Good vision is a fundamental component of safe driving, being one of the most important sensory factors for this activity, accounting for about 95% of all sensory requirements.1 For this reason, drivers with good vision have an advantage over those with poor vision as far as Road Traffic Accidents (RTAs) are concerned. RTAs are a major health problem worldwide,2 and a leading cause of death from trauma,3–6 with an estimated annual death of 1.2 million and up to 50 million injuries worldwide.7

In Ghana, RTAs, with their associated fatalities have been on the increase over the years, and are reported to be higher than in most West African countries, being second after Nigeria.8,9 The rise in the number of RTA in Ghana is exemplified by the fact that there were 14,914 road accidents involving 21,817 vehicles which claimed 2249 lives and caused 14,181 injuries in 2012, compared to 13,572 accidents involving 19,530 vehicles in 2011.10 These data reveal the huge burden of RTAs to the Ghanaian economy, as it results in the loss of 1.6% of her Gross Domestic Product (GDP), with an average of 1800 deaths annually of whom 60% are in the productive ages of 18–55 years.11

Commercial vehicles are the major source of motorized transport in many low-income countries.1 In the absence of good public transport infrastructure such as railways, most people rely on them for commuting, with the role of commercial driver being evident. Therefore, maintaining optimum visual function, such as visual acuity, colour vision, depth perception (stereopsis), contrast sensitivity and peripheral vision, is essential.12,13

Colour vision plays an important role in driving, as a defect leads to difficulty in recognizing traffic signs and signals, as well as signals from other vehicles.14 Studies on the relationship between colour vision defects and road traffic accidents have been ambivalent, as some have indicated that colour vision deficiencies increase the risks of road accidents while others do not support this assertion. For instance, a study on accident rates15 found an increased risk for a small sample of colour deficients15 while Norman found no such effect.16 Another study reported that protans are precluded from holding a commercial driver's license in Australia because they have a substantially reduced ability to see red lights and had more road accidents involving signal lights.17

Stereopsis, the ability to appreciate depth (i.e. the ability to distinguish the relative distance of objects with an apparent physical displacement between the objects)18 is equally important for driving. According to Omolase et al.,19 it is needed to accurately judge distances, overtake other vehicles and change lane, especially in busy traffic. This pre-supposes that drivers with defective stereopsis may have difficulty judging distances in their quest to overtake other vehicles and change lanes,12,19 which could lead to the occurrence of RTAs.

The relevance of these visual function parameters among commercial drivers cannot be underestimated. However, there is a paucity of knowledge on the extent to which they affect commercial drivers in Ghana. This study was therefore conducted to investigate the relationship between some visual function parameters and the occurrence of road traffic accidents among commercial drivers in the Central Region of Ghana.

The findings of this study does not provide evidence to support a relationship between the occurrence of RTAs and refractive errors and stereopsis anomalies. However, the study found that protans were more likely to report RTAs among participants who had colour vision defects. Nearly 2.5% of the drivers had visual acuity worse than the minimum required for driving in Ghana (6/9 Snellen acuity).

The study provides valuable information on the eye health of commercial drivers in the central region of Ghana. The results of this study confirm that commercial driving is dominated by males in the central region of Ghana, as is the case in the entire country.23 This may be due to the fact that driving is perceived to be a risky venture, which males are better placed to endure than females.24 The dominance of an active workforce with a mean age of 39.23±10.96 years is also a reflection of the demanding nature of commercial driving in Ghana, requiring a strong and energetic workforce who can endure the stress related to this occupation.25

Of the 37 drivers who had colour vision defects, 13 had a positive history of RTAs. Among the 13 drivers, protans were more likely to report RTAs than deutans (χ2=6.194, p=0.034), which has been reported by other authors17,26 who asserted that colour vision defects may constitute hazard to safe driving due to a difficulty in identifying road signs and signals, as well as recognizing traffic light. In the case of this study, drivers with protanopia were more likely to miss the red signal from the traffic light that could have led to RTAs. However, other authors27–29 have found no association between RTAs and colour vision defects. The prevalence of colour vision defect in the current study (7.1%) was approximately two times higher than that reported by Ovenseri-Ogbomo and Adofo30 in the Cape Coast Municipality of Ghana, and that of Pepple and Adio12 and Emerole and Nneli31 in Nigeria. This may be due to the differences in instruments used, as the current study utilized the HRR pseudoisochromatic plate, which is capable of revealing both congenital and acquired defects compared to the use of the Ishihara pseudoisochromatic plate used in the previous studies, which is only sensitive to congenital defects.32

Abnormal stereopsis was found in 15.4% of all the drivers. All drivers with monocular blindness were also found to have abnormal stereopsis, mainly because it is a function of binocular vision. Although there was no statistically significant association between abnormal stereopsis and the occurrence of RTAs (χ2=0.34, p>0.05), as reported by other authors in Nigeria,13,26 as well as Ovenseri-Ogbomo and Adofo30 in Cape Coast, Ghana, the rate cannot be overlooked, as 5.2% of the drivers admitted to having difficulties in correctly judging distances when driving. Further studies are needed in this area to establish the contribution of stereopsis anomalies to RTA in Ghana in a much larger driver population.

It must be noted that attempts to find a link between simple visual function deficits and accidents have proved largely fruitless.33 This may be because drivers with worse visual capabilities may be aware of their abilities and drive within them. Alternatively, it may be that the visual capabilities measured are too simple. While a driver's task is a complex one, involving both visual and cognitive factors, the failure to relate simple visual function to accidents does not mean that visual capabilities and lighting conditions that support them are unimportant. Rather, the evidence of the effect of visual capabilities on driving performance means that visual capabilities and lighting conditions that support them are part of the problem and part of the solution, but not the whole problem nor the whole solution.33

In spite of the above, there was a statistically significant association between colour vision defect and stereopsis anomaly (r=0.371, p=<0.005). This may be due to the fact that colour vision and stereopsis occur within the extrastriate cortex, a structure beyond the striate cortex that contains cells that differ in selectivity for different features of stimuli such as colour, motion and stereopsis.34 Hence, any insult to that region of the brain affects both colour vision and stereopsis. There may therefore be the need to include these tests in any routine examination of drivers to provide further evidence in this regard. Although the rates of colour vision defects and stereopsis anomaly was high in this study, most of the participants were unaware of the existence of colour vision disorders. There is therefore a need for ocular health education for drivers in order to inform them of this information and the potential dangers they might pose.

Ingesting alcohol while on duty has been implicated in RTAs by several reports.12,35,36 Alcohol depresses the central nervous system, with a subsequent release of inhibition, thus causing the driver to overestimate his ability while underestimating his deficiencies.12 This study found evidence to support this assertion, as those who reported alcohol consumption were 1.64 times more likely to have been involved in RTAs than those who did not consume alcohol. This suggests a need to intensify campaigns against drink-drive among commercial drivers in Ghana, as has been championed by the National Road Safety Commission of Ghana.37 Regular spots checks to arrest offenders of this law on drink-drive by the Motor Traffic Unit of the Ghana Police Service may also need to be intensified.

Approximately 2.5% of the drivers had a visual acuity worse than the minimum required for driving in Ghana (6/9), which was comparable to that reported in Nigeria.33 However, this was lower than that reported by Ovenseri-Ogbomo and Adofo30 in the Cape Coast municipality in Ghana. This may be due to an increased awareness on the vision requirement for attainment of driver's license in Ghana, as well as a possible stricter regime that enforces the law on driving in Ghana. However, the rate of worse vision recorded in this study indicates that some drivers acquire their license without meeting the minimum standard criteria, among them were 5 (1.0%) who were monocularly blind. This may be due, in part, to the use of unskilled persons in conducting vision assessments in most parts of Ghana, other than the prescribed professionals; ophthalmologists or optometrists, as stated in the law.22 Drivers with monocular vision pose a great risk to themselves, their passengers and other road users, as they may have considerable limitations in undertaking their task of driving accurately. There is therefore the need for the Drivers’ and Vehicular License Authority (DVLA) to enforce the law on vision testing by eyecare professionals, as this will promote road safety. This should be done with strict compliance, as drivers who did not go through eye examinations were 2.13 times more likely to be involved in RTAs (OR: 2.13, 95% CI: 1.16–3.91, p=0.013).

Visual impairment of 2.5% found in the current study was higher than the 1.7% reported by Pepple and Adio12 in a similar population. It was however, lower than the Ibadan study,38 with a prevalence of 3.1%. The prevalence of refractive error in this study (20.4%) compared favourably with a study conducted by earlier authors12,13 in urban Africa, but lower than that reported among drivers of public institutions in Nigeria.39 This group of drivers may presents a risk to themselves, passengers and other road users, as they require optical corrections for driving and yet most did not utilize their prescribed spectacles.12 In spite of this assertion, there was no statistically significant association between refractive error and reported RTA in this study (χ2=3.090, p=0.388).

The current study found an association between protanopia and the occurrence of RTAs. A relatively high prevalence's of colour vision defects and stereopsis were recorded in this study. However, few participants were aware of the existence of colour vision anomalies, while a limited number reported that they were unaware of stereopsis anomalies. This highlights the need for intensified health education to encourage drivers who experience these defects to seek prompt medical help and advice. The prevalence of refractive errors and monocular blindness among the study population also indicates the need to enforce the law on comprehensive eye examination by appropriate professionals, as enshrined in the law, rather than just measuring visual acuity prior to acquiring or renewing drivers’ licenses, as is currently the case.

None declared.