to mitigate these impacts, more comprehensive environmental-assessment analysis is needed to back up such decisions.

Transportation infrastructures, whether for public or private vehicles, can last decades, yet environmental assessments often fail to consider how expensive these infrastructures are in terms of initial investment, or long-term changes in how these systems are used. Building a new bus or rail line, for example, requires a lot of energy and generates greenhouse gases and other emissions. So calculating payback times for this initial energy consumption and later emissions is important for meeting regional environmental-policy targets.

The Arizona–California team assessed how much energy was consumed, the amount of greenhouse gas emitted and how much photochemical smog could potentially be emitted over the entire life cycle of Los Angeles' new bus rapid-transit and light-rail lines. The analysis included initial construction costs of the infrastructure in terms of energy and emissions and how much it would cost to manufacture the vehicles employed on these lines in the long term. The researchers also calculated the reduction in the number of automobiles in Los Angeles and journeys made by these vehicles thanks to the new transit lines.

In the near-term, even when infrastructure, vehicle and fuel impacts are included, the Los Angeles transit systems will cost less in terms of energy and greenhouse-gas emissions per passenger and per kilometre energy use compared with the latest higher-efficiency automobiles. The transit lines should also reduce smog.

Using imported coal to power the rail lines – as would be the case in the near-term – might increase respiratory problems for some users but the city of Los Angeles is already investing less in such an energy source. In the long term, the energy and environmental impacts of transit will decrease by 42–93%, largely as a result of improvements in vehicle technology and by using more renewable fuels and natural gas.

Transit infrastructures will allow for long-term health and environmental benefits. However, the huge amount of concrete used to build the rail tracks will create relatively more negative impacts than the asphalt used in the bus infrastructure. The bus system will then have a shorter payback time – within a decade – when it comes to greenhouse-gas emissions compared to the light-rail system, which has a payback time between 30 and 60 years. The use of low carbon-dioxide concrete or advanced emission-control technologies during material production could also help lower infrastructure impacts and improve payback time.

As cities consider deploying transit systems to reduce air emissions and the number of personal vehicles on the road, planners should try to better understand the upfront infrastructure and long-term technology and behaviour changes that result from such investment decisions. Using only figures per passenger kilometre travelled might lead to inaccurate assessments, so future analyses should instead rely on complete life-cycle evaluations to determine how energy use and emissions change as a result of expanding public-transit networks.