New report proposes safer, fairer colour vision testing for main line train drivers

A new report by researchers from City St George’s, University of London for the Rail Safety and Standards Board (RSSB) has revealed the potential for safer and fairer colour vision assessment protocols for train drivers.

The study, Defining Colour Vision Requirements for Train Drivers, suggests adopting a new two-step testing approach that enhances both operational safety and inclusivity for those with mild colour vision deficiencies. The new protocol significantly reduces discrimination, as the researchers found that the number of people with red/green (RG) colour vision deficiencies who pass the new protocol increases by more than half.

Equally importantly, safety is much increased since the severity of colour vision loss in the most affected applicants who pass the new protocol is reduced three-fold.

Making railways safer

Coloured lights are critical for safe operations on the railways, particularly for tasks such as interpreting signal lights and indicators. Current testing for train driver applicants typically relies on the Ishihara pseudoisochromatic test plates to detect red/green (RG) colour vision deficiencies. In particular, the Ishihara 24-plate edition is widely used in current assessments. This ensures high sensitivity for congenital RG colour deficiency (when no errors are allowed), but it also disqualifies around 11% of normal trichromats (people who have the three types of normally functioning cone cells in the eye that enable them to see different colours).

To accurately investigate the current colour vision assessment protocols, including their effectiveness and limitations, the researchers – Dr Marisa Rodriguez Carmona and Professor John Barbur – analysed existing Ishihara test results from a large dataset of 2,095 participants, including individuals with normal colour vision (trichromats) and those with congenital red/green (RG) colour deficiencies (e.g. 1,200 deutans and 539 protans).

In addition, nine Ishihara-based protocols were reviewed, varying in the number of plates used and the maximum errors allowed for passing. Based on this analysis, the researchers calculated an ‘equivalent RG threshold’ (RG_E), measured in CAD (Colour Assessment and Diagnosis) units to be used in the new two-step protocol.

The report recommends replacing the current single-test protocols with a two-step process. Step one involves using a simplified version of the Ishihara test as a screener (the first 13 plates of the 24-plate edition, with no errors), followed by the CAD test developed at City St George’s for those who fail step one. CAD passes all normal trichromats (including those who fail step one) and ensures accurate measurement of the severity of colour vision loss. All colour-deficient applicants with a threshold below RG_E (i.e., the equivalent threshold) pass.

Fairer recruitment practices

The findings indicate that the two-step protocol significantly reduces discrimination. Compared to current practices, it was seen that this approach increases the pass rate for applicants with mild deficiencies by 47.5% for those less able to differentiate red and green colours due to issues with their green-sensitive cone cells (i.e. deutans) and 66.7% for those who have difficulty distinguishing between red and green due to issues with their red-sensitive cone cells (i.e. protans). At the same time, the severity of colour vision loss among those who pass is reduced at least three-fold, ensuring enhanced safety standards.

Professor Barbur, co-author of the report and Professor of Optics & Visual Science at City St George’s, said:

“This research represents a step forward for the rail industry. By improving the precision and fairness of colour vision testing, we can ensure safety-critical roles remain accessible to a broader pool of candidates without compromising safety.

“The proposed changes could have wide-ranging benefits for the rail sector. Not only would the two-step protocol reduce variability in testing outcomes, but it also eliminates reliance on methods with large variability that fail to quantify adequately severity of colour vision loss. The new protocol ensures all applicants are assessed consistently, with much reduced variability which leads to enhanced safety and reduced discrimination.”

Dr Rodriguez Carmona, Senior Lecturer at City St George’s, said:

“This research demonstrates that it’s possible to balance inclusivity with safety in rail recruitment. Modernising colour vision testing not only ensures a fairer assessment process for applicants with mild deficiencies but also enhances safety in visually-demanding, colour related tasks.”

The RSSB is now urging rail operators and occupational health providers to adopt this scientifically backed, two-step approach. The new protocol would initially utilise existing testing infrastructure, such as the Ishihara test with zero errors as a pass, while incorporating the CAD test for precise evaluation of severity of RG colour vision loss in only those applicants who fail the first step.

The report also acknowledges that further research is needed to fine-tune severity thresholds and explore the impact of colour vision loss on safety-critical tasks. However, the findings offer immediately a clear pathway to a safer, more equitable future for train driver recruitment. In addition, the new approach can also be applied to any environment that currently uses the Ishihara test with a fixed number of errors as a pass.

For more information, the full report is available on the RSSB research catalogue: https://www.rssb.co.uk/research-catalogue/CatalogueItem/T1279