Your CO2 Emissions Per Mile May Vary

Back at ya, Sightline.  There are obvious conclusions to be drawn from the bar chart above, but when the interplay of land use patterns is also considered, the case for transit over cars becomes even stronger.

Since transit typically serves areas with higher levels of density and land use mix, transit trips tend to be shorter than car trips.  One credible commenter noted that in the U.S. the average commute distance by car is 15 miles, while by transit it is only two miles (if anyone has references to related data please comment). Thus compact, mixed-use development enables the double whammy of travel modes that emit less greenhouse gases (GHGs) per passenger-mile, coupled with fewer miles traveled.

And compact development is also essentially a prerequisite for what are by far the two most environmentally benign transportation modes — walking and biking.

As shown in the chart above, rail transit and a bus 3/4 full emit about half the GHGs per passenger-mile as does a Toyota Prius.  Some take this to mean that if we can double the gas mileage of a Prius — which shouldn’t be that hard — then why bother pushing for transit and compact development as a strategy for reducing GHG emissions?

The first and most obvious flaw in the above line of thought is that if we can double the efficiency of a car, then surely we can also double the efficiency of a bus or train.

The second flaw is that we know what happens to per-capita vehicle miles traveled (VMTs) when we build sprawl — they go up.  As noted in the book Growing Cooler, since 1980 VMTs in the U.S. increased at three times the rate of population growth.  Assuming business as usual, i.e. sprawling development, VMTs are projected to rise by 48 percent by 2030.  In this scenario, the average fuel efficiency of our fleet will have to be boosted by one half just to keep GHG emissions from rising above current levels.

The third flaw is that building sprawl not only increases VMTs, it locks in land use patterns that eliminate the possibilities both for transportation modes that are more efficient than cars, and for shorter trip distances.  When roads and parcels are laid out, they persist for hundreds of years.  While there is a severe urgency to address climate change quickly, the problem is not going to disappear in a hundred years, and all indications are that we will need to apply every strategy we can over the long term.

Compared to sprawl, compact development supports a much more balanced range of transportation mode options.  It does not render cars unusable — even in the densest U.S. cities large fractions of the population drive cars.  Meanwhile, transit and density are mutually catalytic and mutually supportive.  And in the U.S, walking and biking as practical transportation are almost nonexistent everywhere except cities with compact, mixed-use land use patterns.

Lastly, compact development and transit have significant benefits beyond the reduction of GHG emissions.  There is no question that improving the efficiency of cars is a strategy we should be aggressively pursuing, and that could yield relatively significant short term results.  But to focus solely on cars while neglecting land-use patterns and alternative transportation modes not only disregards the long-term perspective on climate change, but also ignores the myriad other negative impacts of a sprawling, car-dependent world.

To take but one local example, sprawling development is one the chief causes of the decimation of the Pacific Northwest salmon runs.  But even though that loss doesn’t appear on any accountant’s balance sheet for building sprawl, it’s a significant cost we all pay, whether or not we enjoy the benefits of spacious yards and convenient travel by car.  How many thousand salmon is one McMansion worth?