  Transportation contributes a substantial fraction of all greenhouse gas
emissions. One approach for reducing such emissions is to modify vehicles'
route choices to minimize their fuel consumption or emission, which is known as
eco-routing. Most eco-routing is based on vehicles choosing routes that
minimize their individual fuel consumption or emissions. The Braess paradox
demonstrates that when vehicles choose routes to minimize their individual
goals, the aggregate effect may paradoxically result in the opposite net effect
due to changes in congestion patterns. We construct a multiclass user
equilibrium model in which some vehicles use eco-routing and others seek to
minimize their individual travel times. Using this model, we show that the
Braess paradox exists for eco-routing. If a large number of vehicles are trying
to minimize their fuel consumption or emissions, the total fuel consumption or
emissions may increase. We then solve the multiclass user equilibrium on
publicly available city network data, and find that eco-routing results in
increases in fuel consumption and emissions on some city networks as well.
