Suggestions for an improved Davidson travel time function

Abstract

Travel time functions (also known as speed/flow curves or congestion functions) form a central and key element of transport planning and policy analysis. This paper considers the performance of the functions incorporated in two widely used transportation planning packages, UTPS and Microtrips, and concludes that they have structural deficiencies which need to be overcome. The Davidson function, especially in the modified form suggested by Akcelik, is discussed as a superior alternative travel time function. We show that the modification given by Akcelik may have potential deficiencies, in particular that the slope of Akcelik's linear extension behaves poorly when the 'quality of service' parameter m approaches its upper limit of 1, and that the modified function is not time-dependent and thus static in nature. We identify an alternative linear extension derived from Akcelik's later and independent work, which avoids these potential deficiencies. Finally, we discuss the interpretation of the terms saturation flow and capacity in the context of the Davidson function and illustrate the capacity overestimation problem that can arise if all Davidson function parameters are estimated simultaneously.