Applying Human Factor Engineering Principals to an ATS Line Overview Display

The Automatic Train Supervision (ATS) system plays a vital role in monitoring the performance of Communication-Based Train Control (CBTC) signaling systems during revenue operations. Despite its significance, Transit Operators often neglect to properly define the ATS user interface (graphical user interface) requirements in their specifications, resulting in a CBTC supplier delivering a cumbersome and tedious user experience that is unacceptable for such a crucial subsystem.

The design of user interfaces for workstations is a well-established field of study known as Human-Computer Interaction (HCI), which falls under the broader discipline of Human Factors Engineering (HFE). The goal of HFE is to eliminate the potential for human error through effective design, which is one of the most effective ways to prevent human error.

Transit Operators must incorporate Human Factors Engineering (HFE) principles when creating an ATS user interface specification. The challenge is to strike a balance between including every colour for each element and taking a high-level, generic approach that captures the look and feel of the interface without specifying too many details.

Human Factors Engineering (HFE) Standards

There are several factors to consider when creating an ATS user interface specification but the first place to start is the Human Factors Engineering standards. The three standards to consider are:

• MIL-STD-1472H: Department of Defense Design Criteria Standard Human Engineering - The MIL standard is the most up to date, last revised in 2020. It’s a heavy standard covering a multitude of topics in just under 500 pages. From a CBTC user interface perspective, section 5.17 (Information Systems), 5.16 (Cyber Security) and 5.3 (Speech and Audio) are relevant to an ATS application from a human factors perspective.
• ANSI/HFES 200 – 2008: Human Factors Engineering for Software User Interfaces - The ANSI/HFES 200 covers many topics related to a software user interface but Part 5 section 7 which describes colour usage on a user interface is an important section. Unfortunately, ANSI/HFES 200 has not been updated since 2008.
• IEEE 1474.2 – 2008: User Interface Requirements for CBTC Systems – The MIL and ANSI standards present the generic human engineering design criteria, principles, and practices but it’s not enough. The missing element is how to apply these human factor criterions in a CBTC application. IEEE 1474.2 covers this aspect of the user interface. It’s a light document providing a basic view of the mandatory information that should be displayed on a CBTC user interface for train borne and non-train borne displays.

MIL, ANSI and IEEE standards describe HFE best practices to consider when creating an ATS user interface specification, but a Transit Operator's specification must delve deeper to capture the CBTC specific aspects.

Line Overview Display

There are several user interface elements displayed on an ATS and I won't go through all of them but hte most critical is the Line Overview Display

The Line Overview is the first ATS image the Operator views to determine the status of the subway (see figure 1 below). At first glance, the line overview will immediately indicate the location of all the trains, status of switches and platforms, any alarms currently active on the line and other details.

Therefore, the line overview should be the primary user interface element to determine status and for executing commands. Otherwise, an under utilized line overview display becomes tedious and cumbersome to use because the Operator must navigate through the menu/ribbon tool bar to determine the status of the system.

Therefore, as a design principle, all critical commands and statuses must be accessible from the Line Overview. The menu/ribbon toolbar should not be used for commands or critical statuses, as operators should not navigate through layers of menus/windows to execute a command or determine a status. Instead, a single mouse click on the line overview display should be the primary mode.

To determine what information should be displayed on the line overview, the first step is to classify all information received by the ATS into four categories:

• Emergency Faults - An emergency fault is a safety or mission critical fault such as a train emergency brake application or a switch losing locked status.?An emergency fault must be displayed in a manner that grabs the operator’s attention immediately regardless of where they are looking on the user interface. The emergency fault requirements must define how the symbol or colour should look and behave that grabs attention such as a blinking red symbol with an audible sound.
• Critical Statuses – Are indications required by the operator to understand the status of the CBTC system and make quick decisions. These indications are related to operations such as the position of switches, location of trains, platform doors open/close status etc. The display requirements for a critical status must define how the symbol and colour should look that is representative of the information it is conveying.
• Non-Critical Statuses – Are indications not immediately required by the operator and a delay in receiving this status will not impact operations such as the speed of the train.?
• Commands - A command is a request from the operator to any CBTC subsystem such as setting a route or holding trains at a station. The operator should only have to click on the line overview to open a window and execute the command.

Based on these four categories, emergency faults, critical statuses, and commands must be easily accessible from the line overview since they are crucial for quick decision-making in high-stress situations; this information cannot be hidden under layers of menus and windows.

Non-critical statuses can be hidden in second- or third-layer menus to avoid cluttering the line overview display. Since the statuses have a low criticality, it will not affect the operation of the CBTC system if the operator is delayed in retrieving this information.

To determine how to display each category of information on the line overview such as colour, sound or animation the MIL and ANSI standards should be used. However, an HFE consultant would be best to review and apply these standards but since their CBTC knowledge is limited, the transit operator should focus on classifying the information.

To create a user-friendly ATS interface, the ATS specification must emphasize easy and quick access to emergency faults, critical statuses, and commands from the Line Overview Display. This leads to faster reaction times and fewer errors under stressful situations.

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