In this paper, recent Canadian work challenging long-held theories about speed-flow-concentration relationships in freeway traffic is verified and extended using data from San Diego. Major conclusions of these studies regarding near-constant free-flow speeds and flow-concentration relationships resembling an inverted V are confirmed; other past findings related to the effect of queuing on downstream free-flow speeds and the effect of secondary bottlenecks on flow-concentration relationships are not confirmed. When data are averaged across all lanes (both upstream and downstream of a bottleneck), speed-flow and flow-concentration relationships are found to be consistent with those predicted by queuing and shock wave theory. The functioning of the bottleneck studied is more complicated than had been assumed, however, and this creates further problems in the interpretation of the data.Finally, the inverted-V model of the flow-concentration relationship is shown to imply a simple and plausible model of driver behaviour in which speeds and spacings are adjusted to keep the average front-to-back time gaps approximately constant until some desired maximum speed is reached.
Samenvatting