Writing requirements rationale is crucial because it explains the reasoning behind each requirement and why it is necessary for the project. This practice helps ensure that requirements are clear, consistent, and aligned with stakeholder needs. Additionally, it supports the creation of testable and achievable requirements within project constraints.
 

<h3>Why Write a Rationale?</h3>

<ul>
 <li>Improves understanding: A rationale clarifies the purpose and importance of a requirement, reducing ambiguity and promoting consistency.</li>
 <li>Facilitates decision-making: It provides key information about the requirement's impact and trade-offs, aiding project decisions.</li>
 <li>Provides context: Rationale offers essential background that ensures alignment with stakeholder needs and project constraints.</li>
</ul>

<h3>Examples</h3>

<ul>
 <li>Example 1: A financial software requirement to encrypt all stored data. The rationale: protecting sensitive customer financial information from unauthorized access.</li>
 <li>Example 2: A mobile app requirement to support offline mode. The rationale: ensuring seamless user experience even without internet connectivity.</li>
 <li>Example 3: A website requirement to be optimized for accessibility. The rationale: making the site usable for users with disabilities.</li>
</ul>

<h3>What About Poor Rationales?</h3>

Consider a requirement for a website to include a scrolling marquee on the homepage because it is "cool" or "trendy." While this might seem appealing, the rationale is weak because it can distract users, reduce readability, be inaccessible to some users, and misalign with project goals. A well-written rationale would explain these issues and why the feature should be reconsidered or rejected.

Writing a rationale explains why a requirement exists, ensuring clarity, alignment, and testability. It improves understanding, supports decision-making, and provides essential context for stakeholders.

The Importance of Writing Clear Requirements Rationale

Over the years working with complex automotive systems—especially road-legal, safety-critical platforms—I’ve seen how much a project’s success depends on the quality of its requirements. Getting them right early saves you massive downstream pain. Whether you're defining system behavior, validating test coverage, or aligning with compliance standards like ISO 26262, well-authored requirements are a cornerstone. Below is a list of practices I’ve found effective in structuring requirements for clarity, traceability, and stakeholder alignment—supported by examples from automotive systems and aligned with <a href="https://www.incose.org/products-and-publications/se-handbook" target="_blank">INCOSE guidance</a>.


<ol>
 <li>
 Start by identifying the stakeholders: 
 First step is always understanding who’s impacted—directly or indirectly. Get input early from all key players.
 Example: On an ADAS project, that might include OEM system engineers, safety certifiers, compliance teams, and of course, the end-users (drivers). Each has a different perspective that shapes what’s needed.
 INCOSE Ref: SE Handbook v4.0, Sec 2.3.4 – Stakeholder Needs and Requirements.
 </li>

 <li>
 Define the project goals and objectives: 
 Goals help ground the scope and keep everyone aligned.
 Example: A goal like “Increase pedestrian safety” becomes actionable when turned into an objective like “Detect pedestrians 30m ahead and alert within 200ms.”
 INCOSE Ref: Sec 2.3.5 – System Requirements Definition.
 </li>

 <li>
 Use a consistent format: 
 Stick to a structured syntax—it’s not about being rigid; it’s about being understandable and testable.
 Example: “The [component] shall [do something] when [condition occurs].” e.g., “The LKAS shall apply torque when the vehicle drifts more than 30cm without indicator use.”
 INCOSE Ref: Sec 5.2 – Requirements Management.
 </li>

 <li>
 Write SMART requirements: 
 Make each requirement Specific, Measurable, Achievable, Relevant, and Time-bound.
 Example: Instead of vague phrases like “quick deceleration,” write: “The system shall decelerate from 50km/h to 0 in under 2.5 seconds on dry asphalt.”
 INCOSE Ref: Appendix C – Characteristics of Good Requirements.
 </li>

 <li>
 Use plain language: 
 Avoid unnecessary jargon. Make it easy for every stakeholder to understand—whether they’re technical or not.
 Example: Replace “CAN FD WUP to BCM” with “Establish communication with the Body Control Module within 100ms after wake-up.”
 INCOSE Ref: Sec 5.2.3 – Requirement Representation.
 </li>

 <li>
 Prioritize the requirements: 
 You can’t deliver everything at once. Rank them based on value, risk, and feasibility.
 Example: Functional safety items (like airbag deployment timing) come before infotainment UI tweaks.
 INCOSE Ref: Sec 5.3 – Requirements Analysis.
 </li>

 <li>
 Validate with stakeholders: 
 This isn’t a checkbox—it’s a conversation. Walk through the requirements with key people and make sure they hold up.
 Example: For FCW systems, I’ve validated detection logic with system engineers and test scenarios with vehicle certifiers.
 INCOSE Ref: Sec 5.6 – Validation Process.
 </li>

 <li>
 Keep requirements up-to-date: 
 Things change—regulations, hardware availability, system boundaries. Your requirements need to reflect that.
 Example: When UNECE revised lighting rules, we updated every affected item and synced the change with impacted teams.
 INCOSE Ref: Sec 5.5 – Configuration Management.
 </li>

 <li>
 Ensure unique identification: 
 Every requirement should have a unique ID. This makes traceability and change impact analysis possible.
 Example: Use structured IDs like “SYS-REQ-045” or “SW-FCW-102” consistently in your tools, test plans, and design docs.
 INCOSE Ref: Sec 5.2 – Identification; also see ISO 26262-8, Clause 6.
 </li>

 <li>
 Document everything clearly: 
 Not just the requirement, but also its rationale, version history, and any related links (e.g. regulations or test cases).
 Example: I’ve used Polarion to organize safety requirement sets for EVs, linking test cases and rationale for auditors.
 INCOSE Ref: Sec 5.2 – Documentation Practices.
 </li>

 <li>
 Continuously improve the process: 
 Don’t let your process go stale. After each major project or audit, review how well the requirements process worked—and refine it.
 Example: We updated our template language and added better traceability fields after the first program review of an EV platform.
 INCOSE Ref: Sec 7 – Technical Management: Process Improvement.
 </li>

 <li>
 Understand the hierarchy: Product vs. System vs. Software vs. Hardware:
 Good traceability means knowing where each requirement lives in the architecture.
 <ul>
 <li>Product: What the whole thing is supposed to achieve.</li>
 <li>System: How subsystems interact to fulfill the product goals.</li>
 <li>Software: What logic, functions, and interfaces are required to realize the system behavior.</li>
 <li>Hardware: What physical or electrical constraints support the system.</li>
 </ul>
 Example – Forward Collision Warning (FCW):
 <ul>
 <li>Product Requirement: “The vehicle shall reduce rear-end collisions by alerting the driver to frontal obstacles.”</li>
 <li>System Requirement: “The FCW system shall detect moving/stationary objects within 60m above 30km/h.”</li>
 <li>Software Requirement (SRD): “The FCW logic shall trigger a warning if Time-To-Collision &lt; 2.5s.”</li>
 <li>Hardware Requirement (HRD): “Radar sensor must have at least 80m range and 20Hz update rate.”</li>
 </ul>
 INCOSE Ref: Sec 4 – Technical Processes: Architecture and Decomposition.
 </li>
</ol>


 These practices aren’t just theory—they’re part of the discipline needed to build safe, compliant, and successful automotive systems. The more intentional you are about how requirements are written, structured, and validated, the more likely your project is to stay on track and deliver what it promises.

Writing clear, traceable requirements is critical in complex systems development—especially for road-legal automotive platforms. In this guide, I share practical strategies I've used across real-world projects to improve requirement quality, ensure stakeholder alignment, and support compliance. Topics include SMART writing, stakeholder validation, traceability, system/software/hardware layering, and lessons grounded in INCOSE best practices. Ideal for systems engineers, requirements authors, and technical leads.

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