Comprehensive Transportation Emission Reduction Planning

Many jurisdictions have ambitious greenhouse gas emission reduction targets and are developing plans to achieve them. Since transportation is one of the largest emission sources, it is a major component of these plans.

There are many possible ways to reduce transportation emissions. These include clean vehicle strategies that reduce emission rates per vehicle mile, and transportation demand management (TDM) and Smart Growth policies that reduce total vehicle travel, as summarized below.

Types of Emission Reduction Strategies

Clean vehicle strategies reduce per-mile emission rates. Vehicle travel reduction strategies reduce total vehicle mileage.

Which strategies are most effective and beneficial overall? That depends on how they are analyzed, including the scope of impacts considered and the methods used to predict those impacts. My new report, Comprehensive Transportation Emission Reduction Planning: Guidelines for Evaluating Transportation Emission Reduction Strategies, explores these issues. It identifies ten factors that should be considered when evaluating and prioritizing potential transportation emission reduction strategies, as summarized in the table below.

Factors to Consider in Emission Reduction Evaluation

These factors should be considered when evaluating transportation emission reduction strategies. Failing to account for them tends to bias emission reduction planning.

As part of this study I reviewed more than two dozen emission reduction plans. I found that most plans are biased in ways that favor clean vehicle subsidies over vehicle travel reduction strategies. For example, few account for embodied emissions, rebound effects, TDM and Smart Growth co-benefits, or the very long time period required for clean vehicles to penetrate the fleet. Many plans assume that clean vehicle strategies are more effective, cost effective, fast, reliable, and popular than vehicle travel reductions. My research challenges those assumptions.

This is not to suggest that clean vehicle strategies are bad and should be rejected. However, this research indicates that their emission reductions are often smaller than predicted. Considering embodied energy, clean vehicles typically reduce per-mile emission rates by half to three quarters, and their annual emission reductions are even smaller due to induced vehicle travel which increases their emissions and external costs. In fact, a large electric SUV or light truck has total emissions comparable to a small gasoline car. As a result, it is an exaggeration to call electric or hydrogen vehicles “zero emissions;” it is more accurate to call them “lower-emission” and “elsewhere-emission” vehicles.

Lifecycle Emissions

Clean vehicles have relatively high unit costs per tonne of emissions reduced, and require significant subsidies. On the other hand, vehicle travel reduction strategies often provide additional, but often overlooked benefits and respond to latent consumer demands for non-auto travel, and tend to be very cost effective.

In 2021, hybrid and electric vehicles represented about 3% of new vehicle sales, and these are mainly cars due to slower development of electric SUVs, light truck and vans. Optimistically, half of new vehicles could be electric by 2030 but realistically it will take longer, and the remaining fossil fuel vehicles will skew to low fuel economy. With current policies, the fleet is unlikely to be fully electric by 2050, as illustrated below.

Many TDM strategies can be implemented quickly, reducing emissions far faster than clean vehicle implementation. An integrated set of TDM and Smart Growth strategies can reduce transportation emissions by 20-50 percent in ways that are cost-effective and beneficial overall, considering all impacts.

The figure below indicates predicted emission reductions that can be achieved by clean vehicles and vehicle travel reductions.

Comparing Emission Reductions

This analysis suggests that to be efficient and equitable transportation emission reduction plans should rely at least as much on vehicle travel reductions as on clean vehicle strategies, with particular emphasis on “quick win” strategies that can be implemented in a few years.

There is a positive message here. With better analysis we can identify true win-win emission reduction strategies that also help achieve other economic, social and environmental goals.