One-way streets enable less vehicle idling at traffic lights; however, they are not an effective strategy to reduce air pollution.
A network of one-way streets requires vehicles to travel circuitously to reach destinations and enables more pollution-generating traffic on a street at a given time. Drivers enjoy the higher speeds enabled by one-way streets at the cost of increased mortality for non-drivers in crashes and reduced comfort for less-polluting alternatives such as walking or bicycling.
Unfortunately, the devil, as always, is in the details.
Michael Baker, Salt Lake City