Safety Analysis of Communication Timeout and Latency in a Positive Train Control System [Research Results]
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Safety Analysis of Communication Timeout and Latency in a Positive Train Control System [Research Results]

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      The goal of this Federal Railroad Administration (FRA)-sponsored study was to provide an independent safety analysis of a Positive Train Control (PTC) system as proposed by North American Joint Positive Train Control (NAJPTC) with regard to the effect of PTC's communication timeout threshold and latency on safety and performance at high speeds, as compared to ATS known levels of safety at speeds of up to 110 miles per hour (mph). In January 1998, FRA, in conjunction with the Association of American Railroads (AAR) and IDOT, began to develop a high-speed PTC project for the Union Pacific Railroad (UP) between St. Louis, MO, and Chicago, IL, which is referred to as the IDOT Corridor (Figure 1). Development of this PTC system was terminated then revived at the Transportation Test Center, Inc. (TTC) in Pueblo, CO. However, analysis in this project using IDOT corridor traffic continues to provide valuable insight into the question of timeout and latency on the safety performance of PTC and other train control systems using wireless communication. The analysis considered the effects of timeout and latency on a traffic mix of 6 passenger trains per day and one freight train almost every day, unequipped freight trains that varied in frequency from one train every three days to more than 2 trains per day depending on the season, and an IDOT sub-corridor under consideration. Train speeds varied between 35 and 110 mph depending on the train type. PTC latency values were allowed to vary from 5 and 20 seconds and communications timeout values extended from 120 and 360 seconds. Initial conclusions from this risk assessment are that for this particular corridor, traffic volume, traffic mix, and PTC latency and timeout values, there was no material effect on safety. Contrary to pre-analysis expectations that safety considerations would be the primary factor in specifying maximum acceptable timeout and latency for a PTC system, non-safety considerations such as route capacity, delay reduction and cost may actually be the governing factors in specifying timeout and latency. The final analysis showed that the PTC system as tested, was as safe as, or safer than the UP cab signal/ATS system.
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