Human Reliability Analysis:Accounting for Human Error on Light Rail

Transportation Systems

SYSM 6309 Advanced Requirements Engineering

By: Michael Murphy

Motivation

Case Study

Human Reliability Analysis (HRA) Tools

Technique for Human Error Rate Prediction (THERP)

Error Rates

Event Tree

Swiss Cheese Model

Possible Light Rail System Solutions

Questions

Agenda

Federal Transit Administration’s (FTA) Office of Safety and Security compiled a report of light rail safety statistics from 2003 to 2008 that grouped light rail accident types with light rail accident causes.

5 Types: Rail Grade Crossing (RGX) Collisions, Non-RGX Collisions, Derailments, Fire and Other

4 Causes: Equipment Failure, Workforce Behavior, Customer Behavior and Public Behavior

Within that time span, there were 3,665 reported accidents.

Of the 3,665 reported accidents, 583 (16%) were caused by Workforce Behavior.

Motivation

Date: July 18, 2009

Time: 2:50 pm, Pacific daylight time

Property Damage: $4.5 million

Injuries: 48 (28 serious)

Accident: L train 1433 struck the rear end of standing K train 1407

K train was at the end (aka platform) of the West Portal Tunnel, waiting to merge onto the K line.

L train, controlled by automatic train control system (ATCS), pulled into tunnel.

L train operator “cut out” of ATCS to manually pull ahead to the platform.

L train was moving about 5 mph when the operator “blacked out” until impact.

Case Study: San Francisco Railroad Accident

Accident Simulation

KL

TunnelPlatform

Inbound

Outbound

K L

KA-BLAM!!!

K Train arrives in tunnel, no cars at platform.K Train pulls ahead to platform.L Train arrives in tunnel, K Train still at platform.L Train changes to manual and pulls ahead to platform.

Human ErrorL Train Operator failed to request permission of OCC

management to disengage ATCS.L Train Operator failed to keep the train in automatic

mode until reaching the station stop at the West Portal platform.

Would have been a safe guard against his “black out”

OCC failure to monitor and enforce requirement that the operator wait until reaching the platform before changing operating mode of the train.

Cause of Accident

Bulletin issued reminding the train operators not to “cut out” of the ATCS without OCC permission.

The final sentence of the bulletin read, “Failure to comply with these existing rules outlined in the Rules and Instruction Handbook will result in discipline, up to and including termination.”

Corrective Action

Human Reliability Analysis (HRA)

Role is to provide the foundation for calculating the probability that the human element of an operator/process or system interaction will fail.

Majority of HRA methods were developed in the 1980’s due to concern caused by the accident at Three Mile Island.

Technique for Human Error Rate Prediction

(THERP) Best known human performance reliability prediction technique

which involves 5 steps:

Define the system or process.

Identify and list all human operations performed and their relationships to the system or process tasks and functions.

Predict error rates for each human operation or group of operations.

Determine the effect of human errors on the system or process -> Event Tree.

Develop and recommend changes that will reduce the system or process failure rate.

Probability that a class of errors will lead to system failure is:

Qi = 1 – (1 – FiPi)^nj

Error Rate Tables http://panko.shidler.hawaii.edu/HumanErr/

Error Rates

Event Tree

Swiss Cheese Model

Utilize THERP when building new or expanding current systems.

Additional training of established operating procedures.Threat of disciplinary action when established operating

procedures are not followed.The German Aerospace Center has developed a Railway

Collision Avoidance System (RCAS)Safety overlay that can be deployed on top of any existing

safety infrastructure in train networks.Provides train position and vehicle dimensions to all other

trains in the area using ad-hoc train-to-train communications.

Possible Light Rail System Improvements

Questions?

“Collision of Two Municipal Transportation Agency Light Rail Vehicles, 7/18/2009, San Francisco, CA,” http://www.ntsb.gov/investigations/reports.html

“Human Reliability,” http://en.wikipedia.org/wiki/Human_reliability

E. Hollnagel “Human Reliability Analysis,” 2002, http://www.ida.liu.se/~eriho/WhatIsHRA_M.htm

“Technique for Human Error Rate Prediction.” http://en.wikipedia.org/wiki/Technique_for_Human_Error_Rate_Prediction

Barbara Gregory, MPH, Victoria S. Kaprielian, MD “Anatomy of Error – Swiss Cheese Model,” http://patientsafetyed.duhs.duke.edu/module_e/swiss_cheese.html

References