Communication Timeout and Latency Effect on Positive Train Control System for the IDOT Corridor, Research Results
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Communication Timeout and Latency Effect on Positive Train Control System for the IDOT Corridor, Research Results

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      The Federal Railroad Administration (FRA) sponsored an independent analysis to evaluate the influence of the communication timeout threshold and latency of the North American Joint Positive Train Control (NAJPTC) system. The analysis focused on the overall safety performance as compared with a cab signal system with continuous Automatic Train Stop (ATS) and a four-aspect cab signal system with speed control, or an Automatic Train Control (ATC) system, configured as currently used in Amtraks Northeast Corridor (NEC). ATS and ATC are known to provide satisfactory levels of safety at speeds up to 110 mph. This study builds upon research described in RR08-01, published in June 2008. The analysis considered the effects of timeout and latency on safety performance with average daily traffic comprised of six passenger trains, between 0.86 and 1.07 Positive Train Control (PTC)-equipped freight trains, and between 0.36 and 2.30 unequipped freight trains, depending on the time of the year and location on the Illinois Department of Transportation (IDOT) Corridor, between N. Ridgley and Mazonia, IL. Maximum speeds considered were 110 mph for passenger trains and 60 mph for freight traffic. PTC latency values (See Background) were allowed to vary from 5 to 20 seconds and communication timeout values extended from 20 to 360 seconds. Conclusions from this risk assessment are for the traffic volume and traffic mix. PTC latency and timeout values considered on this particular corridor, did not have a material effect on safety. Instead non-safety considerations such as route capacity, delay reduction and cost may be the governing factors in specifying timeout and latency. This is contrary to pre-analysis expectations where safety considerations were the primary factors in specifying the maximum acceptable timeout and latency for a PTC system. The analysis also showed that the NAJPTC system, as analyzed, passed the test of being as safe as, or safer than, either the cab signal system with ATS or the NEC ATC system.
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