November 14, 2024

Some harsh facts about carbon mitigation in transportation

Allow me to be the turd in the punch bowl. There are some common misconceptions about potential “solutions” to GHG emissions in the transportation sector. In this post I’ll share some facts that show why these are insufficient to meet the challenge of deep decarbonization. Credit to Daniel Malarkey for helping with some of the analysis in this post.

Fact #1: Bus transit is more carbon intensive than driving in the U.S.

The math on this is pretty simple. Using data from the National Transit Database, we added up each transit agency’s consumption of various fuels for motor bus, commuter bus, and bus rapid transit services. We then multiplied each agency’s consumption of each fuel by the following GHG intensity values:

Fuel	GHG Intensity (kg CO2-equiv/gal)
Diesel	10.19
Bio-diesel	10.19
Compressed Natural Gas	6.56
Gasoline	8.49
Liquefied Petroleum Gases	5.75
Electricity	0.39 kg/kWh

Adding these up gives us the GHG emissions from each agency’s bus services. Finally, we divided these total emissions by the total passenger miles of service each agency’s buses provided, to determine an agency-level average GHG emissions per passenger mile. We also divided the total emissions from all agencies’ buses by total PMT across all agencies, to get a national average emissions intensity for bus transit.

The result? A national agency average of 610 g CO2-equivalent per passenger mile!

The histogram below shows the distribution of agency-level GHG intensity values:

In addition, we found that:

• The median bus system in the US averages more than 489 g / passenger mile.
• 67% of agencies emit more than 400 g / passenger mile – higher than driving alone in a 22 mpg Ford Explorer. Together, these agencies account for 23% of bus passenger miles.
• 90% of agencies emit more than 267 g / passenger mile – higher than a 22 mpg Ford Explorer with 1.5 occupants (the average automobile occupancy in the US). Together, these agencies account for 62% of bus passenger miles.
• 99% of agencies emit more than 103 g / passenger mile – higher than a 57 mpg Toyota Prius with 1.5 occupants (the average automobile occupancy in the US). Together, these agencies account for 98% of bus passenger miles.

Transit boosters talk about transit as a climate solution. A diesel bus at 100% of seated capacity emits something like 40 g / passenger mile. A hybrid bus at 100% of seated capacity is even lower – around 30 g / passenger mile. But bus transit as it actually exists in the US emits more CO2 than driving. If we want transit to be a climate solution, we need to make it a climate solution. That means providing bus service that people want to use, in places with enough people to fill up the buses. Until we do that, bus transit won’t be a climate solution, it’ll continue to be a climate delusion.

Fact #2: Short trips are a large share of trips, but a small share of emissions

Proponents of active transportation (i.e. biking and walking) and micromobility love to point out that we make a LOT of short trips. And they’re right! According to the 2017 National Household Travel Survey, more than 1/3 of trips in the US were 2 miles or less, and 60% of trips were 5 miles or less:

But here’s what that perspective misses: those short trips account for a very small share of total miles, precisely because they are short! In fact, only 3% of all miles are accrued on those trips under 2 miles, and 11% of miles are on trips under 5 miles.

Even if you switched every single trip under 5 miles in the US from car to bike, you would be talking about 11% of miles, and therefore about 11% of emissions. There are some great reasons to support active transportation and micromobility – such as supporting a healthy public and improving safety for these road users – but climate mitigation isn’t one of them.

Fact #3: Changes in land use are too slow to meet climate mitigation goals

Some advocates claim we need to change our land use patterns to support less carbon-intensive transportation. This would have been a good idea to follow 80 years ago. Today, we are so locked into current development patterns that changing our land use patterns is unlikely to deliver meaningful cuts in carbon emissions in the time required.

Time is running short to make deep cuts in carbon emissions. The Mercator Research Institute maintains a carbon clock counting down the time until we exceed a 2°C stabilization target, given current emission rates and carbon budgets identified by the IPCC:

We have about 20 years remaining with emissions at current rates (50 billion tonnes / year) until exceeding a 2°C target. If we began a linear ramp-down of emissions today, we would have about 40 years until they would need to be zeroed out, to hit the 2°C target.

Back in 2009, the Transportation Research Board issued a report, Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use and CO2 Emissions. They considered a scenario with a very aggressive push toward compact, mixed use development – putting 75% of new housing into such locations. Drawing on prior research, they took 25% as an upper bound on the reduction in driving among residents of such developments. Given the rate of growth and turnover of the housing stock, they projected up to an 11% reduction in emissions over a 40 year timeframe:

	Moderate	Upper Bound
% of new housing that is compact, mixed-use	25%	75%
Driving reduction in compact, mixed-use development	12%	25%
20-year reduction vs BAU	1%	7-8%
40-year reduction vs BAU	1.3-1.7%	8-11%

Housing prices in the US have risen quickly and there is a need for more home construction. There are many good reasons to allow and encourage growth in compact, mixed-use developments. But making a significant and timely dent in GHG emissions isn’t one of them.

People like to talk about us being in a climate crisis. If that’s true, we need to start acting like it, by targeting our mitigation efforts toward activities that can deliver large reductions in emissions over timescales that matter.