Travel Time Reliability Reference Manual/Availability of Real-Time Data
NPMRDS Real-Time Data Availability
[edit | edit source]The charts below outline the availability of NPMRDS real-time data on two freeway corridors in January 2012.
The following two graphs show the availability of real-time data by hour of day in January 2012 for the 6-lane urban freeway and the 4-lane inter-regional freeway. The availability of real-time data is higher during daytime hours when vehicle volumes are higher and lower overnight. Overall, the 4-lane inter-regional corridor reported more real-time measurements. Although the urban freeway experiences higher vehicle volumes, the inter-regional freeway likely experiences higher commercial truck volumes, which are often a source of probe data. Another possible factor is the shorter TMC lengths along the urban freeway. The inter-regional freeway is 20 miles long with 4 TMC segments, while the urban freeway is 4 miles long with 6 TMC segments. It's unknown whether there is a correlation between TMC length and availability of real-time data.
The two charts below show the frequency at which varying amounts of real-time data were available along the corridors during January 2012. For example, 100% availability corresponds to all TMCs reporting (4/4 or 6/6 for these two corridors) real-time data during a given time interval, 50% availability to half of TMC segments (2/4 or 3/6) reporting real-time data etc. The frequency at which eac of these occur is plotted along the y-axis. The inter-regional freeway consists of 4 TMCs making up 20.18 miles, the urban freeway 6 TMCs totaling 4.35 miles. The greater number of TMC segments along the urban corridor is one factor leading to the smaller frequencies of availability. However, with fewer real-time data points (as shown in the graphs above), it is expected that the frequencies will also be lower on the urban corridor than the inter-regional.
6 TMC segments along 4.35 miles of corridor
4 TMC segments along 20.18 miles of corridor