While the intentions of refashioning urban areas into complete streets are noble, the immediate effect is an increase in pollution stemming from the new construction. While one could reasonably argue that the long term gains from the reconstruction outweigh the pollution, a) it is still important to recognize the damage and b) becoming cognizant of the negative effects lets you plan ahead and build a complete street upon a greener foundation.
A complete street might require: new sidewalks, repaved lanes, and special lanes for busses. All of which requires aggregate rock, cement or asphalt.
For something like a major road, the carbon footprint adds up very quickly. In total, a single 1 lane-mile of freeway pavement can consume up to 12,000 tons of raw materials and emit enough pollution in terms of Global Warming Potential to equal 1200 tons of CO2.[i]
Starting from the bottom, beneath of all of the pavement, there typically lays a layer of aggregate rock – the most mined material on earth. Though recycled aggregates are available, most aggregates are fresh. Each ton of that rock must be hauled in diesel truck and then spread over the sight using special equipment, burning hundreds of gallons of diesel fuel, releasing tons of emissions and putting excess strain on construction budgets. Per cubic meter (which weighs about 2.2 tons), this is will be the most environmentally friendly layer but will still contain 15.8kg of CO2.[ii] That doesn’t sound like a lot until you realize that given a subbase 8 meters wide (2 lanes), and .2 meters deep, per mile this adds up to 89,695 pounds of CO2. That’s 20 metric tons of global warming causing pollution per lane per mile just from the road’s foundational layer.
Next, there’s the paving, the really dirt part.
Paving with either asphalt concrete or cement concrete requires a number of diesel-burning construction machinery such as concrete mixer trucks, asphalt pavers, and vibrating rollers. Per ton of material, concrete requires .5 gallons of diesel, hot mix asphalt requires 2.9. [iii]
According to the Athena study, at installation asphalt containing 20% reclaimed asphalt pavement (RAP) will posses about 118 pounds of CO2 per ton (1 cubic meter weighs 2420 kg and posses 129.74 kg of embodied CO2).
The other option, concrete paving, is much more energy-intensive during the construction process. The industry as a whole employs .1% of the US labor force but generates 1.5% – 2.0% of all US CO2 emissions.[iv],[v] One report found that that to produce and lay down enough cement for a 1km (.62 miles), 4 lane highway required the equivalent energy of 1049 tons of oil.[vi] And emissions, when measured in CO2 equivalence (Different green house gasses have different strengths i.e. one kilogram of nitrous oxide, N2O, has as much global warming power as 310kg of CO2) range from 308lbs per ton of cement concrete to 440kg per ton of continues reinforced concrete.[vii]
You can see how this very quickly can add up, especially considering at the US alone builds at least 15,000 lane miles per year.[viii] Finding alternative methodologies is critical if we are to build new roads or to rebuild improperly utilized urban areas in a manner that is congruent with sustainable living.
[i] http://pavementinteractive.org/images/6/6c/Greenroads_summary.pdf
[ii] http://www.mrmca.com/paving/athena.pdf
[iii] http://www.acpa.org/Downloads/QDs/QD023P%20-%20Conserving%20Fuel%20in%20the%20Road.pdf
[iv] http://www.cement.org/econ/industry.asp
[v] http://www.concretethinker.com/technicalbrief/Concrete-Cement-CO2.aspx
[vi] http://www.lcarc.re.kr/Korean/staff%20list/papers/ASCE_2003.pdf
[vii] http://www.pavementpreservation.org/publications/getfile.php?journal_id=1283
[viii] http://www.bts.gov/publications/national_transportation_statistics/html/table_01_01.html
