AN ANALYSIS OF THE TEMPORAL AND SPATIAL VARIABILITY OF FREE-SPEED ALONG A FREEWAY SEGMENT

In order to determine the speed-flow relationship for a highway section, a number of parameters must be estimated. These include the free-speed, speed-at-capacity, the capacity, as well as the jam density. Due to fluctuations in demand, variations in driver behaviour, as well as geometric and environmental conditions, these parameter values may vary both spatially for different stations and temporally for different days. If these speed-flow relationships are to be utilized to estimate link travel times, diversion capacities, or if they are to be used in incident detection algorithms, it is important that these spatial and temporal variations in the speed-flow relationship be quantified so that accurate estimates of the relevant traffic parameters can be made. This paper presents a statistical analysis of the variability of free-speed estimates for 24 stations along a section of I-4 in Orlando, Florida during a four month time period. This analysis is a first step in performing similar analyses of capacity, speed-at-capacity, and jam density. In the analysis presented in this paper, it was found that free-speed estimates along I-4 had a standard deviation of 4.7 km/hr and were most dependent on the location at which they were observed