Visual function of commercial drivers and its relationship with road traffic crashes

INTRODUCTION

The road traffic system is a dynamic system consisting of people, motor vehicles, tricycles, motorcycles, bicycles, roads and other infrastructures on the road. It is the most common means of transportation in Nigeria like most other developing countries.^[1,2] This is due to the high cost and poor function of other means of transportation such as air, sea and rail systems in Nigerian cities. Commercial transport is used by many people in carrying out their daily activities.

Good vision is very vital for safe driving because most of the information obtained in the course of driving is through the visual sense; as such, there is a legal minimum standard of vision required for all drivers.^[3,4] Vision encompasses more than mere distance and near acuity; it also entails contrast sensitivity, visual field (VF), colour vision and stereopsis.

Visual impairment has been shown to be an important risk factor for road traffic accidents in some studies.^[1,5] However, this result has not been consistent as other studies showed no association.^[6] Although the state of the road viz-a-viz the terrain and illumination as well as the mental and physical health of the driver are contributing factors to the risk of road traffic crash (RTC), good vision still remains an important prerequisite for the safety of all road users.

Both central and peripheral visions are very vital for driving performance. A driver must have vision good enough to be able to see road signs, roadside markings and furniture, traffic lights, pedestrians and other vehicles on the road (parked or moving) at a distance and not until when close, to avoid bumping into them.^[6] Furthermore, the vision of a motorist should be optimal under varying illumination (daylight and night) and weather conditions (rain, harmattan, etc.).^[7]

RTC is a public health problem, and in Nigeria, it is the third-principal cause of overall deaths, the leading cause of injury-related deaths and the most common cause of physical disability.^[8] The causes are multifactorial (road, vehicular and driver factors) and require diverse interventions to reduce the rate. Visual screening is one way to address these risks through driver licence renewal policies because vision has a key role to play in driving performance and is thus crucial to the assessment of driving fitness.

Over the years, there have been several discussions and research on the need for vision evaluation of drivers, for issuance or renewal of licence as well as the choice of the screening tools that will be relevant in assessing safe driving performance. The easy-to-perform static visual acuity assessment alone has been the most widely used tool in many countries; however, some countries include other vision assessment tools such as dynamic visual acuity, contrast sensitivity, useful field of vision, colour vision and stereoacuity.^[6]

Vision zero was adopted by the Swedish parliament in 1997; it was borne out of the need to reduce the frequency as well as the gravity of RTC.^[9] Its sole objective is to ensure no mortality or mild morbidity from road accidents by the year 2020. Their transport system has since then been modified to achieve this and ensure road safety.^[10]

Vision screening is one of the prerequisites documented in the guidelines for the National Driver’s Licence Scheme by the Federal Road Safety Commission (FRSC) for licensing and registration (best corrected visual acuity of at least 6/12 in the better eye and 6/36 in the poorer eye, for private motor drivers and minimum visual acuity of 6/9 in the better eye and 6/24 in the poorer eye with or without glasses, for commercial drivers).^[11] However, compliance with this policy cannot be ascertained. It is thus important to evaluate the level of compliance; it is not enough to have evidence-based tools for visual function evaluation for driving included in the driving scheme but to ensure it is being observed.

Visual disability is an acknowledged safety problem with serious consequences; however, its sole contribution to the risk of RTC has only been documented by few researchers.

MATERIALS AND METHODS

RESULTS

A total of 124 commercial drivers were enrolled, all of them were males. The mean age was 43.66 years, with an age range between 24 and 63 years. Majority of the participants were in the age groups 31–40 years and 41–50 years, respectively. Most of the participants, 52 (41.9%), have secondary education and 4 (3.2%) had no formal education. Majority of the participants have driven for 10–19 years, with mean number of driving years being 18.34 years and most of them (60.5%) drive daily. A large proportion, 86 (69.4%) of the participants are aware that vision screening is a pre-requisite for driving; however, only 41 (33.1%) have had vision screening in the past, and majority of them (51.2%) had the screening done outside the licence-related process [Table 1].

Most of the participants (95, 76.6%) had no history of RTC. Of the 23.4% with RTC history, three-quarters had only one incident and the others had experienced RTC 2–4 times. None of the participants has been involved in road traffic more than 4 times.

The presenting visual acuity of majority of the participants (95.2%) was normal in both eyes; however, 2.4%, 3.2% and 0.8% mild, moderate and severe visual impairment had in the right eye, respectively, while 3.2%, 5.6% and 1.6% had mild, moderate and severe visual impairment in the left eye [Table 2].

The visual function tests were all done monocularly and recorded as such. Over a hundred participants, 116 (93.5%) and 118 (95.2%) in the right and left eye, respectively, had normal contrast sensitivity (score of >2.0) on the PelliRobson chart while visual impairment (a score of 1.5–2.0) was recorded in the right and left eyes of 3 (2.4%) and 4 (3.3%) participants, respectively. Visual disability (a score of <1.5) was recorded in the right eye of 5 participants (4.0%) and the left eye of 2 participants (1.6%) [Table 3].

Defective colour vision (Red-Green defect) was recorded in the right eye of 11 (8.9%) participants and the left eye of 8 (6.5%) participants. A small proportion of the participants had VF defect; 10 (8.1%) and 8 (6.5%) in the right and left eyes, respectively [Table 3].

Most of the participants with visual impairment (82.4%) drove weekly rather than daily when compared with their normal counterparts (18.7%).

Less than one third (25.7%) of participants with normal visual acuity had a prior history of involvement in RTC while only a little above one tenth (13.3%) of those with abnormal visual acuity had had crash before. About 25% of participants with normal VF have had RTC in the past but just 9.1% of their counterparts with VF defect have had RTC. Furthermore, 24.1% of participants with normal contrast sensitivity and colour vision gave a history of previous RTC compared to 12.5% and 16.7% of those with abnormal contrast sensitivity and colour vision, respectively, thus there was no clinically significant relationship between the various visual functions assessed and RTC [Table 4].

It was also found that participants with abnormal visual acuity did not have more RTC than those with normal visual acuity (28.6%). However, participants with abnormal contrast sensitivity and colour vision had a greater number (50%) of RTC than their normal counterparts but the difference was not statistically significant.

DISCUSSION

Road transportation is unarguably very important and the most commonly used means of transportation in middle- and low-income countries like ours. Driving is largely a visual task, and as such, the importance of vision screening in ensuring good vision of all road users, particularly the drivers, cannot be overemphasised.^[2,13]

Several studies have been done to assess visual functions and relate visual impairment to RTCs. However, their findings vary widely. While some found an association between visual function and road crashes, others did not demonstrate a statistical correlation between visual impairment and RTC.^[1,2,3,9]

In this study, it was found that majority of the participants have good visual function status based on the FRSC standard as well as the international standard. The percentages of the participants with normal visual acuity, contrast sensitivity, colour vision and VF are as follows: 87.9%, 93.5%, 90.3% and 91.1%, respectively. This may be because the majority of the participants are young [Figure 1]; thus, age-related ocular pathologies have not set in yet. However, 8 (6.4%) and 13 (10.4%) had visual impairment in the right and left eye, respectively, and 4.0% had monocular blindness. Meanwhile, 90.5% of the participants of a study done in Edo had normal vision, 8.9% had mild to moderate visual impairment and 0.6% had severe visual impairment.^[14] This has a higher prevalence of visual impairment, which may be due to the large sample size which is more than twice the sample size used in this study. Probably, if more drivers had been examined in this study, many more with visual impairment could have been picked as well.

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Figure 1: Percentage distribution of participants visual function in both eyes. (Y-axis: Number of participants).

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According to FRSC, the visual acuity standard recommended for commercial drivers is 6/9 in the better eye and 6/24 in the poorer eye with or without glasses.^[8,9] Based on this, 87.9% of the participants were found to have normal visual acuity, and of this, 29 (26.6%) gave a verbal report of previous involvement in RTC while only few of the participants (12.1%) had poor visual acuity and 2 (13.3%) of them had had RTC in the past. Thus, the relationship between visual acuity and involvement in RTC was not found to be statistically significant (p: 0.110). This could be because participants with visual impairment were found to drive less often than their counterparts with good vision. This means that their awareness of the driving difficulty associated with their visual impairment has made them to self-regulate and restrict their driving based on these self-acknowledged functional limitations and develop specific driving habits such as restricted days of driving/week, fewer mileages, slow driving, driving only in familiar terrain, daytime only driving and restricted driving space to compensate for their defect. This can significantly reduce their driving exposure and subsequently reduce their crash risk to a level comparable or even below that of participants with no visual impairment. Similarly, Ovenseri-Ogomo and Adofo, as well as Omolase et al., found that visual impairment was not significantly associated with a positive history of RTC.^[15,16] On the contrary, Oladehinde et al.^[1] found a statistically significant relationship between visual impairment in the better eye and RTC with an increased risk of 4.16.

From this study, it was noticed that the participants with visual impairment still got licence to drive despite not meeting the required criteria. This shows the need for drivers and vehicle licensing authority to enforce strict proper screening before issuing licence. Dairo et al. in a survey in Ibadan, south western Nigeria, found that (52, 15.3%) of the commercial drivers had a valid licence despite not meeting the minimum legal requirement to drive.^[3]

In this study, ten of participants were found to have a defect in the right eye and 8 in the left eye; however, only 1 (9.1%) of the participants with VF defect said that he has had RTC before compared to 28 (26.2) of their counterparts with no VF defect. The relationship between VF and involvement in RTC was not significant, with a p = 0.290. Ovenseri-Ogomo and Adofo^[15] did not also find a link between VF defect and RTC from their study. On the contrary, Abraham et al., in a study done in eastern Nigeria, reported a positive correlation between VF and RTC.^[17]

Participants of this study were assessed for red-green colour defect with the Ishihara chart, and eleven of them were found to have a colour defect in the right eye and eight in the left eye. Out of the participants with colour defect, only 2 (16.6%) gave a report of previous RTC. On the contrary, 113 participants had normal colour vision and 27 (24.1%) of them have had RTC in the past. Thus, colour vision defect was not found to be associated with RTC (p = 0.728). Similarly, only seven (3.4%) out of 206 subjects interviewed and examined in a study in Ghana had colour vision deficiency and none of them reported a history of RTC. Boadi-Kusi et al.^[18] gave an entirely different report which is that protan colour defect is significantly associated with RTC traffic crash, although deutan and tritan were not found to have significant correlation with RTC.^[13]

Regarding contrast sensitivity, twenty-eight (24.1%) participants with normal contrast sensitivity and one (12.5%) with abnormal contrast sensitivity have been involved in RTC. Hence, abnormal contrast sensitivity was not correlated with RTC (p = 0.677).

Evaluating the crash rate and its relationship with impairment in the visual function; in this study, participants with poor visual function (visual acuity, VF, colour vision and contrast sensitivity) were not found to have more crashes than their counterparts with normal visual function. Pepple and Adio^[12] did not also found impaired visual function to be statistically significant for RTC, although contrast sensitivity was not included in the visual function assessed in their study.^[9]

CONCLUSION

Many of the participants have never had vision screening either at licence issuance or renewal, yet hold a valid licence and have been driving for years; thus, vision screening is yet to be fully accepted and implemented in Nigeria.

Furthermore, from the study, there is no relationship between visual impairment and previous RTC, although the role of visual impairment cannot be completely ruled out as participants with visual impairment were found to drive less often than their counterparts with good vision, thus significantly reducing their driving exposure and crash risk to a level comparable or even below that of participants with no visual impairment.

There was no association between the various tested visual functions of commercial drivers and RTC; hence, other factors such as the mental state of the driver and the condition of the road and vehicle may play a more important role in RTC.