Using observed trac volumes to improve ®ne-grained regional emissions estimation D.A. Niemeier a, * , K. Lin b , J. Utts c a One Shields Ave., Department of Civil and Environmental Engineering, University of California, Davis, 95616 USA b Transportation Planning Division, Institute of Transportation, Ministry of Transportation & Communications, Taipei, Taiwan c Division of Statistics, University of California, Davis, USA Abstract When translating travel demand model output to photochemical model input, period-based network assignment volumes must be converted to gridded-hourly vehicle emissions. A post-processor, such as the California Direct Travel Impact Model (DTIM2), is frequently used to disaggregate the period-based travel demand assignments to the ®ne grained spatial and temporal resolution required by the photochemical models. A recent theoretical enhancement proposed re®ning the temporal and spatial resolutions of travel demand model predictions using observed count data. This method provides a technique for disaggregating the period-based travel demand model assignments (e.g., AM peak, PM peak) into the hourly summaries required by most photochemical model (Lin and Niemeier, 1997). In this study we present a methodological framework for applying the new theory and discuss the results of a large-scale application empirical comparison between the standard and proposed methods for estimating regional mobile emissions in Sacramento, California. The standard method produced slightly higher estimates of daily emissions (about 1%) when compared to the emissions estimated using observed count data. However, the two approaches produced hourly emissions estimates that diered by as much as 15% in some hours. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Travel demand; Air quality; Multivariate multiple regression 1. Introduction Developing stronger theoretical and empirical links between travel activities and emissions generation is widely recognized as being critical to re®ning mobile source emissions estimates for mesoscale air quality analysis (Transportation Research Board, 1997; Cadle et al., 1998). Recent Transportation Research Part D 4 (1999) 313±332 www.elsevier.com/locate/trd * Corresponding author. Tel.: +916 752 8918; fax: +530 752 8924; e-mail: dniemeier@ucdavis.edu 1361-9209/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII:S1361-9209(99)00011-5