Several recent strategic studies have used linear relationships between system speed (veh-km/veh-h) and veh-km to represent the demand-supply interaction. These "area speed-flow relationships" are based on empirical work in the 1970s, but there has been no detailed investigation of the way in which such relationships are generated, or how they are affected by the nature of the network or the pattern of demand. This paper discusses the background to the problem, and presents results of an EPSRC funded study which is investigating the interaction between system speed and vehicle kilometres in different types of network. Four relationships are being obtained, between (i) speed (veh-km/veh-h) and actual flow (veh-km/veh-h) and actual flow (veh-km/h), (ii) speed (veh-km/veh-h) and demanded flow (veh-km/h), (iii) time/km (veh-h/veh-km). The data is collected directly from the micro-simulation package NEMIS for an origin-destination matrix which is factored to represent different levels of demand. The modelling and data collection methodology is described and results for various networks presented. The implications of these results are discussed in terms of definition of network capacity and in implications for strategic modelling and evaluation. (A)
Abstract