Objective

The aim of the design phase is to develop detailed technical specifications for each part of the system so they can be built, coded, or procured. This includes hardware, software, and any commercially available components that will be integrated into the solution.

High-level design

Description

The high-level design activity marks the transition from defining what the system must do (requirements definition) to defining how the system will achieve the requirements. This phase involves evaluating architectural alternatives (eg vehicle-based vs. infrastructure-based alert delivery), and selecting an overall architecture defining the major system components and their interactions. This may include selecting internal and external interfaces, and how these interact with other sub-systems such as in-vehicle units, roadside devices, and backend services.

Key activities

1. Develop and evaluate architecture options. Explore system structure alternatives (eg wired vs. wireless detection), considering performance, cost, and safety implications.
2. Decompose functions and allocate to sub-systems. Allocate key tasks such as detection, alerting, and monitoring to vehicle units, roadside infrastructure, and backend systems.
3. Identify and evaluate interfaces. Define communication protocols (eg API between app and backend), mechanical interfaces (eg mounting hardware), and compliance with standards.
4. Apply industry standards. Identify and adopt proven standards to support future scalability and interoperability.
5. Select and document the preferred design. Present the preferred system architecture to stakeholders for review. Revise the architecture as needed and finalise the high-level design.  
6. Conduct preliminary design review. Review and validate the architecture and subsystem plans with project stakeholders.

Human Factors contributions to high-level design

Human Factors plays a critical role in ensuring the system architecture supports intuitive, safe, and effective human interaction. Human Factors inputs at this stage reduce usability risks, address performance limits, and shape early interface designs.

1. Architecture evaluation. Assess architectural options (eg in-vehicle vs. infrastructure-based alerts) using Human Factors criteria such as user understanding and response time, cognitive load and attention demands, and alert credibility and trust. Support simulations or trade-off analyses to compare usability and workload.
2. Function allocation to sub-systems. Apply task analysis to ensure that functions allocated to each sub-system align with user capabilities and limitations. Identify key areas of human-system interaction and assess whether the design could lead to Human Factors issues such as excessive workload, poor situation awareness, or lack of understanding of failure modes. Consider use of the Human Factors Risk Assessment Prompts to identify potential issues.
3. Interface identification. Lead or support identification of user-facing interfaces (visual, auditory, haptic) and ensure early Human Factors input to their layout and usability. Apply relevant standards and guidelines as well as inclusive design principles (eg accommodating older users, colour blindness). See the Interface Design Guidance for Level Crossing Warnings and Alerts for more information.
4. Risk of negative transfer. Identify risks where existing habits may interfere with new system interactions. The Generic Level Crossing Task Analyses may be useful to considering potential changed interactions that could lead to negative transfer risks. If issues are identified, advise on design adaptations or risk controls.
5. Standards and design compliance. Evaluate applicable HF-related standards and recommend adoption where appropriate. Further, consider interoperability and compliance of system elements with ergonomic principles and cognitive guidelines (eg signal detectability, error tolerance).
6. Stakeholder engagement. Facilitate workshops or structured walkthroughs with representative primary and secondary users to elicit feedback on operational feasibility, clarity, and early usability.
7. Preliminary design review. Contribute to preliminary design review process by presenting Human Factors findings and design implications. Ensure that unresolved Human Factors risks and issues are logged in the HFIR to enable tracking and later resolution.

Detailed design

Description

This stage translates the high-level system design into detailed instructions and specifications that engineers or manufacturers can follow to build the technology. All components (eg sensors, warning lights, software modules, controllers) must be clearly defined in terms of their function, interface, and performance. For low-cost level crossing technologies, this could incorporate off-the-shelf components that reduce custom development time and cost.

Key activities

1. Evaluate off-the-shelf products. Involve stakeholders to assess candidate products and decide whether to adopt, modify, or replace.
2. Perform detailed design. Develop technical specifications for software, hardware, communication protocols, and databases, ensuring all designs are implementation-ready.
3. Conduct technical reviews. Regularly review progress, defects, and coverage of documented requirements.
4. Critical design review. Conduct final review to confirm design readiness, validating that all requirements and considerations have been addressed.

Human Factors contributions to detailed design

Human Factors specialists help to ensure that the design supports user needs, limits human error and enables error detection and recovery, and aligns with usability and other relevant standards. Human Factors input is important in contributing to the layout and behaviour of user interfaces, guiding evaluations of off-the-shelf components, and ensuring that safety and usability considerations are embedded in the final specifications.

1. Off-the-shelf product evaluation. Support product evaluation workshops using structured checklists (eg visual clarity, workload, feedback mechanisms), consider vendor’s HFI processes and identify any Human Factors-related limitations in vendor specifications.
2. Detailed interface design and specification. Guide the layout and behaviour of human-machine interface components (eg in-vehicle displays, roadside signage, mobile apps). Refer to the Interface Design Guidance for Level Crossing Warnings and Alerts for further guidance. Conduct iterative mock-ups, low/high-fidelity prototypes, or simulations with representative end users to assess and refine the design. Consider unintended consequences of different design options such as skill degradation or system dependency. Human error identification methods (eg SHERPA, HAZOP) can be useful to support an understanding of potential errors and make recommendations for re-design.
3. Usability evaluation. Conduct formative usability tests with representative users, ensuring to consider both primary and secondary users. Consider evaluations of aspect such as alert comprehension, distraction potential, and response time in operational contexts. Capture both objective performance metrics (eg time to respond, error rate) and subjective feedback (eg perceived clarity or trust).
   See the Human Factors Guidance for Evaluating Innovative Level Crossing Technologies for more considerations.
4. Anthropometric and accessibility review. Confirm human-machine interfaces accommodate the expected range of users (eg percentile-based reach, visibility) and address relevant accessibility standards (eg DSAPT compliance).
5. Training and manual design impacts. Identify training implications of design decisions (eg cognitive complexity, addressing negative transfer) and conduct training needs analysis for primary and second users, considering initial training and skills maintenance. Provide early input into system manuals/maintenance procedures development to align with Human Factors principles.
6. Technical reviews and critical design review. Participate in technical design reviews to verify Human Factors requirement coverage and confirm design solutions address known Human Factors issues. Present Human Factors evaluation outcomes, outstanding issues/risks and proposed design mitigations/additional controls at critical design review.

Next, Phase 2: Development, build and integration