This section discusses general findings about the literature and GHG mitigation strategies as a whole, and addresses some of the questions raised earlier:
Sections 5-7 present a review of individual strategies.
GHG mitigation is a relatively recent but rapidly growing area of research. The literature on GHG mitigation strategies has started to develop recently relative to the literature on other major transportation concerns such as safety and air quality. This is because climate change itself has emerged recently as a critical transportation issue relative to these other areas.
Nevertheless, this is becoming an increasingly active area of research and there is much valuable information already about GHG mitigation strategies. The literature includes, for instance, many case studies that examine the effects of GHG mitigation strategies that have been piloted or implemented in different jurisdictions. There are also many case studies of the GHG effects of policies that were implemented to address other transportation concerns, but which simultaneously have an effect on GHG emissions. This includes, for instance, efforts to increase fuel economy for the purpose of national energy security, which has the co-benefit of reducing GHG emissions. In sum, for most strategies, there are some examples of practical implementation and effects.
The amount of and nature of knowledge varies across strategies. Some strategies have, thus far, only been assessed with transportation, economic, or other models, primarily because they have yet to be implemented fully in practice. For example, this is the case with tax incentives for alternatively-fueled vehicles: alternatively-fueled vehicles have only recently entered the market, and so the research community has, until recently, been unable to estimate the effect of policies designed to encourage their ownership. Strategies such as these are an important area of future research. Thus, the amount and nature of knowledge varies widely across strategies.
The effects of each strategy may vary widely depending on the implementation context. As in other areas of transportation such as air quality or congestion, the effects of a particular GHG mitigation strategy depend significantly upon the transportation and broader social and economic context in which the strategy is implemented. For instance, transit improvements can apply to a variety of modes and include increases to the frequency of service on existing routes, system-wide route optimizations, the addition of new routes, and improvements to transit information and comfort. The effects of a particular type of transit improvement additionally depend on the state of the existing transit system, demographic and economic trends, and land use patterns. Even for strategies that are well defined, the use of roundabouts instead of traffic signals for example, the effects depend on traffic patterns in the region and may vary greatly. Thus, for almost all strategies, including those that have been studied extensively, there is a wide range of possible GHG reductions, costs, and other effects. The very same strategy may reduce emissions in one context, have no effects in another, and even increase emissions in a third. This variability cannot be reduced, but it can be better understood through additional research of these strategies in a still wide range of contexts. This also implies that transportation agencies should carefully evaluate each strategy in the context of their own jurisdiction, and agencies ought not adopt or discard strategies solely because they have or have not been effective in other areas. Finally, strategies that involve changes to infrastructure create GHG emissions due to construction, which can reduce or even negate the overall benefits of an action.
When strategies reduce emissions, the reductions attributable to individual strategies are typically modest relative to total emissions from all surface transportation sources. Another observation about GHG mitigation strategies is that most strategies will, at best, have a small impact on emissions relative to total emissions-reductions comprising a few percent of the total surface transportation emissions. This is because most strategies affect certain parts of the transportation system (some subset of drivers, of the traffic system, or of the fleet) and those effects are themselves modest. For example, telework strategies, which seek to encourage employers to enable employees to work from home, affect some portion of businesses for which telework is feasible, and some portion of the employees of those businesses. Further, for those employees who choose to telework, it may only affect their commute on certain days and have little effect on other days or on other types of travel.
Some strategies can achieve potentially significant reductions. While these strategies may be more difficult to implement, agencies should consider them and seek ways to address the associated concerns. Importantly, the effects of many strategies increase the more intensely they are implemented, and not all strategies are inherently limited to having small effects. When implemented at a high intensity, some strategies have the potential to achieve major reductions in emissions. For example, road pricing strategies include charging for driving in certain corridors at certain times of the day (usually peak hours), thus reducing demand. If the added cost is low, there may be little to no effect on the vehicle miles traveled. However if the cost is high there may be a significant decline in vehicle miles traveled and, consequently, GHG emissions on that corridor. For illustration, one can imagine if the cost were prohibitively high for almost everyone, virtually no driving would take place. Road pricing raises important concerns about equity and the social and economic impacts of limiting vehicular travel, and therefore it may be difficult to implement even at modest levels. With strategies such as this, the effects are typically limited to modest reductions in practice because of concerns about social, economic, and other impacts. While they may be challenging to implement, agencies should consider strategies that can achieve potentially significant reductions, and agencies should seek ways to address associated social and economic concerns. For example, it has been suggested that inequity in road pricing may in part be addressed by using the revenue from road pricing to improve transit and other modes of transportation, which are primarily used by low-income drivers.
The net GHG effect of some strategies is unknown-even when considered in a specific implementation context-because the strategies have multiple and complex effects that have rarely been evaluated. Most of the findings about a strategy focus on its immediate and intended effect on GHG emissions. For example, the research on road resurfacing as a mitigation strategy seeks to establish the extent to which smooth roads improve fuel economy and thus reduce GHG emissions. Similarly, research on incident management seeks to estimate how much congestion can be avoided when non-recurring incidents (like crashes and cargo spills) occur, and the corresponding reductions in GHG emissions. Both of these are key steps in determining the effects of these strategies.
Yet all strategies are complex and may have multiple, and sometimes unintended, effects that are difficult to assess and are not often accounted for in the literature. These unintended effects can reduce or negate the emissions reductions achieved by a strategy. Road resurfacing itself creates GHG emissions, and those emissions are usually not taken into account in research that assesses the effect of smooth roads on fuel economy. This means that it is not yet known whether road resurfacing as a mitigation strategy decreases, increases, or has no effect on GHG emissions. Incident management may reduce congestion initially, but some or all of those gains may be lost if driving increases as travelers discover that the transportation system has become more efficient and are induced to use it further. Thus, the overall effect of many strategies that have complex effects is currently unknown. This phenomenon of complex, multiple consequences is a crosscutting concern discussed in detail below in the section titled Cross-Cutting Issues in Determining the GHG Effects of Strategies.
As discussed below, some studies have shown that bundled strategies can achieve substantial GHG reductions and agencies seeking to reduce GHG emissions should consider a multi-strategy approach, evaluating the interactions of strategies in the context of their own jurisdiction. The fact that most strategies at best produce modest GHG reductions implies that agencies seeking to reduce GHG emissions significantly will have to take a multi-strategy approach that simultaneously addresses different aspects of transportation emissions with a range of mechanisms. Furthermore, because the effects of each strategy can vary widely, agencies should evaluate options carefully in the context of their own jurisdictions, using the literature as a basis for such analysis.
Importantly, strategies that are implemented at the same time may interact with each other and increase, inhibit, or otherwise alter their effectiveness in reducing emissions. For example, anti-idling regulations, which administer penalties for idling, and eco-driving education, which encourages drivers to reduce idling among other things, may each reduce some emissions when implemented individually. When implemented together, however, the combination of the "stick" (regulations and penalties for idling) and the "carrot" (encouragement) could result in greater reductions than the sum of the effects each strategy in isolation. On the other hand, strategies may have diminishing returns when combined. For example, strategies such as traffic signal optimization seek to improve system performance and reduce fuel consumption. They produce the greatest reductions when GHG emissions from driving are high; for example, when vehicles have poor fuel economy or the fuels they burn have high carbon content. Therefore, as strategies that seek to increase fuel economy such as fuel economy standards are implemented, the reductions from system optimization strategies decrease. The reductions from the two strategies together are typically higher than if only one strategy were implemented, but the absolute effect of the two strategies in combination is less than the sum of the effects of the individual strategies. Thus, in order for agencies to use a multi-strategy approach, it is very important that they assess the interactions of strategies.
The literature cannot yet offer strong evidence about interaction effects. There is not yet sufficient evidence in the literature to determine how strategies interact, for several reasons. There are few real-world examples in which the same bundle of strategies has been implemented in similar enough ways to generalize the effects of those bundles. Moreover, to fully understand the interaction between strategies, the effects of each strategy in isolation would ideally also be known. Yet in most real-world cases, strategies cannot be implemented both in isolation and as bundles.
Sometimes, the conditions under which strategies are implemented or the nature of the strategies themselves are not conducive to rigorous analysis. For instance, eco-driving education campaigns often use television and other media to educate citizens about the benefits of better driving practices. However, accurately attributing observed changes in the general population's driving habits to the campaign (as opposed to other causes) is very difficult. Accurately attributing the combined effect of anti-idling regulations and eco-driving together may be still more difficult.
It is possible to model the interactions among some strategies (e.g., ramp metering and capacity expansion, which both involve transportation system improvements), and there are studies that do this. In many cases, however, the research tools and knowledge are not yet available to do so. Some studies have sought to model the effects of policy bundles at a national level, but their findings may not be immediately useful for State DOTs and MPOs because of difficulties in disaggregating the findings and applying them to a state or local level. Thus, while agencies should consider any available literature on strategies' interactions, it may not offer strong evidence and they will need to conduct their own analyses.
At this time, it is more appropriate to describe what is and is not known about strategies and the key factors in assessing them, rather than to rank or quantify them. Given the variability of each strategy's effects, knowing that each strategy is complex and may have unintended consequences, and seeing that new findings about strategies are continually emerging, it is not appropriate to quantify the effects of mitigation strategies or to rank strategies at this time. This sourcebook instead describes what is and what is not known about different strategies and provides insights about the various factors that agencies should consider when evaluating each.
Stakeholders may strongly oppose strategies that, for example, raise transportation costs, reduce transportation convenience, unfairly affect some portion of the population, are too costly to the public, create inconvenience for travelers, or increase GHGs in that particular context. These concerns may be widespread, or they may be voiced by a small but important or powerful minority. Although these barriers may be difficult to overcome, the strategies should not be discarded outright: as noted earlier, some can be very effective and agencies should seek ways of implementing them.
It is important but difficult to assess total GHG effects in order to determine the true impacts of a strategy in mitigating emissions. Almost all GHG mitigation strategies produce some emissions as a side effect, in addition to reducing emissions as intended. These net effects must be analyzed to know the true measure of a strategy's effectiveness in mitigating emissions. For example, road resurfacing and replacing intersections with roundabouts requires new construction and maintenance of the transportation system. In scrappage programs, consumers are encouraged to replace vehicles that have low fuel economy (typically older vehicles) with new vehicles that have higher fuel economy. The process of scrapping older vehicles produces emissions and scrappage programs result in additional manufacturing of new vehicles, above and beyond business-as-usual.
Strategies can also produce emissions in other parts of the transportation system. For example, ramp metering regulates the flow of vehicles onto highways, thereby reducing congestion and delays and reducing GHG emissions. However, ramp metering increases idling when vehicles wait for their turn to enter the highway at the ramp meters, and, by reducing congestion, may enable drivers to travel at higher speeds that reduce fuel economy. Both of these effects increase GHG emissions (in comparison to not idling or driving slower) and are unintended consequences of ramp metering.
These added construction, manufacturing, idling, and speed effects create emissions that offset some of the sought-after reductions from the strategy-fuel economy gains from smoother roads or newer vehicles, or smoother traffic flow from roundabouts and less congested highways. These unintended or secondary consequences may reduce or negate the gains from the intended effects of a strategy, meaning that a strategy may reduce, have no effect on, or even increase emissions. For example, some studies have found that the unintended emissions from ramp metering outweigh the emissions reductions, and therefore the strategy may not be effective at mitigating emissions (Cambridge Systematics, 2001).
Life-Cycle GHG emissions analyses seek to determine the net GHG emissions from all effects attributable to a product or process, including these unintended emissions. Life-cycle emissions are the true measure of a strategy's effectiveness in mitigating emissions and combating climate change. Importantly, life-cycle emissions are usually not considered in assessments of GHG mitigation strategies in the literature. In some cases, this is because the focus of a study is on establishing the intended effects (e.g., the relationship between smooth roads and increased fuel economy). In many cases, however, life-cycle emissions are extremely difficult to estimate because the second, third, and nth order effects of strategies are difficult to trace and even more difficult to quantify.
By the very definition of the phrase "unintended consequences," any strategy may produce significant emissions that are not currently evident. Therefore, all strategies should be assessed carefully and second and third order effects should be traced to the extent possible. In Column E, the table highlights strategies that are known or expected to have large unintended consequences, where the strategy's life-cycle effect is often to have no change in emissions or even to increase emissions. These strategies have nevertheless been included in this sourcebook because transportation agencies have considered them as a way to reduce GHGs, and because they may in some circumstances be effective. When considering these strategies in particular, agencies should carefully take into account the known indirect effects.
It is important to assess the possibility that gains from some strategies-those that make transportation by roadway faster, easier, or less costly-may be reduced or lost to induced demand or rebound effects. There is an economic phenomenon that when the price of a good decreases, perhaps because the supply increases, consumption of that good increases. This phenomenon affects GHG emissions in transportation in two ways. First, policies that reduce highway congestion increase the supply of transportation. The benefits of these policies may be partially offset by additional driving that occurs in response to the improved travel conditions. This additional driving is known as "induced demand" and can be an important consideration in estimating the travel and emissions impact of traffic congestion management and other transportation system improvements.
The most often-cited example of induced demand is highway capacity expansion. When new highways are built or new lanes added to existing highways, emissions might initially decrease as congestion on other routes decreases. However, this improved efficiency decreases the cost of road travel in terms of travel time and fuel spent. This lower-cost capacity is often quickly used by new users (e.g., those who were previously not traveling or those who were using other modes) (Leeming, 1969). The net effect may be that more vehicle miles are driven than before the expansion occurred, and therefore the strategy could increase GHG emissions overall.
Capacity expansion is one cause of induced demand. However, any strategy that makes the transportation by roadways faster or easier-by reducing the number of vehicles on the roadways or improving transportation system performance-can potentially induce demand. Strategies that are vulnerable to induced demand are noted in Column F. While capacity expansion often results in a net increase in emissions, most strategies do not create new capacity. Therefore, for most strategies, induced demand may reduce or negate GHG reductions from the strategy, but it is unlikely to result in greater emissions. Additionally, many strategies that free up capacity are likely to have a very small effect (such as car sharing programs), and it is expected that the newly freed capacity may be too small to be observable and so there may be little or no induced demand. This is an area of further research. Induced land development, which can occur as a result of either new roadway capacity or new transit capacity, can also be a source of induced vehicle travel.
Because reductions in congestion brought about by these strategies can be partially offset by additional travel from drivers who are attracted to the less congested roads, careful analysis of the direct and indirect travel activity effects of a project is warranted. The induced travel is likely to come partly from changes in travel patterns (new trips and longer trips), and partly from shifts of travelers from other times of day, routes, and modes (such as transit). Accurate project evaluation must consider the impact of induced demand; otherwise, the benefits may be overestimated. Once properly accounted for, minimizing induced travel often depends on the quality of alternatives and complementary strategies for implementation. If the alternatives to traveling in congested conditions are inferior, a high time savings or price benefit is needed to change traveler behavior. In contrast, if alternatives are attractive, they are more likely to be successful, resulting in less induced demand and lower congestion.
A second phenomenon is known as the rebound effect and is associated with gains in energy efficiency or other mechanisms that reduce cost. When energy efficient technologies or systems are introduced, the cost of using them is less than their less-efficient counterparts, so they are used more. Thus, the sum of the energy reduction is lost to increased use. In transportation, the rebound effect is associated with increases in fuel economy: More-efficient vehicles are driven more than less-efficient vehicles, and some of the net energy benefits of switching to a more efficient vehicle are lost to the increase in use.
Importantly, induced demand and the rebound effect can be managed: the decrease in cost of driving that results from these strategies can be offset with an increase elsewhere in the system (e.g., through road pricing). In sum, even though strategies may be vulnerable to induced demand and rebound effects, the extent of the effects should be evaluated carefully because reductions may still be made, particularly when coupled with market strategies.
The many uncertainties in the current state of knowledge present both a need and an opportunity for future research. The sourcebook discusses opportunities for near-term research on individual strategies in the strategy reviews; here it briefly presents examples of research needs that are relevant to GHG mitigation strategies as a whole.
The variability of strategies' effects suggests that transportation agencies would benefit from tools that enable them to estimate effects of strategies in their own jurisdictions. Another component of this larger research project is aimed at developing a GHG policy analysis tool to meet this need. Additionally, research on interactions between strategies is currently limited. As GHG mitigation strategies are implemented more frequently, new opportunities will emerge for careful study of combinations of multiple strategies. Such research would provide much needed knowledge about how bundles of strategies may work. Similarly, agencies would benefit from guidance on how to assess interactions among strategies in their own jurisdictions, and how to choose bundles of strategies for long-term implementation.
There is also great value in research that assesses the life-cycle GHG effects from different strategies and that develops tools for transportation agencies to assess life-cycle effects. Such analyses are complex, so there is also a need for methodological innovations. Similarly, research is needed on the effects of induced demand for strategies other than capacity expansion, which has been widely studied. FHWA has undertaken a research project intended to further investigate the GHG reduction potential of highway operation and management strategies, which will include the effects of induced demand. The study is scheduled to be completed in 2012.
Finally, there is increasing recognition nationally and internationally that climate change is a critical concern of our time. Correspondingly, the research and practice in GHG mitigation is accelerating and new findings are continuously emerging. The authors of this sourcebook are hopeful that many outstanding uncertainties will be resolved in time, and recognize that conclusions drawn today about the effects and effectiveness of these strategies may tomorrow be reevaluated. The current uncertainties about the strategies, coupled with the dynamism of the body of research, suggest that a summary of the literature may need to be updated frequently in order to remain current. A final area of study, then, is in the methods by which information on GHG mitigation strategies can be kept up-to-date efficiently, and how this knowledge can be delivered to the broader community of practitioners, researchers, educators, and students.
Such methods could take several forms and serve many purposes. A review could evolve as the Highway Capacity Manual has, improving over several decades as the result of voluntary contributions of improvements by users of successive editions. Alternatively, it could capitalize on today's information technologies that make such participatory efforts highly engaging and productive. One can imagine a wiki sourcebook with a large community of continuously contributing authors, an environment for discussion, and a clearinghouse for research needs and publications. Such an approach could have great utility for the transportation community.
Cambridge Systematics, Inc. et al. (2001). Twin Cities Ramp Meter Evaluation, Executive Summary. Minnesota Department of Transportation. http://www.dot.state.mn.us/rampmeter/pdf/finalreport.pdf.
Leeming, J. J. and McKay, G. M. (1969). Road Accidents: Prevent or Punish. Cassell.