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U R B A N A RT E R I A L S P E E D F L O W E Q U AT I O N S F O R T R AV E L D E M A N D M O D E L S 113 SCAG Arterial Speed Study 0.6 0.5 0.4 Travel Time (h) 0.3 0.2 Fitted Akcelik Classical Queue Theory 0.1 Fitted BPR Field Data Points Standard BPR 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 One-Hour Volume/Capacity Ratio FIGURE 3 Speedflow equations versus queuing theory. factor of the segment length, as is done in the BPR and Andrade, and the other staff at the City of Los Angeles exponential equations. Department of Transportation for their assistance in pro- In evaluating the speedflow equations against theo- viding real-time traffic and speed data for city streets. retical delays for hourly demands that exceed hourly Robert Dulla and Thomas Carlson of Sierra Research capacities, only the Akcelik equation produced the prepared the sampling plan for the study and the recom- expected delays due to oversaturated conditions at the mended sampling plan for monitoring of regional travel downstream signal on a street segment. The other equa- speeds on an ongoing basis in the future. Moses Wilson tions significantly underestimated delay within the 1.00 of Wiltech led the floating-car and traffic count data col- to 2.00 v/c range, the BPR curve eventually surpasses the lection in the field. Chris Ferrell and David Reinke of delay estimates produced by queuing theory and the Dowling Associates led the data analysis effort. Akcelik equation. REFERENCES ACKNOWLEDGMENTS Akcelik, R. 1991. Travel Time Functions for Transport Plan- The authors thank Deng-Bang Lee, Mike Ainsworth, ning. Australian Road Research, Vol. 21, No. 3, pp. 4959. Hong Kim, Gouxiong Huang, Huasha Liu, Teresa Wang, Highway Capacity Manual. 2000. TRB, National Research Dale Iwai, and many others at SCAG for their technical Council, Washington, D.C. assistance, advice, and review throughout this project. Special Report 87: Highway Capacity Manual, 2nd ed. 1965. Gouxiong Huang, in particular, advised the authors on Highway Research Board, Washington, D.C. how the link capacities for the model had been deter- Traffic Assignment Manual. 1964. U.S. Bureau of Public mined. The authors also thank Verej Janoyan, Ray Roads. Washington, D.C.