Travel Time Reliability Reference Manual/INRIX Data Types
INRIX Speed Data Types
INRIX speed data includes a code with each speed observation. This code represents the type of speed: reference, historical or real-time.
Reference speed: based on the typical free flow speed for the segment - related to the speed limit
Historical speed: based on the historical speed data for that particular segment during the same time of day
Real-time speed: based on real-time speed measurements from probe, gps or other real-time measurements
The reference speed is constant across all times and varies only by TMC segment. The historical speed is time dependent and may capture recurring congestion along a segment. INRIX uses reference and historical speeds to fill in gaps in the real-time data. INRIX does not reveal the threshold necessary to provide real-time data, nor does INRIX reveal when reference data is used instead of historical data for filling gaps in real-time data. It appears, however, that reference data is used during the night and other off peak times in order to reduce processing when speeds are likely constant, whereas, historical data is likely used during hours when speeds may vary but no real-time data is available.
Data Type by Year
Below are graphs representing the distribution of data types for freeways and arterials in Wisconsin by year. The data does not include all INRIX freeway and arterial coverage in Wisconsin, but a subset. The graphs reveal an increase in the availability of real-time data with time. It is expected that this trend has continued with 2013 and 2014 data.
Data Type by Hour
The graph below shows the distribution of real-time data along a 4-lane inter-regional freeway in Wisconsin by hour of the day. Real-time data is most prevalent during daytime hours when more vehicles are present on the roadway.