Why are the Illinois Tollway congestion levels and travel times sometimes less accurate?
The Gateway system gets data from a variety of sources – Illinois DOT, Illinois Tollway, Indiana DOT, etc. These agencies use a variety of mechanisms to provide their raw data – pavement loop detectors, transponder toll tag timestamps, etc. In the case of toll tag transponders, the data is by nature coarser and potentially less timely than direct roadside-based systems. In particular, these data can suffer from latency issues because of delays in receiving the same. For example, if motorists are delayed by heavy congestion, the system is initially unaware of this problem until the motorist passes through the congestion and reaches a toll plaza (or exit) where the toll tag is read. That situation inevitably results in extra delay before the end point timestamp is read. Additionally, the system is unable to determine the precise location of congestion since the data only indicate a delay between two points sometimes spaced far apart (for example 2 toll plazas). This results in long map s
