Field Location and Data Collection
To obtain meaningful speed-flow-density measurements, the data collection must be taken at the right location during the appropriate time periods. The location and time period of field measurements significantly affect the resulting speed-flow-density measurements. May (May 1990) gives an illustrative example on this matter:
Consider the directional roadway shown at the top of Figure 7 (below) with traffic moving from left to right. There are no entrances or exits along the roadway, and in the middle portion of the study section the capacity has been reduced due to a lane drop. To describe the example clearly, assume three-lane sections at the upstream and downstream portions and a two-lane section in the middle portion. Measurement stations are established at locations A, B, C, and D. Speeds, flows, and densities are measured independently at each station. Station A is at the upstream end of the study section, away from any influence due to the lane drop location. Station B is in the three-lane section, but just a short distance upstream of the lane drop location. Station C is in the two-lane section, while station D is in the downstream three-lane section. Theoretical speed-flow, flow-density, and speed-density curves are also shown on Figure 7 directly below the four measurement stations.
Figure 7 (May, 1990)