If it’s the first week of May, that can only mean one thing! No, not May Day. No, not Star Wars Day. No, not Cinco de Mayo. No, not Mother’s Day. Look, clearly you’re not going to get this on your own.
That’s right – it’s Air Quality Awareness Week. The U.S. EPA has designated this year’s theme as “Show How You Care About The Air.” EPA and various other government entities that work on air quality, including NOACA, are encouraging people to take a few simple steps throughout the course of the week that can have a positive, tangible impact on air quality.
One of these actions is changing your commute mode. The overwhelming majority of Americans (76.4% in 2013, to be exact) drive alone to work. Here in Northeast Ohio, that number is significantly higher, with values ranging from 79.9% in Cuyahoga County to 87.9% in Lake County. If you total the five counties in the NOACA region, 772,262 of the 938,244 workers over the age of 16 – 82.3% – drive alone to work. Given that transportation accounts for a significant portion of key pollutants in the region – 50% of nitric oxides (NOx) and 15% of fine particulate matter (PM2.5) – reducing the share of single-occupancy vehicles (SOV) on the region’s roads has the potential to improve air quality.
The question becomes by how much. While active transportation undoubtedly holds the potential to cut mobile emissions, some research suggests its immediate impact is somewhat limited. As I’ve shown, increasing overall fuel economy can do more to mitigate climate change than land use planning.
Moreover, research from the Federal Highway Administration (FHWA) shows that bike and pedestrian are not the most cost-effective way to cut emissions. According to the agency’s analysis of projects funded through the Congestion Mitigation and Air Quality Improvement Program (CMAQ), active transportation lowers emissions far less, per dollar spent, than diesel vehicle retrofits, truck stop electrification, or idle reduction projects. This may help to explain why bicycle and pedestrian projects accounted for just 7% of CMAQ funding in FY2013.
Short trips and cold starts
On the aggregate, it’s likely true that, at least in the short-term, retrofitting diesel engines in heavy-duty vehicles or reducing the amount of fuel that truck drivers use overnight may be a more effective way to cut emissions. But personal vehicles account for a much larger share of mobile emissions, and a significant share of these emissions come from short trips.
According to the 2009 National Household Transportation Survey (NHTS), the median distance of a light-duty vehicle trip in the U.S. was just four miles; nearly half of all personal trips (43.4%) were less than 3.2 miles. These short trips account for an outsized share of vehicle emissions due the issue of cold starts.
A cold start occurs when both the car engine and its catalytic converter have cooled to within 10℉ of the ambient air temperature. In order for an engine to operate at peak efficiency, it needs to warm to roughly 140℉. Until it reaches this point, the vehicle will fail to fully combust gasoline, ensuring that it releases emissions at a higher rate.
One recent study (PDF) notes that cold engines can emit four times as many hydrocarbons, three times as much carbon monoxide (CO), and twice as much NOx as a warm engine. All told, the authors conclude that excess emissions attributable to cold starts account for 10-30% of total mobile emissions.
The benefits of mode shift on a national scale
Given these facts, it appears that shifting travel mode for short trips could go a long way to improving air quality. Additional research backs up this hypothesis.
In a 2010 article (paywall) in the journal Transportation Research Part D, Audrey de Nazelle and her colleagues examined the benefits of shifting short vehicle trips to active transportation. While their travel data were older (they used the 1995 NHTS), they found that 62.5% of all trips less than 0.5 miles occur in cars. This share that climbs to 87.1% for 0.5 to 1-mile trips, 92.2% for 1- to 2-mile trips, and 94.3% for 2- to 3-mile trips.
The authors examined the effects of shifting 35-70% of short social trips and 15-45% of commutes, respectively, from driving to active transportation. Nationwide, this mode shift would cut daily VOC emissions by 30-70 tons, CO emissions by 400-900 tons, and NOx emissions by 15-35 tons. It would also reduce vehicle miles traveled (VMT) by 0.8-1.8%, cutting greenhouse gas emissions (GHGs) by 20,000-46,000 tons per day. They compared these results to emissions reductions from existing CMAQ projects, finding that promoting widespread mode shift for short trips could lead to emissions reductions that were “orders of magnitude greater.”
How can mode shift improve air quality and public health in Cleveland?
But that study looks at the U.S. as a whole. I often hear people from people that the weather in Northeast Ohio is too harsh, making it impossible to walk or bike for 6-9 months a year. The deck is also stacked heavily towards driving in this region, as our SOV mode share attests. Are national estimates really applicable here? Surely things are different here than in Portland or Austin or San Diego.
Fortunately, a group of researchers from the University of Wisconsin-Madison already considered this issue. In a 2012 study, they analyzed the impact of replacing half of all vehicle trips less than four kilometers (2.4 miles) with biking in the 11 largest metropolitan areas in the Midwest, including Cleveland. And they assumed this mode shift would only occur during cycling season, which they defined as April-October.
The authors estimated that eliminating these short car trips would slash residential vehicle use in these cities by one-fifth. This outcome would reduce the frequency of cold starts from 59.9% to 21.9% in urban Census tracts and from 55.6% to 20.3% in suburban tracts. Across the entire study area, PM2.5 concentrations would fall by 1-2%, while NOx and VOC levels would fall by 5-12% and 10-25, respectively.
Based on their findings,
Eliminating short car trips and replacing 50% of them by bicycle would result in mortality declines of approximately 1,295 deaths per year, including 608 fewer deaths due to improved air quality and 687 fewer deaths due to increased physical activity…We estimate that the combined benefit from improved air quality and physical fitness for the region would exceed $8.7 billion/year, which is equivalent to about 2.5% of the total cost of health care for the five midwestern states in the present study.
Here in Cleveland, PM2.5 values would fall by 0.05 micrograms per cubic meter (µg/m3), preventing 53 premature deaths, 184 asthma attacks, and 1,405 lost workdays per year. The additional physical activity would save another 42 lives per year, increasing the total benefits to $664 million annually.
And these numbers don’t account for the health benefits of increased physical activity. That prevents another 687 premature deaths and provides $3.8 billion in total benefits each year. This mode shift would further reduce GHG emissions by 3.9 billion pounds.
Clearly, the air quality benefits cities can obtain by promoting mode shift for short trips are significant. While mode shift, on its own, cannot bring every city into attainment for air quality standards or halt climate change, it is an important component of a comprehensive approach to both issues. Increasing the mode share of active transportation can produce additional dividends, as it benefits public health, enhances the livability of neighborhoods, improves safety for all road users, and just generally elevates the quality of life in communities around the country.
So show you care about air quality this week and take shorter trips on foot or by bike. Even if the weather isn’t perfect, it will be well worth it.