Guensler, Randall L. and Daniel Sperling (1993) Congestion Pricing and Motor Vehicle Emissions: An Initial Review. Institute of Transportation Studies, University of California, Davis, Presentation Series UCD-ITS-RP-93-09
An exploratory analysis of four congestion pricing scenarios is conducted. Using projected changes in vehicle operating conditions from the literature percent changes in emission rates are estimated for the four scenarios. A range of emission impact estimates are provided based upon confidence intervals associated with the existing modeled relationships between average vehicle speed and emissions. The high degree of uncertainty in emission estimates is evidenced by the large range of empirical results. The actual uncertainty in emission impact estimates is even greater than indicated by the ranges provided in this paper due to the innumerable sources of emission calculation uncertainty that are discussed in recent emission inventory uncertainty literature.
The results of the impact assessment indicate that our ability to estimate the emission impacts of congestion pricing are questionable at best. However, based upon our knowledge of the cause effect relationships at work we can make a few qualitative conclusions with some degree of confidence: 1) where congestion pricing causes automobile vehicle trips and vehicle miles of travel to decline, emission benefits for all pollutants will accrue to the extent that they are not offset by increased emissions from alternative modes; 2) where congestion pricing reduces congestion and smoothes traffic flow emission rates per mile of travel will likely decline for carbon monoxide and hydrocarbons, but will likely increase for oxides of nitrogen; 3) if congestion pricing yields increased vehicle activity at high speeds in excess of 55 mph emission increases are likely for all pollutants; and 4) where congestion pricing increases congestion and lowers average operating speeds on local roads, emission rates per mile of travel will likely increase for all pollutants.
In practice the net effects of congestion pricing on traffic flow are likely to be limited to the areas surrounding those freeway links that are currently severely congested or expected to be congested in the future (i.e. mitigating future congestion increases). Hence, emission impacts are localized and limited to the VMT occurring during peak periods at these locations.