Quantitative modelling in cognitive ergonomics: predicting signals passed at danger

Distances and speeds are given both in Imperial and Metric units since there was no uniformity in the sources on which the paper is based. For the sources of data, see the Acknowledgement section of this paper. The paper analyses the 1999 Ladbroke Grove accident in detail, but the emphasis is less on the state of the railway infrastructure, at that time than on the methodology used. Considerable changes have been made to the rail infrastructure since 1999.

Abstract

This paper shows how to combine field observations, experimental data and mathematical modelling to produce quantitative explanations and predictions of complex events in human–machine interaction. As an example, we consider a major railway accident. In 1999, a commuter train passed a red signal near Ladbroke Grove, UK, into the path of an express. We use the Public Inquiry Report, ‘black box’ data, and accident and engineering reports to construct a case history of the accident. We show how to combine field data with mathematical modelling to estimate the probability that the driver observed and identified the state of the signals, and checked their status. Our methodology can explain the SPAD (‘Signal Passed At Danger’), generate recommendations about signal design and placement and provide quantitative guidance for the design of safer railway systems’ speed limits and the location of signals.

Practitioner Summary: Detailed ergonomic analysis of railway signals and rail infrastructure reveals problems of signal identification at this location. A record of driver eye movements measures attention, from which a quantitative model for out signal placement and permitted speeds can be derived. The paper is an example of how to combine field data, basic research and mathematical modelling to solve ergonomic design problems.

Keywords:

• Railway ergonomics
• cognitive ergonomics
• accidents
• attention dynamics
• mental models
• eye movements
• mathematical models

Acknowledgements

This paper is based on an internal report originally prepared for the Health and Safety Executive of the UK. We thank Mr. S. Jones for permission to use that source and for the opportunity to ride in the cab of a train over the route on which the accident occurred in order to view the signal infrastructure and to obtain photographs for analysis. Except for the data on driver eye movements, the technical data are derived from the final report of the Public Enquiry (The Cullen Report) and evidence cited therein. The recording of in-service train drivers’ eye movements was a challenging task, and relied on the goodwill and cooperation of drivers, train companies, trade unions and Railway Safety, who sponsored the empirical research. We thank the Railway Unions and their members for their cooperation, especially those drivers who participated in the eye movement research. Natasha Merat and David Field collected and coded the data. Finally, our then colleague and co-investigator on the Railway Safety project, Dr. Mark Bradshaw, died soon after that project was completed, and we dedicate this paper to him.

Notes

Distances and speeds are given both in Imperial and Metric units since there was no uniformity in the sources on which the paper is based. For the sources of data, see the Acknowledgement section of this paper. The paper analyses the 1999 Ladbroke Grove accident in detail, but the emphasis is less on the state of the railway infrastructure, at that time than on the methodology used. Considerable changes have been made to the rail infrastructure since 1999.

1. Our italics.

2. Since the Public Inquiry, large boards carrying track numbers have been placed beside each set of lamps. Other changes are planned in the modernisation of the railway. This paper is primarily concerned with the state of the system at the time the Ladbroke Grove accident occurred.