The Spectral Analysis of Speed Fluctuations on a Freeway

Two completely independent sets of observations of speed histories collected in the center lane of a freeway facility are analyzed using spectral analysis techniques. These observations were obtained from two entirely different vantage points, one from within an instrumented vehicle moving with the traffic stream, and the other from fixed roadway sites. This information has been used to calculate the autocorrelation, coss-correlation, and spectral density functions. These calculations demonstrate that a strong correlation, exists between the speed histories at locations separated by distances up to about 0.8 mi. Furthermore, it is shown that for both data sets the dominant frequencies are those with periods in the range of 3 to 6 min. The calculations are also used to obtain an estimate of the speed of propagation of fluctuations in a traffic stream with respect to the roadway. This speed of propagation is estimated to be 12 mph. However, because of the short duration of the samples used, such results therefore are only indicative.