Travel Time Reliability Reference Manual/Surface Plots of travel time and VMT
TTRMS tool is able to provide surface plots of travel time and VMT as well. Those plots can give user a visual view of the reliability of travel by time of day and day of the year. Instead of displaying travel time on the plots, TTRMS analysis tool uses travel time index (TTI). The travel time index is the ratio of the observed travel time divided by the speed limit (free-flow) travel time. This allows for an easier comparison between each of the corridors regardless of length or free-flow speed. The following thresholds were used for the travel time surface plots:
• Speed limit travel time
• The 45 miles per hour (mph) travel time. This threshold was chosen because MnDOT defines congestion as 45 mph or less.
• 1.5-2.0 times the TTI
• 2.0-2.5 times the TTI
• 2.5-3.0 times the TTI
• 3.0-3.5 times the TTI
• 3.5-4.0 times the TTI
• Greater than 4.0 times the TTI
Figure 1 is a example of TTI surface plot along I-94 Westbound from I-494 to TH 101 in 2012. The colors in the legends represent different TTI levels. The plots can automatically updated by different free-flow travel time input.
Figure 1: 2012 WB I-94 Travel Time Surface Plot
Figure 2 is a example of vehicle miles traveled (VMT) surface plot along I-94 Westbound from I-494 to TH 101 in 2012. In this plot, each five-minute observation is displayed as a unique value. Seven VMT bins with increments of 750 vehicle-miles were selected for the VMT surface. In addition, the plot and bins can be updated by entering different "base VMT" to the TTRMS analysis tool.
Figure 2: 2012 WB I-94 VMT Surface Plot