Many bowtie diagrams treat human error as a threat leading directly to a Top Event. This approach highlights specific actions—such as pressing the wrong button or skipping a critical step—that can initiate failure. However, a more nuanced perspective treats human error not as a threat in itself, but as a factor that degrades the effectiveness of controls designed to prevent or mitigate system threats.

This alternative view recognizes how human performance can undermine the viability of barriers. For example, where a maintenance procedure is in place (a control), but a worker misunderstands instructions, or supervision is inadequate, resulting in a control being weakened or bypassed. Here, human error acts more as a vulnerability (or “Escalation”) that reduces control integrity, rather than as a standalone initiating threat.

By modelling human error in this way, the bowtie diagram becomes more dynamic and realistic. It encourages a focus on improving the design, clarity, and usability of controls—addressing the conditions under which people are more or less likely to perform reliably.

This aligns with modern human factors thinking, which emphasizes system design and context over blaming individuals. It also promotes stronger, more resilient risk management strategies.

There remains a case for using a bowtie diagram to illustrate the effect of human performance on the controls themselves. In what the Center for Chemical Process Safety calls a “Level 1 bowtie”, the failure of a barrier itself is treated as a “top event”. Breaking out this element into a separate chart helps to maintain readability of the original bowtie diagram and develop understanding of this vital source of failure.

A visible example of where the aircraft industry began to go down this path is the Civil Aviation Authority (CAA) series of bowtie diagrams, notably those focused on human factors, such as Operation of Large CAT Fixed wing aircraft – human performance. (If you find the PDF hard to read, that’s one issue which Bowtie Designer could help with)

Human factors can be both active (immediate errors) or latent (systemic weaknesses in training, supervision, or culture). Understanding and correcting these is key to ensuring effective barriers which prevent a hazardous event from emerging, or translating into bad outcomes.